Chemical Reactions – SS2 Chemistry Lesson Note (Second Term)

The rate of a chemical reaction is the number of moles of reactants converted, or products formed, per unit of time. It is found by measuring the change in concentration or mass of a reactant or product with time. According to collision theory, the rate depends on particle energy, collision frequency and activation energy.

Behavioural objectives

By the end of the lesson, students should be able to:

• define the rate of a reaction and calculate it from given data;
• list the properties that can be used to measure the rate of a reaction;
• describe the features of a rate curve;
• state the collision theory and define activation energy;
• explain how nature of reactants, concentration, surface area, temperature, light and catalysts affect the rate of a reaction.

What is the rate of reaction?

The rate of a chemical reaction is the number of moles of reactants converted or products formed per unit of time.

The rate of reaction is usually determined experimentally by measuring the change in concentration of one of the components in the reaction with time. Thus:

Rate of reaction = change in concentration of reactant or product (mol dm^-3) ÷ time taken for the change (seconds)

The unit of the rate of reaction is mol dm^-3 s^-1 or g dm^-3 s^-1.

The rate of reaction can also be expressed as:

Rate of reaction = change in the number of moles or mass of a reactant or product ÷ time taken for the change

In this case, the unit of rate is mol s^-1 or g s^-1.

Worked example

Question: When 0.5 g of calcium trioxocarbonate (IV) was added to excess dilute hydrochloric acid, carbon (IV) oxide was evolved. The complete reaction took 5 minutes. What was the rate of reaction?

Solution:

Rate of reaction = change in mass of reactant ÷ time taken for the change

= (0.5 – 0) g ÷ (5 × 60) s

= 0.5 ÷ 300

= 1.67 × 10^-3 g s^-1

How is the rate of reaction measured?

Concentration is one of the properties of a reaction that can change with time. The following properties can also change with time, so they can be used to measure the rate of reaction:

1. Decrease in mass of the reaction system
2. Volume of gaseous product
3. Amount of precipitate formed
4. Change in colour intensity
5. Change in pH
6. Change in total gas pressure

What is a rate curve?

The rate curve is a graphical illustration of the rate of a reaction. The graph below illustrates the rate curve.

Features of a rate curve

1. It passes through the origin, because there is no change in concentration or mass at the start of the reaction.
2. It is steep at first, because the rate is fast at the beginning.
3. It becomes less steep later, because the rate slows down.
4. It finally becomes horizontal, because the reaction has reached its end point.

What can be determined from the rate curve?

1. Average rate of reaction:
   Average rate = total number of moles or mass involved ÷ time taken
2. Rate at a particular instant during the reaction:
   Rate at an instant = gradient of the tangent at a point on the curve

Rate and concentration

When the rate of reaction varies directly with concentration, then:

Rate of reaction ∝ [A]

R ∝ [A]

R = k[A]

where k is called the rate constant.

What is the collision theory?

The collision theory states that for a chemical reaction to occur, the reactant particles must collide, and they must collide with a certain minimum amount of energy known as activation energy.

Reacting particles are in continuous motion, so they possess energy and they collide with one another. Not all collisions result in chemical reactions. Collisions that result in chemical reactions are called effective collisions.

The minimum amount of energy required by reacting particles for a chemical reaction to occur is called activation energy. It is the energy barrier the reactants must overcome for the reaction to occur. It is the minimum energy required for bond breaking.

Chemical reactions occur only when the energy of the colliding reactant particles is equal to or more than the activation energy.

Every reaction has its own activation energy. Reactions with low activation energy have a high rate of reaction and occur spontaneously. Reactions with high activation energy have a low rate of reaction and are not spontaneous.

The graph below illustrates activation energy for endothermic and exothermic reactions.

From the graph, it can be seen that:

1. Both exothermic and endothermic reactions generally require an initial input of energy to overcome the activation energy barrier.
2. Exothermic reactions, once started, proceed without any further external energy supply, e.g. burning of kerosene.
3. Endothermic reactions proceed with a continuous supply of external energy, e.g. cooking of rice.

What factors affect the rate of reaction?

From the collision theory, the rate of reaction depends on the following features:

1. The energy of the particles.
2. The frequency of collision of the particles.
3. The activation energy of the reaction.

These features are in turn affected by some factors, which change them and so change the rate of reaction. Some important factors are:

1. Nature of reactants
2. Concentration, or pressure for gases, of reactants
3. Surface area of reactants
4. Temperature of the reaction mixture
5. Presence of light
6. Presence of catalysts

To study the effect of any one of these factors on the rate of reaction, all other factors must be kept constant.

Effect of nature of reactants

If all other factors are kept constant, different substances react at different rates with dilute HCl, for example. When dilute HCl reacts with zinc, iron and gold under the same conditions, hydrogen gas evolves fast with zinc, slowly with iron and no gas evolves with gold.

The difference in rate of reaction is due to the chemical nature of the elements, as they naturally possess different amounts of energy content.

Effect of concentration of reactants

The frequency of collision among particles is high when the particles are crowded in a small space, i.e. at high concentration. This leads to more effective collisions and thus a high rate of reaction. An increase or decrease in the concentration of the reactants results in a corresponding increase or decrease in effective collisions, and hence in the reaction rate.

Effect of surface area of reactants

This is a very important factor when a solid is involved in a chemical reaction. Lumped solids offer a small surface area of contact for reaction, while powdered solids offer a large surface area. The rate of reaction is slow with lumped solids but high with powdered solids.

Effect of temperature

Increasing the temperature of a system can increase the reaction rate in two ways. When the temperature is raised, energy in the form of heat is supplied to the reactant particles, so that:

1. The number of particles with energy equal to or greater than the activation energy increases.
2. The velocity of all the reactant particles increases because of the greater kinetic energy, leading to a higher frequency of collision.

As a result, the number of effective collisions increases and the reaction proceeds at a faster rate. A decrease in temperature leads to a decreased rate of reaction.

Effect of light

Some reactions are influenced by light. The rate of reaction is high when the light intensity is high, low when the intensity is low, and the reaction does not proceed at all in the absence of light. Such reactions are known as photochemical reactions. Examples include:

1. Reaction between hydrogen and chlorine
2. Decomposition of hydrogen peroxide
3. Reaction between methane and chlorine
4. Photosynthesis in plants
5. Conversion of silver halides to grey metallic silver

Effect of a catalyst

A catalyst is a substance that alters the rate of a reaction but does not itself undergo any change at the end of the reaction.

A positive catalyst increases the rate of reaction by lowering the activation energy of the reaction. A catalyst that increases the activation energy is known as a negative catalyst or an inhibitor.

The diagram below shows the energy profile for catalysed and uncatalysed exothermic and endothermic reactions.

Evaluation

1. State three properties, other than concentration, that can be used to measure the rate of a reaction.
2. List the four features of a rate curve.
3. What is activation energy?
4. Explain how an increase in temperature increases the rate of a reaction.
5. What is the difference between a positive catalyst and a negative catalyst?

Assignment

1. (a) Define the rate of reaction.
   (b) State the collision theory.
2. (a) Explain, in terms of collision theory, how the rate of a gaseous reaction is affected by an increase in pressure.
   (b) Give the reason why red-hot iron wool reacts more readily with oxygen than a red-hot iron nail.