rates of reaction chapter 16

factors affecting rates of reaction

nature of reactants

particle size

concentration

temperature

catalysts

homogenous catalyst

reactants and catalyst in same phase

heterogenous

reactants and catalyst in different phases

eg oxidation of methanol

adsorption

lower activation energy due to the platinum catalyst present

autocatalysis

where one of the products of the reaction acts as a catalyst

surface adsorption theory

adsorption, reaction on surface, desorption stage

catalytic converters

platinum, palladium, rhodium

carbon monoxide to carbon dioxide

catalyst poisons

lead

effective collison

is one that results in the formation of products

activation energy

is the minium energy that colliding particles must have for a reaction to occur

rate of reaction

change in concentration per unit time of any one reactant

How are rates of reaction measured?

a) The formation of a product per unit time can be measured

b) The loss of a reactant per unit time can be measured

Note:

- Reactions will not have the same rate throughout

- Two types of ‘rate’ can be measured

Average rate of reaction

What is meant by instantaneous rate of reaction?

Instantaneous reaction is the rate of a reaction at any one particular time during the reaction

Finding the instantaneous rate of reaction from a graph

a) Draw a tangent to the curve at the time asked

b) Choose two good points on the tangent

c) Find the slope of the tangent using:

Instantaneous rate of reaction = slope at that point

What determines whether a reaction does or does not take place i.e. if products are formed or not?

• Reactants will collide with each other when mixed

• These reactants have a certain amount of energy

• If the colliding reactants meet the required energy (activation energy), it will be an effective collision

What is activation energy?

Activation energy is the minimum combined energy of colliding particles for effective collisions/for a reaction to take place

What is meant by an effective collision?

An effective collision is one that reaches activation energy and that results in the formation of products

Using reaction profile diagrams to represent the energy changes in a reaction

1) Large activation energy Vs small activation energy

Reaction profile diagram for an exothermic reaction

Produces and gives out heat to its surroundings

A + B → C + D + Energy

Reactants → Products

Notice: In an exothermic reaction

- Energy of products is less than energy of reactants

△H = Energy of products - Energy of reactants

(△H will be a negative value)

Reaction profile diagram for an endothermic reaction

Takes in heat from its surroundings

A + B → C + D + Energy

Reactants → Products

Notice: In an endothermic reaction

- Energy of products is greater than energy of reactants

△H = Energy of products - Energy of reactants

(△H will be a positive value)

Factors affecting the rate of a reaction: 1) NATURE OF REACTANTS

a) Reactions involving ionic compounds in solution - Faster reactions

Reason:

- In solution, ionic compounds are dissociated, and present as free ions

- No bond breaking is required, the activation energy is lower, and the reactions are fast

Example: Reaction of solutions of silver nitrate and sodium chloride

Note:

- Silver nitrate will not react with sodium chloride at all when they are in the solid state

- In the solid state the ions are not dissociated and are held tightly by ionic bonding in a crystal lattice

Factors affecting the rate of a reaction: 1) NATURE OF REACTANTS

b) Reactions involving covalent compounds → Slower reactions

Reason:

- Covalent compounds are not dissociated

- The covalent bonds need to be broken and new ones reformed, the activation energy is higher, and the reactions are slow

Example: Reaction of solutions of hydrochloric acid and sodium thiosulfate