Chemical Reaction

In a chemical change, the atoms of the substances combine, break apart or exchange partners. This process is called a chemical reaction. For example:

1. Hydrogen gas + oxygen gas → water

(The atoms of hydrogen and oxygen combine.)
2. Calcium carbonate → calcium oxide + carbon dioxide

(Calcium carbonate decomposes, breaks apart, when heated.)
3. Magnesium + hydrochloric acid → magnesium chloride + hydrogen gas

(Magnesium and hydrogen exchange places: magnesium takes hydrogen’s place beside chlorine.)

Reactants and products

When carbon burns, it combines with oxygen from the air and forms carbon dioxide:

carbon + oxygen → carbon dioxide

• The substances that take part in a reaction, and are used up, are the reactants: here carbon and oxygen.
• The new substances formed by the reaction are the products: here carbon dioxide.

sodium + chlorine	→	sodium chloride
reactants		product

We can write this reaction in words or in formulae:

Zinc + hydrochloric acid → zinc chloride + hydrogen gas
$$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$$

Writing a chemical reaction in words or in formulae is called a chemical equation. The reactants are written on the left of an arrow and the products on the right. The arrow is read as “gives” or “forms”.

Word equation

When calcium carbonate is heated, it gives calcium oxide and carbon dioxide:

Calcium carbonate → calcium oxide + carbon dioxide

A chemical equation written with the names of the reactants and products is a word equation.

Formula equation

The same reaction written with symbols and formulae is a formula equation:

$$\text{CaCO}_3 \xrightarrow{\text{heat}} \text{CaO} + \text{CO}_2$$

A condition needed for the reaction (heat, light, a catalyst, high pressure) is written over the arrow. Heat is often shown by the sign Δ.

Unbalanced chemical equation

Count the atoms of each element on both sides of these equations.

1. Zn + HCl → ZnCl₂ + H₂

Element	Atoms in the reactants	Atoms in the products
zinc	1	1
hydrogen	1	2
chlorine	1	2

Zinc is the same on both sides, but hydrogen and chlorine are not.

2. Mg + O₂ → MgO

Element	Atoms in the reactants	Atoms in the products
magnesium	1	1
oxygen	2	1

Magnesium is the same on both sides, but oxygen is not.

3. KClO₃ → KCl + O₂

Element	Atoms in the reactants	Atoms in the products
potassium	1	1
chlorine	1	1
oxygen	3	2

Potassium and chlorine are the same on both sides, but oxygen is not.

A chemical equation in which the number of atoms of some element is not the same on both sides is an unbalanced chemical equation (also called a skeleton equation). It is not yet a true picture of the reaction, because atoms can’t vanish or appear from nowhere. It needs to be balanced.

Balanced chemical equation

Now look at the same reactions, with numbers written in front of some formulae:

1. Zn + 2HCl → ZnCl₂ + H₂

Element	Atoms in the reactants	Atoms in the products
zinc	1	1
hydrogen	2	2
chlorine	2	2

2. 2Mg + O₂ → 2MgO

Element	Atoms in the reactants	Atoms in the products
magnesium	2	2
oxygen	2	2

3. 2KClO₃ → 2KCl + 3O₂

Element	Atoms in the reactants	Atoms in the products
potassium	2	2
chlorine	2	2
oxygen	6	6

A chemical equation with the same number of atoms of each element on both sides is a balanced chemical equation.

The number written in front of a formula is its coefficient. It multiplies every atom in the formula: 3O₂ means three oxygen molecules, 3 × 2 = 6 oxygen atoms; 2KClO₃ has 2 K, 2 Cl and 2 × 3 = 6 O.

A balanced equation should also show the physical state of each substance, in brackets after its formula: (s) for a solid, (l) for a liquid, (g) for a gas, and (aq) for a substance dissolved in water (aqueous):

$$\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

How to balance a chemical equation

In a chemical reaction, matter is neither created nor destroyed: the total mass of the products equals the total mass of the reactants. This is the law of conservation of mass, and it is why an equation must balance. The usual way to balance it is by hit and trial (trial and error):

• a.Write the reaction as a word equation.
• b.Write the correct formula of each reactant and product.
• c.Never change the small numbers (subscripts) inside a formula: that would change the substance itself. H₂O is water; H₂O₂ is hydrogen peroxide, a bleach.
• d.Put suitable coefficients in front of the formulae until every element has the same number of atoms on both sides. Use the smallest whole numbers that work.

Some more balanced equations:

1. Calcium carbonate → calcium oxide + carbon dioxide

$$\text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2$$
2. Hydrogen peroxide → water + oxygen

$$2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$$
3. Sodium hydroxide + hydrochloric acid → sodium chloride + water

$$\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$
4. Aluminium + nitrogen → aluminium nitride

$$2\text{Al} + \text{N}_2 \xrightarrow{\Delta} 2\text{AlN}$$
5. Sodium + oxygen → sodium oxide

$$4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}$$

Information from a balanced chemical equation

A balanced chemical equation tells us:

• a.the names and formulae of the reactants and products;
• b.the number of molecules and atoms of each reactant and product;
• c.the ratio of the masses of the reactants and products;
• d.the type of chemical reaction.

How is the food we eat digested? How does a medicine cure us? How does milk become curd? All of these happen through chemical reactions. Without chemical reactions, life itself would be impossible:

• Digestion: enzymes in your stomach and intestines break down starch, proteins and fats into small molecules your body can absorb.
• Respiration: in every cell, glucose reacts with oxygen to release the energy you live on.
• Photosynthesis: green plants make glucose from carbon dioxide and water, using sunlight; every bite of food you eat starts here.
• Medicines: they react with chemicals in the body or in germs; an antacid neutralizes extra stomach acid.
• Fuel: burning wood, kerosene, LPG or biogas releases heat to cook and keep us warm.
• Food: milk turning into curd, dough rising, rice cooking, pickles (achar) souring.
• Industry and farming: making cement, steel, soap, fertilizers, plastics and medicines.

Chemical reactions can be divided into two groups by whether they give out or take in heat. In Activity 15.4, heat is given out. A chemical reaction that releases heat is called an exothermic reaction (exo = out, thermic = heat).

More exothermic reactions:

• Respiration:

$$\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} + 6\text{O}_2\text{(g)} \rightarrow 6\text{CO}_2\text{(g)} + 6\text{H}_2\text{O(l)} + \text{heat}$$

• Slaking of lime:

$$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{heat}$$

• Burning of carbon (charcoal):

$$\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + \text{heat}$$

• Neutralization of an acid by a base:

$$\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{heat}$$

All burning (combustion) is exothermic: a gas stove, a candle, a forest fire.

To cook food, you must keep on supplying heat. On a gas stove, the burning of the gas is an exothermic reaction: it gives out heat. But the cooking of the food takes that heat in: as long as the flame is on, the rice softens; turn it off, and the cooking stops. A chemical reaction that absorbs heat is called an endothermic reaction (endo = in).

More endothermic reactions:

• Steam passed over red-hot coke (carbon) gives a mixture of gases called water gas:

$$\text{C(s)} + \text{H}_2\text{O(g)} \xrightarrow{\text{heat}} \text{CO(g)} + \text{H}_2\text{(g)}$$

• Nitrogen and oxygen combine only at very high temperatures (in a lightning flash, or an engine) to form nitric oxide:

$$\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \xrightarrow{\text{heat}} 2\text{NO(g)}$$

• Limestone breaks down into lime in a kiln:

$$\text{CaCO}_3\text{(s)} \xrightarrow{\text{heat}} \text{CaO(s)} + \text{CO}_2\text{(g)}$$

• Photosynthesis takes in energy too, as light rather than heat:

$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$

1. Choose the correct answer

2. Write the differences between

3. Answer the following questions

4. Change these word equations into balanced formula equations

5. Balance these equations

6. Complete these equations and balance them