7.5 Rate of Reaction

In any chemical reaction, reactants convert into products. Some reactions occur in less than 1 second, while others take much longer. The amount of products produced from reactants per unit time is called the rate of reaction.

For example, when AgNO₃ is added to NaCl solution, it takes less than 1 second to form a white precipitate of AgCl. In contrast, it takes years for a bridge to rust. Reactions that take less time have a higher rate of reaction, while reactions that take more time have a lower rate of reaction.

Experiment on Rate of Reaction:

• Take four test tubes and label them 1, 2, 3, and 4.

• Add approximately 0.5 mg of sodium carbonate (Na₂CO₃) or washing soda in each test tube.

• Add normal water to tubes 1 and 2, and hot water to tubes 3 and 4.

• Add 1 mL of lemon juice (citric acid) or vinegar (4-10% acetic acid) to tubes 2 and 4.

Observe the following:

1. Which test tubes produce gas bubbles?

2. Which test tube does not produce gas bubbles?

3. Which test tube produces the maximum number of gas bubbles?

4. Which test tube produces the minimum number of gas bubbles?

Explanation:

The experiment shows that not all test tubes produce the same amount of product in the same time.

7.5.1 Le Chatelier’s Principle

Some reactions are reversible, meaning the products can react and turn back into reactants. The forward reaction produces a new substance, while the backward reaction reverts the products back into reactants. Initially, the forward reaction occurs at a higher rate, but this decreases over time. The rate of the backward reaction starts slow but increases as time progresses. Eventually, both rates become equal, and the system reaches a state of equilibrium.

At equilibrium, the concentrations of both reactants and products remain unchanged.

Effect of Changes on Equilibrium:

Le Chatelier’s principle states that when any factors (temperature, pressure, or concentration of reactants) are altered, the equilibrium will shift in a way that counteracts the change, returning the system to equilibrium.

Effect of Heat:

Consider the reversible reaction:

N₂ + 3H₂ ⇌ 2NH₃ + 92 kJ (exothermic forward reaction)

• If heat is applied, the system will shift towards the backward reaction to absorb the excess heat, driving the equilibrium to the left (towards N₂ and H₂).

• Conversely, if heat is removed, the equilibrium will shift towards the forward reaction to release heat, driving the equilibrium to the right (towards NH₃).

For another example:

N₂ + O₂ ⇌ 2NO + 180 kJ (endothermic reaction)

• If heat is applied, the forward reaction will be favored, shifting equilibrium towards NO production.

• If heat is removed, the backward reaction will increase, shifting equilibrium towards N₂ and O₂.

Effect of Pressure:

In gaseous reactions, changes in pressure affect the equilibrium. When pressure is applied, the system shifts to neutralize the pressure change.

For the reaction:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• The left side has 4 moles of gas (1 + 3), while the right side has 2 moles. Increasing pressure will shift the equilibrium towards the side with fewer moles (right side) to neutralize the pressure increase.

• If pressure is decreased, the equilibrium shifts towards the side with more moles (left side).

For another reaction:

N₂ (g) + O₂ (g) ⇌ 2NO (g)

• Both the reactants and products have 2 moles of gas, so pressure changes have no effect on this reaction.

Effect of Concentration:

When the concentration of a reactant or product changes, the equilibrium will shift to neutralize the effect of the change.

• If the concentration of a reactant increases, the equilibrium shifts towards the product side to consume the excess reactant.

• If the concentration of a product increases, the equilibrium shifts towards the reactant side to reduce the excess product.

In both cases, the equilibrium shifts to restore balance.