Chemical Reaction

You met these in earlier classes. A quick reminder:

The substances you start with in a chemical change are the reactants; the new substances formed are the products.

During a chemical reaction, atoms don’t appear or disappear. The reactants lose, gain or share electrons; their bonds break, and the same atoms join up in new ways: reactants combine, break apart, or swap their ions to form the products. This process is a chemical reaction.

Writing the reaction down in words, symbols or chemical formulae gives a chemical equation. The same reaction three ways:

• Word equation: magnesium + oxygen → magnesium oxide
• Skeleton equation (formulae, not yet balanced): Mg + O₂ → MgO
• Balanced equation: 2Mg + O₂ → 2MgO

The reactants go on the left and the products on the right, with an arrow (→, read “gives” or “forms”) between them. Conditions are often written over the arrow: heat (or Δ), light, a catalyst, high pressure. Sometimes the state of each substance is added in brackets: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

Balancing an equation

In the skeleton Mg + O₂ → MgO, there are 2 oxygen atoms on the left and only 1 on the right. An oxygen atom can’t vanish, so the equation is wrong. A balanced equation has the same number of atoms of each element on both sides. This is the law of conservation of mass written in symbols: the mass of the products equals the mass of the reactants.

To balance, you change only the big numbers in front (the coefficients), never the small numbers inside a formula. Changing H₂O into H₂O₂ would turn water into hydrogen peroxide, a different substance.

By the kinds of reactants and products, reactions are sorted into a few types. Figure 15.1 shows them with colored balls.

1. Combination reaction

A reaction in which two or more reactants combine to form only one product is a combination reaction (also called an addition or synthesis reaction). Its general form is A + B → AB.

2. Decomposition reaction

A reaction in which one reactant breaks down into two or more products is a decomposition reaction. Its general form is AB → A + B. Most need energy to break the reactant apart: heat (thermal decomposition), light or electricity.

$$\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$$
$$\text{2NaOH}(s) \rightarrow \text{Na}_2\text{O}(s) + \text{H}_2\text{O}(l)$$
$$\text{H}_2\text{CO}_3(aq) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(l)$$
$$\text{2KClO}_3(s) \rightarrow \text{2KCl}(s) + \text{3O}_2(g)$$

The first is how lime (CaO) is made from limestone, in kilns like the ones beside Nepal’s cement factories. The third happens every time you open a bottle of soda: the carbonic acid in it breaks into fizz (carbon dioxide) and water.

3. Displacement reaction

A reaction in which an atom or radical of one reactant is pushed out (displaced) by an atom or radical of another reactant is a displacement reaction. There are two kinds.

i. Single displacement reaction

One element (or radical) displaces another element from its compound. Its general form is AB + C → A + BC: element C takes B away from A, and A is left on its own.

$$\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)$$
$$\text{Zn}(s) + \text{2HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$$
$$\text{Cu} + \text{2AgNO}_3 \rightarrow \text{2Ag} + \text{Cu(NO}_3)_2$$
$$\text{Ca} + \text{2H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$$
$$\text{Cl}_2 + \text{2NaBr} \rightarrow \text{2NaCl} + \text{Br}_2$$
$$\text{Br}_2 + \text{2KI} \rightarrow \text{2KBr} + \text{I}_2$$

Which element wins? The more reactive one always displaces the less reactive one, never the other way round. For metals, chemists list them in order, the reactivity series:

K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au

Zinc is above hydrogen, so it pushes hydrogen out of acids. Copper is below hydrogen, so it doesn’t react with dilute hydrochloric acid at all, but it is above silver, so it pushes silver out of silver nitrate. Among the halogens, reactivity goes F > Cl > Br > I (as you saw in Unit 14), so chlorine displaces bromine, and bromine displaces iodine, but iodine can’t displace either.

ii. Double displacement reaction

Two compounds swap their atoms or radicals with each other, forming two new compounds. Its general form is AB + CD → AD + CB. Often one of the new compounds doesn’t dissolve in water and appears as a cloudy solid, a precipitate:

$$\text{NaCl} + \text{AgNO}_3 \rightarrow \text{AgCl} \downarrow + \text{NaNO}_3$$
$$\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 \downarrow + \text{2NaCl}$$
$$\text{Pb(NO}_3)_2 + \text{2KI} \rightarrow \text{2KNO}_3 + \text{PbI}_2 \downarrow$$

The first gives a white precipitate of silver chloride (a test for chlorides), the second a white precipitate of barium sulphate (a test for sulphates), and the third a brilliant yellow precipitate of lead iodide.

4. Acid–base reaction

The reaction of an acid with a base to form a salt and water is an acid–base reaction. It is also a kind of double displacement (the acid’s H and the base’s metal swap partners). The acid and the base destroy each other’s properties and form neutral products, so it is also called a neutralization reaction:

acid + base → salt + water

$$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$
$$\text{H}_2\text{SO}_4 + \text{2NaOH} \rightarrow \text{Na}_2\text{SO}_4 + \text{2H}_2\text{O}$$
$$\text{HNO}_3 + \text{KOH} \rightarrow \text{KNO}_3 + \text{H}_2\text{O}$$
$$\text{2HCl} + \text{CaO} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$$

In the first, HCl is the acid, NaOH the base, NaCl the salt. A base doesn’t have to be a hydroxide: calcium oxide, CaO, is a base too.

Some reactions are over in a flash: an acid and a base neutralize each other the moment they meet, and a firework explodes. Others take days or years: an iron gate rusts, and a banana ripens. The rate of a chemical reaction is how fast the reactants change into products: the amount of reactant used up, or of product formed, per unit time.

At the start, the amount (concentration) of the reactants is greatest and there is no product at all. As the reaction goes on, the reactants are used up and the products build up, as in Figure 15.5. The reaction is usually fastest at the start, when there are most reactant particles to meet, and slows down as they run out.

Why reactions happen at all: collisions

Particles can only react when they collide. But most collisions are just bumps: the particles bounce off unchanged. A collision makes products only if the particles hit hard enough, with at least a certain minimum energy, called the activation energy. It is like pushing a stone over a hill before it can roll down the other side (Figure 15.6).

So anything that makes particles collide more often, or hit harder, or lowers the hill, speeds a reaction up. That one idea explains every factor below.

1. Catalyst

A catalyst is a substance that changes the rate of a chemical reaction without itself undergoing any permanent chemical change. It is written above the arrow, not as a reactant, because it comes out at the end just as it went in.

• A positive catalyst speeds a reaction up. Examples: manganese dioxide (MnO₂), vanadium pentoxide (V₂O₅, in making sulphuric acid) and iron (Fe, in making ammonia).

$$\text{2KClO}_3 \xrightarrow{\text{MnO}_2,\ \Delta} \text{2KCl} + \text{3O}_2$$

• A negative catalyst slows a reaction down. Phosphoric acid (H₃PO₄) is added to hydrogen peroxide to slow its decomposition, so it keeps longer in the bottle.

$$\text{2H}_2\text{O}_2 \xrightarrow{\text{H}_3\text{PO}_4} \text{2H}_2\text{O} + \text{O}_2 \quad \text{(slower)}$$

2. Heat (temperature)

Generally, the rate of a reaction increases as the temperature rises. Hotter particles move faster, so they collide more often, and, much more importantly, a bigger share of the collisions have enough energy to get over the hill. For many reactions, a rise of just 10 °C roughly doubles the rate. Potassium chlorate gives off oxygen only when heated:
$$\text{2KClO}_3 \xrightarrow{\text{heat}} \text{2KCl} + \text{3O}_2$$

This is also why we cook food (the reactions that soften rice are fast at 100 °C and very slow at room temperature), why a pressure cooker cooks faster (water boils at about 120 °C inside it) and why a refrigerator keeps milk from going sour (the reactions of the bacteria in it slow down in the cold).

3. Pressure

For reactions between gases, the rate depends on the pressure. Squeezing a gas into a smaller space crowds its molecules together, so they collide more often. Generally, the rate of a gaseous reaction increases with pressure. Ammonia is made from nitrogen and hydrogen at a very high pressure, usually about 200 times the pressure of the air (different factories have used anywhere from 200 to 900 atm):
$$\text{N}_2(g) + \text{3H}_2(g) \xrightarrow{\text{Fe, high pressure, 450 °C}} \text{2NH}_3(g)$$

4. Surface area

Only the particles on the surface of a solid can be hit by the other reactant; those inside have to wait their turn. Break a lump into small pieces and many more particles are on a surface. So the larger the surface area of contact, the faster the reaction.

5. Light

Some reactions need light, and light increases their rate. Photosynthesis is the most important:
$$\text{6CO}_2 + \text{6H}_2\text{O} \xrightarrow[\text{chlorophyll}]{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + \text{6O}_2$$
carbon dioxide + water → glucose + oxygen. Silver nitrate also breaks down in light, which is why it is kept in dark bottles:
$$\text{2AgNO}_3 \xrightarrow{\text{light}} \text{2Ag} + \text{2NO}_2 + \text{O}_2$$
(silver nitrate → silver + nitrogen dioxide + oxygen). Old camera film worked this way: light turned silver compounds into tiny dark grains of silver, making the picture.

6. Concentration (one more factor)

Concentration matters just as much: the more concentrated a solution, the faster the reaction. More particles in the same volume means more collisions every second. Marble chips fizz gently in dilute acid and furiously in concentrated acid.

1. Choose the best option

2. Give reasons

3. Write differences between

4. Balance these equations and name their types

5. Answer these questions