# Writing and balancing chemical equations

> Count the atoms on both sides


## Objectives

- Name the parts of an equation: reactants, products, coefficients, subscripts, conditions over the arrow.
- Balance an equation by changing coefficients only, never subscripts.
- Check a balanced equation by counting each kind of atom on both sides.

## 1. Properties of iron




## 2. Do the atoms add up?

Predict: count the atoms. Is this equation balanced?

- Yes: N and H appear on both sides
- We can't tell without the conditions
- No: the atom counts differ ✓

The equation is not balanced: the left side has 2 N atoms and 2 H atoms, while the right side has 1 N atom and 3 H atoms. Atoms are not created or destroyed in a reaction, so coefficients are needed to make the counts equal.

[ar-EG] الـ equation دي مش balanced: على الشمال فيه 2 ذرة N و 2 ذرة H، وعلى اليمين ذرة N واحدة و 3 ذرات H. والذرات مش بتتخلق ولا بتختفي في الـ reaction، عشان كده محتاجين coefficients تخلّي الأعداد متساوية.
## 3. Coefficients, not subscripts

Coefficient The number written before a formula in an equation. It multiplies the whole formula. You can balance an equation by changing a subscript, for example $H_2O$ → $H_2O_2$. Conditions, like a catalyst or temperature, go over the arrow: $\ce{->[\Delta]}$.


## 4. Worked: water

Not balanced yet Count: H 2 and 2; O 2 on the left, 1 on the right. O is not balanced. Put 2 before $H_2O$: O is now 2 and 2. But H is now 2 on the left and 4 on the right. Put 2 before $H_2$: H is 4 and 4. Check every atom: H 4 = 4, O 2 = 2. Balanced: $\ce{2H2 + O2 -> 2H2O}$. Fixing one atom can unbalance another. So always recount at the end.


## 5. Balance it yourself

Set the coefficients for $\ce{N2 + H2 -> NH3}$. Make both sides match. Coefficient of $\ce{N2}$ Coefficient of $\ce{H2}$ Coefficient of $\ce{NH3}$ N left N right H left H right Balanced Balanced! N 2 = 2, H 6 = 6 with the lowest whole numbers.


## 6. New equations

To balance $\ce{Mg + O2 -> MgO}$, a student writes $\ce{Mg + O2 -> MgO2}$. What is wrong?

- A coefficient was put before $O_2$
- The arrow should point the other way
- The subscript was changed, so the product is no longer magnesium oxide ✓
- Nothing: the atoms are now equal on both sides

Changing the subscript turned the product into $MgO_2$, which is a different substance from magnesium oxide. Only coefficients may change when balancing: $\ce{2Mg + O2 -> 2MgO}$.

[ar-EG] تغيير الـ subscript خلّى الـ product يبقى $MgO_2$، ودي مادة تانية غير الـ magnesium oxide. في الـ balancing الـ coefficients لوحدها هي اللي مسموح تتغير: $\ce{2Mg + O2 -> 2MgO}$.
## 7. Summary

Reactants → products; conditions are written over the arrow. Balance by changing coefficients only, never subscripts. Check: each kind of atom has the same count on both sides. Back to the hook: $\ce{N2 + 3H2 -> 2NH3}$ has 2 N and 6 H on each side. No atom is lost. Key words: **balanced equation**, **coefficient**, **conditions over the arrow**. Can you explain why changing $H_2O$ to $H_2O_2$ is not a way to balance an equation? A subscript is part of the formula: changing $H_2O$ to $H_2O_2$ turns water into a different substance. Only coefficients may change: $\ce{2H2 + O2 -> 2H2O}$.



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-chem.equation-balancing