4.6 The rate and extent of chemical change

True or false: all chemical reactions occur at the same rate

False

All chemical reactions occur at vastly different rates

How to calculate the rate of chemical reaction?

Measure the amount of reactant used or product formed

Divide it by the time taken

Mean rate of reaction = quantity of reactant used or product formed / time

Or record the visual change in reaction (e.g. colour change, precipitate formation). However, this may be subjective and cannot be presented on a graph.

What are the units for quantity of reactants and products?

Mass in grams

Volume in cm³

Moles

What are the units for rate of reaction?

g/s, cm³/s, mol/s

How to measure the rate of reaction from a graph?

Draw a tangent to the graph at the point where you want to measure rate of reaction

Calculate the gradient of the tangent

State the factors which affect the rate of reaction

• concentration of reactants in solution

• pressure of reacting gases

• surface area of solid reactants

• temperature

• presence of catalysts

What increases the rate of a reaction (that all the factors cause)?

Increasing the frequency of successful collisions

How does increasing the concentration of solution increase the rate of reaction?

• A higher concentration means more particles in the reactant in the same volume

• Because there are more particles, they collide more frequently

• More frequent collisions means there’s a greater amount of successful collisions

• This leads to an increased rate of reaction

How does increasing the pressure of a gas affect the rate of reaction?

• Increasing pressure reduces the space in which gas particles can move, this means that the particles are closer together

• When the particles are closer together, they collide more frequently

• More frequent collisions means more successful collisions, so the rate of reaction increases

How does increasing the surface area of solid reactants increase the rate of reaction?

• Increasing the surface area, exposes more particles of the solid

• With more exposed particles that are more frequent collisions between the reactant particles, as the particles around it will have a greater area to work on

• More frequent collisions means more successful collisions, so the rate of reaction increases

How does increasing temperature affect the rate of reaction?

• When temperature increases, particles gain more kinetic energy

• This makes them move faster

• Because they move faster, they collide more frequently

• Also a greater proportion of collisions have enough energy to be successful (they have enough energy to overcome the activation energy)

• This increases the rate of reaction

How does the presence of catalyst affect the rate of reaction?

• A catalyst provides an alternative reaction pathway

• This pathway has a lower activation energy

• Because the activation energy is lower, more particles have enough energy to react when they collide

• This means that there are more frequent successful collisions, so the rate of reaction increases

Describe collision theory

According to this theory, chemical reactions can only occur when reacting particles collide with each other and with sufficient energy.

What is activation energy?

The minimum amount of energy the particles must have to react

what do catalysts do?

Catalyst change the rate of reaction without being used up.

Different reactions need different catalyst. Enzymes act as catalyst in biological systems.

How do catalysts increase the rate reaction?

They provide a different reaction pathway for the reaction that has a lower activation energy. This means that more particles now have enough energy to react, this means that a greater proportion of collisions are successful. Therefore, the rate of reaction increases even though the number of collisions stay the same.

What are reversible reactions?

Reactions where the products of their reaction can react to produce the original reactants.

Draw a symbol equation of a reversal reaction.

How can the direction of a reversible reaction be changed?

By changing the conditions

If a reversible reaction is exothermic in the forward direction, what is it in the backward direction?

Endothermic, the same amount of energy is transferred in each reaction

when is equilibrium reached in a reverse reaction?

• when the reaction occurs an operators which prevents the escape of reactants and products

• When the reactions occur at the exact same rate

What do the relative amounts of all the reactants and products at equilibrium depend on?

The conditions of the reaction

What happens when a system is at equilibrium and a change is made to any of the conditions?

The system responds to contract the change

What is Le Chatelier’s principle?

It explains how irreversible reaction equilibrium responds to changes in conditions; if a change is made to a system at equilibrium, the position of equilibrium shift to oppose the change

Briefly describe what will happen to equilibrium if the concentration of one of the reactants or products is changed

The system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again

What is the effect on the increase in the concentration of reactant particles in a system at equilibrium?

Equilibrium will shift to the right to speed up the reaction in the forward direction. Therefore more products particles will be formed until equilibrium is reached again.

What is the effect of an increase in the concentration of product particles in a system at equilibrium?

Equilibrium will shift to the left to speed up the reaction in the backwards direction. Therefore more products particles will react to form reactant particles until equilibrium is reached again

What is the effect of an increase in temperature of a system at equilibrium?

• the relative amount of products at equilibrium increases for an endothermic reaction.

• The relative amount of products at equilibrium decreases for an exothermic reaction

what is the effect of a decrease in temperature of a system at equilibrium?

• the relative amount of products at equilibrium decreases for an endothermic reaction.

• The relative amount of products at equilibrium increases for an exothermic reaction

what is the effect of an increase in pressure for a gaseous reaction at equilibrium?

an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction.

what is the effect of a decrease in pressure for a gaseous reaction at equilibrium?

a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.