Iron with hot concentrated sulphuric acid

Two salts and sulphur dioxide

1. Properties of iron

Iron with hot concentrated sulphuric acid

2. Same acid, but hot and concentrated

With dilute

H2SO4 H X_{2} SO X_{4}

, iron gives

FeSO4 FeSO X_{4}

and hydrogen. Now use hot, concentrated

H2SO4 H X_{2} SO X_{4}

Pick the right answer

Predict: which gas forms now?

A. Hydrogen, as with the dilute acid
B. Sulphur dioxide, $SO2 SO X_{2}$
C. Oxygen
D. No gas

Explanation

Hot and concentrated,

H2SO4 H X_{2} SO X_{4}

acts as an oxidizing agent: its sulphur is reduced from +6 to +4, so the gas formed is sulphur dioxide,

SO2 SO X_{2}

, not hydrogen. No oxygen gas is released.

3. The book's equation

aligned&3Fe + 8H2SO4 →[conc.,\ Δ]\\&FeSO4 + Fe2(SO4)3\\& + 4SO2 + 8H2Oaligned

Iron reacts with hot concentrated sulphuric acid producing iron (II) sulphate, iron (III) sulphate, sulphur dioxide and water.

True or false?

Hot concentrated

H2SO4 H X_{2} SO X_{4}

gives

H2 H X_{2}

gas with iron, like the dilute acid.

Explanation

The products with hot concentrated

H2SO4 H X_{2} SO X_{4}

are

FeSO4 FeSO X_{4}

Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}

SO2 SO X_{2}

and water: no hydrogen. The acid's sulphur is reduced from +6 to +4 in

SO2 SO X_{2}

4. Two salts, not one

FeSO4 FeSO X_{4}

iron (II) sulphate: iron +2

Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}

iron (III) sulphate: iron +3

3 Fe atoms react. One ends in

FeSO4 FeSO X_{4}

, two end in

Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}

True or false?

Iron with hot concentrated

H2SO4 H X_{2} SO X_{4}

forms only iron (III) sulphate.

Explanation

Both salts form: of the 3 Fe atoms, one ends in iron (II) sulphate,

FeSO4 FeSO X_{4}

, and two in iron (III) sulphate,

Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}

5. Worked: check the balance

A long equation is easy to misremember. Count each atom on both sides.

Atom	Left	Right
Fe	3	1 + 2 = 3
S	8	1 + 3 + 4 = 8
H	16	16 (in 8 H₂O)
O	32	4 + 12 + 8 + 8 = 32

3Fe + 8H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4SO₂ + 8H₂O

Tip: the 8 appears twice, for the acid and for the water.

6. Rebuild it

Balance the equation

Fill in the coefficients of the book's equation.

Fe + H2SO4 →[conc.,\ Δ] FeSO4 + Fe2(SO4)3 + SO2 + H2O

Balanced equation

3Fe + 8H2SO4 →[conc.,\ Δ] FeSO4 + Fe2(SO4)3 + 4SO2 + 8H2O

Explanation

3 Fe share out as 1 + 2. 8 S share out as 1 + 3 + 4. 16 H make 8 water.

Need a hint?

Iron first: one Fe in the iron(II) salt, two in the iron(III) salt.
Sulphur: count S in the two salts, then add the SO₂ molecules.
The acid and the water share one coefficient.

7. Dilute or hot concentrated?

Sort into groups

Sort each product of iron with

H2SO4 H X_{2} SO X_{4}

$H2 H X_{2}$ $SO2 SO X_{2}$ $FeSO4 FeSO X_{4}$ $Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}$

Pick the right answer

Copper does not react with dilute

H2SO4 H X_{2} SO X_{4}

, but it reacts with hot concentrated

H2SO4 H X_{2} SO X_{4}

. Which gas forms?

A. $SO2 SO X_{2}$
B. $H2 H X_{2}$
C. $O2 O X_{2}$

Explanation

Copper is below hydrogen, so it does not react with dilute

H2SO4 H X_{2} SO X_{4}

. The hot concentrated acid acts as an oxidizing agent: it oxidizes copper and its sulphur is reduced to

SO2 SO X_{2}

8. Summary

Product	Dilute $H2SO4 H X_{2} SO X_{4}$	Hot concentrated $H2SO4 H X_{2} SO X_{4}$
$FeSO4 FeSO X_{4}$	yes	yes
$Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}$	no	yes
$H2 H X_{2}$	yes	no
$SO2 SO X_{2}$	no	yes
water	no	yes

Iron with sulphuric acid: products (book p.21)

Iron + hot concentrated $H2SO4 H X_{2} SO X_{4}$ → $FeSO4 FeSO X_{4}$ + $Fe2(SO4)3 Fe X_{2} (SO X_{4}) X_{3}$ + $SO2 SO X_{2}$ + water.
No hydrogen: the acid's sulphur is reduced, +6 → +4 in $SO2 SO X_{2}$ .
Iron + dilute $H2SO4 H X_{2} SO X_{4}$ → $FeSO4 FeSO X_{4}$ + $H2 H X_{2}$ : an iron (II) salt, not an iron (III) salt.

Can you explain why the hot concentrated acid gives no hydrogen gas?

Concepts in this lesson