Transition Elements · تدريب

كل الأسئلة بإجاباتها · Transition Elements

1. Which of the following is a compound?

   • B. Oxygen gas, O2$\mathrm{O}{}_2$
   • C. Sand mixed with salt
   • A. Magnesium oxide, MgO$\mathrm{MgO}$
   • D. Iron, Fe$\mathrm{Fe}$

   الإجابة: Magnesium oxide, MgO$\mathrm{MgO}$

   A compound is two or more elements chemically combined in a fixed ratio. MgO has two kinds of atom, Mg and O, bonded 1 : 1. O2$\mathrm{O}{}_2$ and Fe each contain one kind of atom, so they are elements. Sand with salt is a mixture: nothing is bonded, and the amounts can vary. Common slip: calling O2$\mathrm{O}{}_2$ a compound because its molecule has two atoms.

2. An iron ore contains iron compounds (mostly oxides) mixed with impurities. Which observation shows that an iron ore is a mixture, not a compound?

   • A. Its iron percentage differs from one deposit to another, and some impurities can be separated by physical means.
   • D. It is a solid with a definite colour.
   • C. It contains iron chemically combined with oxygen.
   • B. It contains more than one element.

   الإجابة: Its iron percentage differs from one deposit to another, and some impurities can be separated by physical means.

   A compound has a fixed ratio, so its composition never changes. A mixture keeps the properties of its parts. Its composition can vary, and its parts are separated by physical means. An ore's iron percentage changes from place to place, and sand can be removed physically. So the ore is a mixture. The iron oxide inside it is still a compound. Common slip: thinking 'more than one element' means mixture.

3. Magnesium sulphate crystals have the formula MgSO4.7H2O$\mathrm{MgSO}{}_4\cdot 7\mathrm{H}{}_2\mathrm{O}$. How many water molecules does the crystal hold for every one MgSO4$\mathrm{MgSO}{}_4$?

   • A. 7
   • D. 1
   • B. 14
   • C. 21

   الإجابة: 7

   In a hydrate formula, the dot joins the salt to its water of crystallization. The number in front of H2O$\mathrm{H}{}_2\mathrm{O}$ counts water molecules. So 7H2O$7\mathrm{H}{}_2\mathrm{O}$ means seven water molecules for every one MgSO4$\mathrm{MgSO}{}_4$. Common slip: counting the 14 hydrogen atoms instead of the 7 molecules.

4. Barium chloride crystals, BaCl2.2H2O$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}$, are heated strongly until all their water of crystallization is driven off. Which equation shows this change?

   • A. BaCl2.2H2O(s) →[Δ] BaCl2(s) + 2H2O(v)$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{BaCl}{}_2(\mathrm{s})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$
   • B. BaCl2.2H2O(s) →[Δ] BaCl2(s) + 2H2(g) + O2(g)$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{BaCl}{}_2(\mathrm{s})+2\mathrm{H}{}_2(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})$
   • D. BaCl2.2H2O(s) →[Δ] Ba(s) + Cl2(g) + 2H2O(v)$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{Ba}(\mathrm{s})+\mathrm{Cl}{}_2(\mathrm{g})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$
   • C. BaCl2.2H2O(s) →[Δ] BaCl2.H2O(s) + H2O(v)$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{BaCl}{}_2\cdot \mathrm{H}{}_2\mathrm{O}(\mathrm{s})+\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$

   الإجابة: BaCl2.2H2O(s) →[Δ] BaCl2(s) + 2H2O(v)$\mathrm{BaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{BaCl}{}_2(\mathrm{s})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$

   Heating drives off water of crystallization as vapour. The salt itself stays as it is. Two water molecules leave for every BaCl2$\mathrm{BaCl}{}_2$, so the equation shows 2H2O(v)$2\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$. The same thing happens when limonite is roasted. Common slip: splitting the water into H2$\mathrm{H}{}_2$ and O2$\mathrm{O}{}_2$.

5. Magnetic separation can separate the two parts of a mixture when ...

   • D. both parts are attracted by a magnet
   • B. both parts are metals
   • A. one part is attracted by a magnet and the other part is not
   • C. the two parts have different colours

   الإجابة: one part is attracted by a magnet and the other part is not

   To separate a mixture, use a property its parts do not share. In magnetic separation, the magnet pulls the attracted part. The other part stays behind. No new substance forms. Common slip: thinking every metal is attracted by a magnet.

6. Iron filings are mixed with copper powder. The particles are about the same size. Which method separates them and keeps both metals unchanged?

   • D. Shaking the mixture through a sieve
   • B. Heating the mixture strongly in air
   • A. Passing a magnet over the mixture
   • C. Adding water, then filtering

   الإجابة: Passing a magnet over the mixture

   Look for a property the two parts do not share. Iron is attracted by a magnet, and copper is not. So a magnet pulls out the iron and leaves the copper. This is a physical method: no new substance forms. Common slip: heating, which turns the metals into new substances.

7. Which of the following equations represents a thermal decomposition reaction?

   • D. KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l)$\mathrm{KOH}(\mathrm{aq})+\mathrm{HNO}{}_3(\mathrm{aq})\to \mathrm{KNO}{}_3(\mathrm{aq})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})$
   • A. MgCO3(s) →[Δ] MgO(s) + CO2(g)$\mathrm{MgCO}{}_3(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{MgO}(\mathrm{s})+\mathrm{CO}{}_2(\mathrm{g})$
   • C. Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s)$\mathrm{Zn}(\mathrm{s})+2\mathrm{AgNO}{}_3(\mathrm{aq})\to \mathrm{Zn}(\mathrm{NO}{}_3){}_2(\mathrm{aq})+2\mathrm{Ag}(\mathrm{s})$
   • B. S(s) + O2(g) →[Δ] SO2(g)$\mathrm{S}(\mathrm{s})+\mathrm{O}{}_2(\mathrm{g})\stackrel{\mathrm{\Delta }}{\to }\mathrm{SO}{}_2(\mathrm{g})$

   الإجابة: MgCO3(s) →[Δ] MgO(s) + CO2(g)$\mathrm{MgCO}{}_3(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{MgO}(\mathrm{s})+\mathrm{CO}{}_2(\mathrm{g})$

   Thermal decomposition breaks one compound into simpler substances by heat. Pattern: one compound in, two or more substances out, with Δ over the arrow. MgCO3$\mathrm{MgCO}{}_3$ breaks into MgO and CO2$\mathrm{CO}{}_2$. Sulphur burning is two substances joining. Zinc with silver nitrate is simple displacement. KOH with HNO3$\mathrm{HNO}{}_3$ is neutralization, a double displacement. Common slip: calling any heated reaction a decomposition.

8. Lead(II) nitrate solution is added to potassium iodide solution: Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) v + 2KNO3(aq)$\mathrm{Pb}(\mathrm{NO}{}_3){}_2(\mathrm{aq})+2\mathrm{KI}(\mathrm{aq})\to \mathrm{PbI}{}_2(\mathrm{s})\downarrow +2\mathrm{KNO}{}_3(\mathrm{aq})$. Which statement describes this reaction?

   • A. Double displacement; PbI2$\mathrm{PbI}{}_2$ is a precipitate
   • B. Simple displacement; potassium takes the place of lead
   • D. Thermal decomposition, because a solid forms
   • C. Double displacement; KNO3$\mathrm{KNO}{}_3$ is a precipitate

   الإجابة: Double displacement; PbI2$\mathrm{PbI}{}_2$ is a precipitate

   Two compounds exchange their partners: AB + CD → AD + CB. That is double displacement. PbI2$\mathrm{PbI}{}_2$ does not dissolve, so it separates as a solid. The ↓ sign marks it as a precipitate. KNO3$\mathrm{KNO}{}_3$ stays dissolved (aq). Common slip: calling it simple displacement, but no free element takes part.

9. Natural gas is mostly methane, CH4$\mathrm{CH}{}_4$. What is the molar mass of methane? (C = 12, H = 1)

   • A. 16 g/mol
   • C. 13 g/mol
   • B. 5 g/mol
   • D. 52 g/mol

   الإجابة: 16 g/mol

   Molar mass is the sum of the atomic masses in the formula. Multiply each atomic mass by its number of atoms, then add. CH4$\mathrm{CH}{}_4$: 1 × 12 + 4 × 1 = 16 g/mol. Common slip: 13 g/mol, from forgetting the subscript 4.

10. Green iron(II) sulphate crystals have the formula FeSO4.7H2O$\mathrm{FeSO}{}_4\cdot 7\mathrm{H}{}_2\mathrm{O}$. Calculate their molar mass. (Fe = 56, S = 32, O = 16, H = 1)

    الإجابة: 278 ± 0.5 g/mol

    Work in two parts. FeSO4$\mathrm{FeSO}{}_4$ = 56 + 32 + 4 × 16 = 152 g/mol. Each H2O$\mathrm{H}{}_2\mathrm{O}$ = 2 × 1 + 16 = 18. Seven of them = 7 × 18 = 126. Add the parts: 152 + 126 = 278 g/mol. The dot means: add the water's mass too. Likely wrong answers: 152 (water left out) or 170 (only one water counted).

11. Which expression gives the mass percentage of iron in iron(II) oxide, FeO$\mathrm{FeO}$? (Fe = 56, O = 16)

    • B. 12 × 100$\frac{1}{2}\times 100$
    • C. 1672 × 100$\frac{16}{72}\times 100$
    • D. 7256 × 100$\frac{72}{56}\times 100$
    • A. 5672 × 100$\frac{56}{72}\times 100$

    الإجابة: 5672 × 100$\frac{56}{72}\times 100$

    % of an element = (mass of the element in one mole ÷ molar mass) × 100. Molar mass of FeO = 56 + 16 = 72 g/mol. One mole holds 56 g of iron. So % Fe = 56/72 × 100, about 77.8%. Common slip: 1/2 × 100, which counts atoms, not masses.

12. Calculate the mass percentage of iron in iron(III) sulphate, Fe2(SO4)3$\mathrm{Fe}{}_2(\mathrm{SO}{}_4){}_3$. (Fe = 56, S = 32, O = 16)

    الإجابة: 28 ± 0.5 %

    Step 1, molar mass: one SO4$\mathrm{SO}{}_4$ group = 32 + 4 × 16 = 96. Three groups = 288. Add 2 × 56 = 112. Total = 400 g/mol. Step 2, iron in one mole = 2 × 56 = 112 g. Step 3: % Fe = 112/400 × 100 = 28%. Likely wrong answers: 14% (one Fe only), or about 54% (the 3 after the bracket ignored).

13. Natural gas is used as a fuel in homes and in industry. What is its main component?

    • B. Carbon monoxide, CO$\mathrm{CO}$
    • D. Carbon dioxide, CO2$\mathrm{CO}{}_2$
    • A. Methane, CH4$\mathrm{CH}{}_4$
    • C. Hydrogen, H2$\mathrm{H}{}_2$

    الإجابة: Methane, CH4$\mathrm{CH}{}_4$

    Natural gas is a fuel found underground. Its main part is methane, CH4$\mathrm{CH}{}_4$. The book gives 93% methane. In the Midrex furnace, natural gas is changed into the gases that reduce iron ore. Common slip: choosing CO, which is made from natural gas, not found as it.

14. Methane, CH4$\mathrm{CH}{}_4$, is called the simplest hydrocarbon. Why?

    • C. Its molecule contains only one hydrogen atom.
    • A. It contains carbon and hydrogen only, and its molecule has only one carbon atom.
    • B. It contains carbon, hydrogen and oxygen in the smallest amounts.
    • D. It is a gas at room temperature.

    الإجابة: It contains carbon and hydrogen only, and its molecule has only one carbon atom.

    A hydrocarbon is a compound of carbon and hydrogen only. Methane, CH4$\mathrm{CH}{}_4$, has one carbon atom bonded to four hydrogen atoms. No hydrocarbon has fewer carbon atoms, so methane is the simplest. Common slip: misreading CH4$\mathrm{CH}{}_4$ as having one hydrogen atom.

15. Choose the correct statement about iron.

    • D. An iron ore is suitable for extraction of iron when its iron percentage is high, whatever its impurities.
    • B. Iron is the most abundant element in the earth's crust, because it is the most important metal in heavy industries.
    • A. Only three elements are more abundant than iron in the earth's crust: oxygen, silicon and aluminum.
    • C. The mass of iron in the earth decreases gradually as we come close to the earth's core.

    الإجابة: Only three elements are more abundant than iron in the earth's crust: oxygen, silicon and aluminum.

    Iron is the fourth most abundant element in the earth's crust, after oxygen, silicon and aluminum. It forms about 5% of the crust by mass. Its mass increases gradually as we come close to the earth's core. An ore's suitability depends on three factors. They are the iron percentage, the impurities' composition and harmful elements (S, P, As). Common slip: 'most important' read as 'most abundant'.

16. Match each ore in column (A) with its chemical formula and colour in column (B).

    الإجابة: Hematite → Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$, blood red · Limonite → 2Fe2O3.3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}$, yellow · Magnetite → Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$, black · Siderite → FeCO3$\mathrm{FeCO}{}_3$, yellowish grey

    Hematite is iron (III) oxide, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$, blood red. Limonite is hydrated iron (III) oxide, 2Fe2O3.3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}$, yellow. Magnetite is magnetic iron oxide, Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$, black, with magnetic properties. Siderite is iron (II) carbonate, FeCO3$\mathrm{FeCO}{}_3$, yellowish grey. Most common slip: swapping the formulas of hematite and magnetite. Another: matching yellow limonite with yellowish grey siderite.

17. Arrange the following events in the order in which they happen as iron ore is prepared and used in the furnace.

    الإجابة: The ore is crushed into pieces of a suitable size. → A huge amount of fine ore particles is obtained. → The fine particles are collected into larger, homogeneous pieces (sintering). → The ore is reduced to iron in the furnace.

    Crushing gives the ore a suitable size, so it is easily reduced. Crushing (and cleaning the furnace) also gives a huge amount of fine particles. These cannot be used directly in the furnace. Sintering collects them into larger pieces of similar size. Later, after roasting too, the ore goes to the furnace for reduction. Most likely wrong order: sintering placed before crushing, as if sintering made the fine particles.

18. Write the balanced equation for the thermal decomposition of siderite during roasting, with state symbols.

    الإجابة: FeCO3(s) →[Δ] FeO(s) + CO2(g)$\mathrm{FeCO}{}_3(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{FeO}(\mathrm{s})+\mathrm{CO}{}_2(\mathrm{g})$

    Siderite is iron (II) carbonate, FeCO3$\mathrm{FeCO}{}_3$. Strong heating breaks it into iron (II) oxide and carbon dioxide gas. Heat (Δ) goes over the arrow. Balance: 1 Fe, 1 C and 3 O on each side. In air, the FeO is then oxidized: 2FeO + 1/2O2 → Fe2O3$2\mathrm{FeO}+\frac{1}{2}\mathrm{O}{}_2\to \mathrm{Fe}{}_2\mathrm{O}{}_3$. Grader looks for: FeCO3$\mathrm{FeCO}{}_3$ alone on the left, FeO and CO2$\mathrm{CO}{}_2$ on the right, Δ. Most likely wrong answer: writing Fe as a product, as if roasting reduced the ore.

19. Explain the role of coke in the blast furnace. Support your answer with equations.

    الإجابة: Key points: Coke is the source of carbon monoxide, the reducing agent. (1) C(s) + O2(g) →[Δ] CO2(g)$\mathrm{C}(\mathrm{s})+\mathrm{O}{}_2(\mathrm{g})\stackrel{\mathrm{\Delta }}{\to }\mathrm{CO}{}_2(\mathrm{g})$ (2) CO2(g) + C(s) →[Δ] 2CO(g)$\mathrm{CO}{}_2(\mathrm{g})+\mathrm{C}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }2\mathrm{CO}(\mathrm{g})$ (3) Carbon monoxide reduces the ore: 3CO(g) + Fe2O3(s) →[higher than 700\,^C] 2Fe(s) + 3CO2(g)$3\mathrm{CO}(\mathrm{g})+\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})\stackrel{\text{higher than }700{}_{}^{}{}_{}^{\circ }\mathrm{C}}{\to }2\mathrm{Fe}(\mathrm{s})+3\mathrm{CO}{}_2(\mathrm{g})$.

    Coke is a form of carbon. It burns to carbon dioxide. The carbon dioxide meets more hot coke and becomes carbon monoxide. Carbon monoxide takes oxygen from iron (III) oxide at higher than 700 °C, giving iron. So coke's role: the source of carbon monoxide, the reducing agent. Grader looks for the three equations and the words 'reducing agent'. Most likely wrong answer: 'coke reduces the ore directly'. Solid carbon never meets Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$ in the book's equations.

20. Explain the role of natural gas in the Midrex furnace. Support your answer with equations.

    الإجابة: Key points: Natural gas (93% methane) is the source of water gas, a mixture of carbon monoxide and hydrogen, which are the reducing agents. (1) 2CH4(g) + CO2(g) + H2O(v) →[Δ] 3CO(g) + 5H2(g)$2\mathrm{CH}{}_4(\mathrm{g})+\mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{v})\stackrel{\mathrm{\Delta }}{\to }3\mathrm{CO}(\mathrm{g})+5\mathrm{H}{}_2(\mathrm{g})$ (2) 2Fe2O3(s) + 3CO(g) + 3H2(g) →[Δ] 4Fe(s) + 3CO2(g) + 3H2O(v)$2\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})+3\mathrm{CO}(\mathrm{g})+3\mathrm{H}{}_2(\mathrm{g})\stackrel{\mathrm{\Delta }}{\to }4\mathrm{Fe}(\mathrm{s})+3\mathrm{CO}{}_2(\mathrm{g})+3\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$.

    Natural gas is 93% methane. Methane reacts with carbon dioxide and water vapour to give water gas: carbon monoxide and hydrogen. Both gases take oxygen from iron (III) oxide, giving iron. So the role of natural gas: the source of the reducing mixture (water gas). Grader looks for: both equations, and 'CO and H2 are the reducing agents'. Most likely wrong answer: 'methane reduces the ore'. Methane never meets the ore in the book's equations.

21. The steel industry depends on two essential processes. What are they?

    • C. Removing every element other than iron, to give pure iron
    • B. Reducing the iron ore by carbon monoxide, and removing impurities from the iron
    • A. Removing impurities from the furnace iron, and adding some elements to it
    • D. Roasting the iron in air, and passing carbon monoxide over it

    الإجابة: Removing impurities from the furnace iron, and adding some elements to it

    After reduction comes the third stage: iron production, such as cast iron and steel. Steel making rests on two processes. First, removal of impurities from the iron that comes out of the furnaces. Second, addition of some elements to give the properties needed for industry. Common slip: thinking steel is pure iron. Elements such as carbon are added on purpose.

22. A scientist finds a piece of metal that is about 90% pure iron. According to the book, where does iron in this form come from?

    • B. A hematite deposit in the Oasis area
    • C. Rock dug from deep in the crust, close to the core
    • A. A meteorite that fell from space
    • D. A magnetite deposit, because magnetite is attracted by a magnet

    الإجابة: A meteorite that fell from space

    In the earth's crust, iron is found as natural ores. These contain different iron compounds (mostly oxides) mixed with impurities. Iron is found as nearly pure metal (90%) only in meteorites. So a 90% iron lump points to a meteorite. Common slip: thinking magnetite, being magnetic, is iron metal. It is an iron oxide.

23. Sample	Ore	Iron compound in the sample (by mass)
    X	Hematite	75% Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$
    Y	Magnetite	70% Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$
    Z	Siderite	85% FeCO3$\mathrm{FeCO}{}_3$

    Three ore samples, each of mass 100 kg, were analysed (see the table). The impurities in them contain no iron. Which sample has the highest percentage of iron? (Fe = 56, C = 12, O = 16)

    • A. Sample X
    • C. Sample Z
    • D. The three samples are equal, because each has a mass of 100 kg
    • B. Sample Y

    الإجابة: Sample X

    Step 1, % Fe in each pure compound. Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$: 112/160 × 100 = 70%. Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$: 168/232 × 100 = 72.4%. FeCO3$\mathrm{FeCO}{}_3$: 56/116 × 100 = 48.3%. Step 2, iron in each 100 kg sample. X: 75% of 70% = 52.5%. Y: 70% of 72.4% = 50.7%. Z: 85% of 48.3% = 41.0%. So X is richest, even though pure magnetite is richer than pure hematite. Likely wrong answers: Y (only the pure compound compared) or Z (only the compound share compared).

24. Give reasons: Sintering processes are carried out on iron ore after crushing.

    الإجابة: Key points: (1) Crushing (and cleaning the furnace) gives a huge amount of fine ore particles. (2) These fine particles cannot be used directly in the furnace. (3) Sintering collects them into larger pieces of similar size (homogeneous), so they can be used.

    Crushing gives pieces of a suitable size. But it also makes a huge amount of fine particles. The furnace cannot use these fine particles directly. Sintering gathers them into larger pieces that are similar and homogeneous. A full answer names the fine particles, says they cannot be used directly, and says sintering collects them. Most likely wrong answer: 'sintering melts the ore' or 'sintering removes impurities'. Removing impurities is the job of concentrating.

25. Explain the role of roasting in the extraction of iron. Support each purpose with one equation.

    الإجابة: Key points: Roasting = heating the ore strongly in air. Purpose (a): drying the ore and expelling humidity, to increase the percentage of iron, e.g. 2Fe2O3.3H2O(s) →[Δ] 2Fe2O3(s) + 3H2O(v)$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }2\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})+3\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$ (or FeCO3 →[Δ] FeO + CO2$\mathrm{FeCO}{}_3\stackrel{\mathrm{\Delta }}{\to }\mathrm{FeO}+\mathrm{CO}{}_2$). Purpose (b): oxidation of some impurities such as sulphur and phosphorus, e.g. S + O2 →[Δ] SO2$\mathrm{S}+\mathrm{O}{}_2\stackrel{\mathrm{\Delta }}{\to }\mathrm{SO}{}_2$ or 4P + 5O2 →[Δ] 2P2O5$4\mathrm{P}+5\mathrm{O}{}_2\stackrel{\mathrm{\Delta }}{\to }2\mathrm{P}{}_2\mathrm{O}{}_5$.

    Roasting improves the chemical properties of the ore. The ore is strongly heated in air. It has two purposes. First, it dries the ore and expels humidity, which increases the iron percentage. Second, it oxidizes impurities such as sulphur and phosphorus. Any book equation for each purpose earns the mark. Most likely wrong answer: 'roasting reduces the ore to iron'. Reduction comes later, in the blast furnace or Midrex furnace.

26. 1. Coke, C → Gas A ((1) + O2$\mathrm{O}{}_2$, heat)
    2. Gas A → Gas B ((2) + hot coke)
    3. Gas B → Fe + gas A ((3) + Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$, higher than 700 °C)

    The diagram shows the reactions in the blast furnace. Identify gases A and B, then write the balanced equation of each of the three steps.

    الإجابة: Key points: A = carbon dioxide, CO2$\mathrm{CO}{}_2$; B = carbon monoxide, CO. (1) C(s) + O2(g) →[Δ] CO2(g)$\mathrm{C}(\mathrm{s})+\mathrm{O}{}_2(\mathrm{g})\stackrel{\mathrm{\Delta }}{\to }\mathrm{CO}{}_2(\mathrm{g})$ (2) CO2(g) + C(s) →[Δ] 2CO(g)$\mathrm{CO}{}_2(\mathrm{g})+\mathrm{C}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }2\mathrm{CO}(\mathrm{g})$ (3) 3CO(g) + Fe2O3(s) →[higher than 700\,^C] 2Fe(s) + 3CO2(g)$3\mathrm{CO}(\mathrm{g})+\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})\stackrel{\text{higher than }700{}_{}^{}{}_{}^{\circ }\mathrm{C}}{\to }2\mathrm{Fe}(\mathrm{s})+3\mathrm{CO}{}_2(\mathrm{g})$.

    Read the clues. Gas A forms when coke burns in oxygen, so A is CO2$\mathrm{CO}{}_2$. Step (3) gives A back, which fits: CO becomes CO2$\mathrm{CO}{}_2$ when it takes oxygen. Gas A plus hot coke gives B, which reduces the ore. So B is CO. Balance step (3): each CO takes one O atom, and Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$ has three. So 3CO give 2Fe and 3CO2$3\mathrm{CO}{}_2$. Most likely wrong answer: swapping A and B, which makes CO2$\mathrm{CO}{}_2$ the reducing agent.

27. In the Midrex furnace, natural gas is changed into water gas. Complete and balance the equation: 2CH4(g) + CO2(g) + H2O(v) →[Δ]$2\mathrm{CH}{}_4(\mathrm{g})+\mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{v})\stackrel{\mathrm{\Delta }}{\to }$ ...

    الإجابة: 2CH4(g) + CO2(g) + H2O(v) →[Δ] 3CO(g) + 5H2(g)$2\mathrm{CH}{}_4(\mathrm{g})+\mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{v})\stackrel{\mathrm{\Delta }}{\to }3\mathrm{CO}(\mathrm{g})+5\mathrm{H}{}_2(\mathrm{g})$

    Water gas is a mixture of carbon monoxide and hydrogen. Count atoms on the left. C: 2 + 1 = 3, so 3CO. O: 2 + 1 = 3, and 3CO hold exactly 3 O. H: 8 + 2 = 10, so 5H2$5\mathrm{H}{}_2$. Grader looks for: CO and H2$\mathrm{H}{}_2$ as products, coefficients 3 and 5. Most likely wrong answer: writing CO2$\mathrm{CO}{}_2$ or H2O$\mathrm{H}{}_2\mathrm{O}$ among the products, which are the products of the reduction step.

28. Which of the following statements about steel production are correct? Choose all that apply.

    • S2. Impurities are removed from the iron that comes out of the blast furnace or the Midrex furnace.
    • S4. Steel comes directly out of the Midrex furnace.
    • S3. Some elements are added to iron to give steel the properties needed by industry.
    • S6. The blast furnace is one of the three furnaces used to produce steel.
    • S1. Steel can be produced in an electric furnace.
    • S5. Steel is iron from which every other element has been removed.

    الإجابة: Impurities are removed from the iron that comes out of the blast furnace or the Midrex furnace.

    The blast furnace and the Midrex furnace reduce the ore to iron. Steel is made from that iron later. Steel making removes impurities from the furnace iron and adds some elements. Steel is produced in one of three furnaces: the oxygen converters, the open-hearth furnace or the electric furnace. Common slips: 'steel is pure iron', and counting the blast furnace as a steel furnace.

29. A blood-red iron ore is mined in the Oasis area (Western Desert). A sample of it contains 80% by mass of the ore's iron compound. The rest is impurities that contain no iron. Calculate the percentage of iron in the sample. (Fe = 56, O = 16)

    الإجابة: 56 ± 0.5 %

    Step 1: a blood-red ore from the Oasis area is hematite, iron (III) oxide, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$. Step 2: % Fe in pure Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$ = (2 × 56) / 160 × 100 = 70%. Step 3: only 80% of the sample is Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$. So % Fe in the sample = 80/100 × 70 = 56%. This fits the book's range for hematite, 50-60%. The impurities add mass but no iron. Likely wrong answers: 70% (impurities forgotten) or 80% (the compound's share taken as iron's share).

30. According to the book, two of the important iron ores are found in the Oasis area. Which statement compares these two ores correctly?

    • D. Both are hydrated iron (III) oxides with the same formula.
    • B. One of them is an iron oxide and the other is an iron carbonate.
    • A. Both contain iron in oxidation state +3, but only one of them contains water of crystallization.
    • C. One of them is black and has magnetic properties.

    الإجابة: Both contain iron in oxidation state +3, but only one of them contains water of crystallization.

    The book places hematite and limonite in the Oasis area (Western Desert). Hematite is iron (III) oxide, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$. Limonite is hydrated iron (III) oxide, 2Fe2O3.3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}$. So iron is +3 in both. Only limonite holds water of crystallization. Magnetite is in the Eastern Desert. Siderite, the carbonate, has no place given. Common slip: putting magnetite in the Oasis area.

31. A magnetite ore is mixed with sand. The sand contains no iron. Which process removes the sand, and what happens to the iron percentage of the ore?

    • B. Concentrating by magnetic separation; the iron percentage stays the same, because no iron is removed.
    • A. Concentrating by magnetic separation; the iron percentage increases.
    • C. Roasting in air; the sand is oxidized and leaves as a gas.
    • D. Sintering; the fine sand particles are collected into larger pieces.

    الإجابة: Concentrating by magnetic separation; the iron percentage increases.

    Sand is an impurity mixed with the ore, not combined with it. Concentrating processes remove such impurities by surface tension, magnetic or electric separation. Magnetite has magnetic properties and sand does not, so a magnet separates them. The mass of iron stays the same, but the total mass falls. So the iron percentage increases. Common slip: thinking the percentage cannot change because no iron is added.

32. Give reasons: The iron percentage of limonite increases when it is roasted, although no iron is added.

    الإجابة: Key points: (1) Heating drives off the water of crystallization as vapour: 2Fe2O3.3H2O(s) →[Δ] 2Fe2O3(s) + 3H2O(v)$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }2\mathrm{Fe}{}_2\mathrm{O}{}_3(\mathrm{s})+3\mathrm{H}{}_2\mathrm{O}(\mathrm{v})$. (2) The mass of iron stays the same. (3) The total mass of the solid decreases, so iron's share (percentage) increases.

    Limonite is hydrated iron (III) oxide. Roasting drives its water of crystallization out as vapour. No iron leaves, so the mass of iron stays the same. But the total mass of the solid falls. The same iron in a smaller mass gives a larger percentage. Key words: water of crystallization, expelling humidity, mass of iron unchanged. Most likely wrong answer: 'heating makes more iron'. A reaction cannot create iron atoms.

33. A student writes: "Coke reduces the iron ore in the blast furnace." Which statement corrects the student, using the book's equations?

    • B. Coke burns to carbon dioxide, and carbon dioxide takes the oxygen from Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$.
    • C. The student is right: hot coke reacts directly with Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$ at higher than 700 °C.
    • A. Coke is first changed into carbon monoxide, and carbon monoxide takes the oxygen from Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$.
    • D. Oxygen of the air reduces Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$, and coke only heats the furnace.

    الإجابة: Coke is first changed into carbon monoxide, and carbon monoxide takes the oxygen from Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$.

    In the book's equations, the ore is reduced by carbon monoxide, not by coke. Coke burns: C + O2 → CO2$\mathrm{C}+\mathrm{O}{}_2\to \mathrm{CO}{}_2$. Then CO2 + C → 2CO$\mathrm{CO}{}_2+\mathrm{C}\to 2\mathrm{CO}$. Carbon monoxide reduces the ore: 3CO + Fe2O3 → 2Fe + 3CO2$3\mathrm{CO}+\mathrm{Fe}{}_2\mathrm{O}{}_3\to 2\mathrm{Fe}+3\mathrm{CO}{}_2$, at higher than 700 °C. CO gains oxygen, so it is the reducing agent. Coke is the fuel and the source of CO. Common slip: naming CO2$\mathrm{CO}{}_2$, whose carbon already holds all the oxygen it takes here.

34. Compare the blast furnace and the Midrex furnace. Classify each statement.

    الإجابة: Blast furnace only: The reducing gas comes from coke., CO2 + C → 2CO$\mathrm{CO}{}_2+\mathrm{C}\to 2\mathrm{CO}$ is one of its reactions. · Midrex furnace only: Natural gas (93% methane) is the raw material of the reducing gases., Hydrogen is one of the reducing agents., Water vapour is one of the products of the reduction step. · Both furnaces: Carbon monoxide is a reducing agent., Iron (III) oxide is reduced to iron.

    Both furnaces reduce iron (III) oxide to iron, and carbon monoxide is a reducing agent in both. They differ in the source of the reducing agent. Blast furnace: coke gives CO, through C + O2 → CO2$\mathrm{C}+\mathrm{O}{}_2\to \mathrm{CO}{}_2$ and CO2 + C → 2CO$\mathrm{CO}{}_2+\mathrm{C}\to 2\mathrm{CO}$. Midrex furnace: natural gas gives water gas, CO + H2$\mathrm{H}{}_2$. Hydrogen takes oxygen and becomes water vapour, so H2O$\mathrm{H}{}_2\mathrm{O}$ is a Midrex product only. Common slip: putting CO in the blast furnace only.

35. 1. Iron ore → Iron (Furnace X: products iron, CO2$\mathrm{CO}{}_2$ and H2O$\mathrm{H}{}_2\mathrm{O}$)
    2. Iron → Steel (Furnace Y: impurities removed, some elements added)

    The diagram shows the production of steel from iron ore. Which choice identifies furnaces X and Y?

    • D. X: electric furnace; Y: Midrex furnace
    • C. X: Midrex furnace; Y: blast furnace
    • B. X: blast furnace; Y: open-hearth furnace
    • A. X: Midrex furnace; Y: oxygen converter

    الإجابة: X: Midrex furnace; Y: oxygen converter

    Two clues. Furnace X gives iron, CO2$\mathrm{CO}{}_2$ and H2O$\mathrm{H}{}_2\mathrm{O}$. Water vapour means the ore was reduced by H2$\mathrm{H}{}_2$ as well as CO: the water gas made from natural gas in the Midrex furnace, 2Fe2O3 + 3CO + 3H2 → 4Fe + 3CO2 + 3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3+3\mathrm{CO}+3\mathrm{H}{}_2\to 4\mathrm{Fe}+3\mathrm{CO}{}_2+3\mathrm{H}{}_2\mathrm{O}$. The blast furnace reduces with CO from coke, so it gives only CO2$\mathrm{CO}{}_2$ with the iron. Furnace Y turns iron into steel by removing impurities and adding elements. Steel furnaces are the oxygen converters, the open-hearth furnace and the electric furnace. So Y can be an oxygen converter. Common slip: calling the blast furnace a steel furnace.

36. An iron ore has a yellowish grey colour. When it is heated strongly, carbon dioxide gas is given off. What is this ore, and what is its formula?

    • B. Limonite, 2Fe2O3.3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}$
    • D. Siderite, Fe2(CO3)3$\mathrm{Fe}{}_2(\mathrm{CO}{}_3){}_3$
    • A. Siderite, FeCO3$\mathrm{FeCO}{}_3$
    • C. Hematite, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$

    الإجابة: Siderite, FeCO3$\mathrm{FeCO}{}_3$

    Two clues. The colour, yellowish grey, belongs to siderite. The gas confirms it: of these ores, only the carbonate gives off CO2$\mathrm{CO}{}_2$ when heated. Siderite is iron (II) carbonate, FeCO3$\mathrm{FeCO}{}_3$. On heating: FeCO3 →[Δ] FeO + CO2$\mathrm{FeCO}{}_3\stackrel{\mathrm{\Delta }}{\to }\mathrm{FeO}+\mathrm{CO}{}_2$. Limonite is yellow, but its heating gives water vapour. Common slip: choosing limonite from the colour alone.

37. A limonite ore contains sulphur as an impurity. The ore is roasted (strongly heated in air). Which gases leave the ore?

    • C. CO2$\mathrm{CO}{}_2$ and SO2$\mathrm{SO}{}_2$
    • D. Hydrogen, oxygen and SO2$\mathrm{SO}{}_2$
    • A. Water vapour and SO2$\mathrm{SO}{}_2$
    • B. Water vapour and sulphur vapour

    الإجابة: Water vapour and SO2$\mathrm{SO}{}_2$

    Use both purposes of roasting on this ore. Drying: limonite loses its water of crystallization as vapour: 2Fe2O3.3H2O → 2Fe2O3 + 3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{Fe}{}_2\mathrm{O}{}_3+3\mathrm{H}{}_2\mathrm{O}$. Oxidation of impurities: sulphur reacts with oxygen of the air, S + O2 → SO2$\mathrm{S}+\mathrm{O}{}_2\to \mathrm{SO}{}_2$. So water vapour and sulphur dioxide leave. Common slip: thinking sulphur simply melts out. It is oxidized.

38. In the blast furnace, carbon monoxide reduces iron (III) oxide. Which choice gives the condition and the products of this reaction?

    • C. Room temperature; Fe and CO2$\mathrm{CO}{}_2$
    • D. Higher than 700 °C; Fe and C
    • B. Higher than 700 °C; Fe and O2$\mathrm{O}{}_2$
    • A. Higher than 700 °C; Fe and CO2$\mathrm{CO}{}_2$

    الإجابة: Higher than 700 °C; Fe and CO2$\mathrm{CO}{}_2$

    The book's equation: 3CO + Fe2O3 → 2Fe + 3CO2$3\mathrm{CO}+\mathrm{Fe}{}_2\mathrm{O}{}_3\to 2\mathrm{Fe}+3\mathrm{CO}{}_2$, at higher than 700 °C. Each CO takes one oxygen atom from the ore and becomes CO2$\mathrm{CO}{}_2$. The iron oxide loses its oxygen and becomes iron. Common slip: writing O2$\mathrm{O}{}_2$ as a product, as if the oxygen simply left the ore.

39. Which gases reduce the iron oxide in the Midrex furnace, and what do they change into?

    • A. CO and H2$\mathrm{H}{}_2$; they change into CO2$\mathrm{CO}{}_2$ and H2O$\mathrm{H}{}_2\mathrm{O}$
    • D. H2$\mathrm{H}{}_2$ only; it changes into H2O$\mathrm{H}{}_2\mathrm{O}$, because the furnace uses no coke
    • C. CO only; it changes into CO2$\mathrm{CO}{}_2$, while H2$\mathrm{H}{}_2$ only heats the furnace
    • B. CH4$\mathrm{CH}{}_4$ and CO2$\mathrm{CO}{}_2$; they change into CO and H2$\mathrm{H}{}_2$

    الإجابة: CO and H2$\mathrm{H}{}_2$; they change into CO2$\mathrm{CO}{}_2$ and H2O$\mathrm{H}{}_2\mathrm{O}$

    Natural gas gives water gas, a mixture of CO and H2$\mathrm{H}{}_2$. Both are reducing agents: 2Fe2O3 + 3CO + 3H2 → 4Fe + 3CO2 + 3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3+3\mathrm{CO}+3\mathrm{H}{}_2\to 4\mathrm{Fe}+3\mathrm{CO}{}_2+3\mathrm{H}{}_2\mathrm{O}$. CO gains oxygen and becomes CO2$\mathrm{CO}{}_2$. H2$\mathrm{H}{}_2$ gains oxygen and becomes H2O$\mathrm{H}{}_2\mathrm{O}$. Common slip: thinking the Midrex furnace has no CO because it uses no coke.

40. Which of the following oxidation states is NOT shown by its element? (₂₁${}_{21}$Sc, ₂₂${}_{22}$Ti, ₂₆${}_{26}$Fe, ₂₉${}_{29}$Cu)

    • B. Titanium (+4)
    • C. Iron (+6)
    • D. Copper (+2)
    • A. Scandium (+4)

    الإجابة: Scandium (+4)

    Scandium is [Ar] 4s² 3d¹$4s^{2}3d^1$. It loses the two 4s electrons and the one 3d electron to give Sc³⁺$\mathrm{Sc}{}^{3+}$. Sc³⁺$\mathrm{Sc}{}^{3+}$ has the stable argon configuration, with no 3d electrons left. A fourth electron would have to come from the complete 3p sublevel of an inner shell. That needs a very large energy, so Sc⁴⁺$\mathrm{Sc}{}^{4+}$ is not formed. Titanium is [Ar] 4s² 3d²$4s^{2}3d^2$: losing four electrons also gives the argon configuration, as in TiO2$\mathrm{TiO}{}_2$. Iron shows +6 in Na2FeO4$\mathrm{Na}{}_2\mathrm{FeO}{}_4$: 2(+1) + x + 4(−2) = 0, so x = +6. Its states above +3 are not important, but they exist. Copper shows +2 in CuO$\mathrm{CuO}$. Group IB is the exception: its maximum state exceeds its group number. Answer: scandium (+4).

41. In which iron ore is all the iron in oxidation state +2? (O = −2; the carbonate group CO3$\mathrm{CO}{}_3$ = −2)

    • B. Hematite, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$
    • D. Magnetite, Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$
    • C. Limonite, 2Fe2O3.3H2O$2\mathrm{Fe}{}_2\mathrm{O}{}_3\cdot 3\mathrm{H}{}_2\mathrm{O}$
    • A. Siderite, FeCO3$\mathrm{FeCO}{}_3$

    الإجابة: Siderite, FeCO3$\mathrm{FeCO}{}_3$

    The sum of oxidation states in a compound is zero. FeCO3$\mathrm{FeCO}{}_3$: Fe + (−2) = 0, so Fe = +2. It is iron (II) carbonate. Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$: 2Fe = +6, so Fe = +3. Limonite is the same oxide plus neutral water: +3. Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$: 3Fe = +8, so not all the iron can be +2. You will see later that it holds both +2 and +3. Common slip: choosing magnetite.

42. Classify the following substances into paramagnetic and diamagnetic substances. (₂₂${}_{22}$Ti, ₂₆${}_{26}$Fe, ₃₀${}_{30}$Zn; Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$ contains both Fe²⁺$\mathrm{Fe}{}^{2+}$ and Fe³⁺$\mathrm{Fe}{}^{3+}$ ions.)

    الإجابة: Diamagnetic: ZnCO3$\mathrm{ZnCO}{}_3$, TiO2$\mathrm{TiO}{}_2$ · Paramagnetic: Siderite, FeCO3$\mathrm{FeCO}{}_3$, Magnetite, Fe3O4$\mathrm{Fe}{}_3\mathrm{O}{}_4$, Hematite, Fe2O3$\mathrm{Fe}{}_2\mathrm{O}{}_3$

    A substance with unpaired electrons is paramagnetic; with all electrons paired, it is diamagnetic. Fe²⁺$\mathrm{Fe}{}^{2+}$ is [Ar] 3d⁶$3d^6$: 4 unpaired. Fe³⁺$\mathrm{Fe}{}^{3+}$ is [Ar] 3d⁵$3d^5$: 5 unpaired. So siderite, hematite and magnetite are paramagnetic. Zn²⁺$\mathrm{Zn}{}^{2+}$ is [Ar] 3d¹⁰$3d^{10}$: all paired. Ti⁴⁺$\mathrm{Ti}{}^{4+}$ is [Ar] 3d⁰$3d^0$: no d electrons. Both are diamagnetic. Common slip: calling every transition-metal compound paramagnetic.