What is the Rate of Reaction?

The rate of a reaction is a major concept in Chemistry that surfaces in several concepts in Chemistry?

What is the rate of a reaction?

The rate of a chemical reaction is defined as the amount of reactants converted to products per unit time.

                                 Or

The rate of reaction is the amount of products formed per unit time

• Mathematically

Rate of a reaction = amount of reactants used/time

        or

Rate of a reaction= amount of products formed/time

Examples

1. If 30g of Silver Chloride was produced in  reaction within 15 seconds. Calculate the rate of reaction.

Solution

 Rate of reaction = amount of products formed / time

 Rate of reaction = 30g /15 s

                            = 2g/s

• Calculate rate of reaction if 20g of zinc reacted completely for 5 seconds with hydrochloric acid to form salt and liberate hydrogen according to this equation.

     Zn + 2HCl  à ZnCl₂ + H₂

Solution

Rate of reaction = amount of reactants /time

Rate of reaction = 20g /5 s

                          = 4g/s

Parameters to Monitor or Measure the Rates of Chemical Reactions

1. Rate at which reactant is decreasing
2. Rate at which product is forming
3. Decrease in the mass of the reaction system
4. Volume of gaseous product formed
5. Amount of precipitate formed
6. Time take to arrive at an early seen stage
7. Change in the intensity of colour
8. Change in pH
9. Change in the total gas pressure

Terms Associated with Rates of Chemical Reactions

1. Collision Theory: The collision theory states that there must be collisions between reactant particles for a chemical reaction to occur.
2. Transition State Theory: The transition state theory states that bonds must be broken in reactants before new bonds will be formed in the products.
3. Activation Energy or Energy Barrier: This is the minimum amount of energy that the colliding particles must possess before reaction will occur.
4. Effective collision: This is the collision that generates the minimum amount of energy required for product formation.

Reaction Rate curve

The rate curve is a graph that illustrates the solubility of salts at different temperatures.

Graph is shown below

Factors that affect the rate of Chemical reactions

1. Nature of reactants
2. Concentration/pressure(gaseous reactants)
3. Surface area of reactants
4. Temperature of reaction mixture
5. Presence of light
6. Presence of catalyst

These parameters govern or influence the factors affecting rates of reaction.

1. Energy content of the particles or average kinetic energy of the particles
2. Frequency of collisions of the reactant particles
3. Activation energy of the reaction

Nature of reactants

The rate of a chemical reaction is dependent on the nature of reactants. When we talk about the nature of reactants we mean reactivity, type of bonds, and position of elements.

Example

Compare the reactions of zinc and iron with acid

Zn + H₂SO₄  à ZnSO₄ + H₂

Fe + H₂SO₄ à FeSO₄ + H₂

The reaction with zinc will be faster than that of iron.

Why?

This is due to the nature of the reactant; zinc is positioned higher than iron in the reactivity series.

Another example

Na + H₂O à NaOH + H₂

Ca + 2H₂O à Ca (OH)₂ + H₂

The reaction with sodium (group I elements) is faster than Calcium (group 2 elements)

This is also due to the fact that metals higher than

Temperature

Reactions generally occur faster when the temperature is increased and slow down at a reduced temperature.

When the temperature is increased, the average kinetic energy of the particles increases leading to an increase in the frequency of effective collisions thus increasing the rates of reactions.

Conversely, if the temperature is decreased rate of reaction also decreases due to a decrease in the frequency of collision and rate of reaction.

Most organic reactions are aided by applying heat using a Bunsen burner though there are reactions that can occur on their own without gid. These reactions that proceed on their own are called spontaneous reactions.

illustration

Concentration of reactant particles

The rate of a reaction depends on the concentration of the reactants.

If the concentration is increased, more particles will be available leading to a greater number of collisions and thus increasing the rate of reaction

Illustration

Example

Surface area/Particle size of reactants

Surface area is inversely related to particle size. The surface area is largest in porous or powdered reactants. Increasing the surface area of the reactant particles will make more particles available and in contact with the medium thus increasing the frequency of collisions and ultimately rate of reaction.

Illustration

Effect of catalyst

A chemical reaction that is influenced by the presence of a catalyst is said to be a catalyzed reaction.

A catalyst alters the rate of a chemical reaction by changing the activation energy; a positive catalyst will speed up the rate of a reaction by decreasing the activation energy while a negative catalyst will decrease rate of reaction by increasing the activation energy.

A positive catalyst is called promoter while a negative catalyst is called inhibitor.

Illustration

What is a catalyst?

A catalyst is a substance that alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction.

Characteristics of a Catalyst

1. A catalyst is specific in function
2. A catalyst remains chemically unchanged at the end of the reaction
3. A catalyst does not affect equilibrium position
4. A catalyst is needed in small quantity
5. A catalyst affects activation energy
6. A catalyst has maximum activity at optimum temperature

Diagram illustrating effect of catalyst on activation energy

Hint: The energy profile diagrams of endothermic and exothermic reactions are not the same.

Presence of light  

A reaction is said to be a photochemical reaction if the reaction rate is influenced by light.

Some reactions are only sensitive to light energy and thus can only proceed or proceed faster in the presence of light.

Examples of photochemical reactions

1. Halogenations of alkanes

Chlorination of methane

CH₄ + Cl₂ à CH₃Cl + HCl

• Decomposition of silver halides(applied in photography)

2AgBr à 2Ag + Br₂

• Formation of ozone

3O₂ à 2O₃

• Photosynthesis

6CO₂ + 6H₂O à C₆H₁₂O₆ + 6O₂

• Decomposition of hydrogen peroxide

2H₂O₂ à 2H₂O + O₂

The rate of a chemical reaction is usually used in increasing most organic reactions because organic reactions are slow reactions.

Also, non-spontaneous reactions are aided by either adding heat or altering other factors affecting rates of reactions.