Rates of reaction and equilibria

Question 1

What does the rate of a reaction measure?

Answer 1

The rate of a chemical reaction measures how fast a reactant is being used up or how fast a product is being formed.

Question 2

What is the equation used to find the rate of a reaction?

Answer 2

Rate = change in concentration / time

Question 3

Describe the changes to rate of a reaction as time increases.

Answer 3

The rate of a reaction is fastest at the start of the reaction as each reactant is at its highest concentration. The rate of reaction slows down as the reaction proceeds, because the reactants are being used up and their concentrations decrease. Once the reactants has been completely used up, the rate of reaction is zero.

Question 4

Give the factors that alter the rate of a chemical reaction.

Answer 4

Concentration, temperature, use of a catalyst and surface area.

Question 5

What two conditions need to have been met in order for a collision to be effective?

Answer 5

The orientation of the particles and the energy the particles have.

Question 6

How does increasing the concentration affect the rate of reaction?

Answer 6

The rate of reaction depends upon the frequency of successful collisions. An increase in concentration increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given period of time there will be more successful collisions which will result in an increased frequency of successful collisions.

Question 7

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

Answer 7

When a gas is compressed into a smaller volume, the pressure of a gas is increased and the rate of reaction increases. The concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently leading to more effective collisions in the same time.

Question 8

Give two ways in which the progress of a chemical reaction can be followed.

Answer 8

By monitoring the removal of reactant or by following the formation of a product.

Question 9

What are the two methods that can be used to determine the rate of the reaction if it is a reaction that produces a gas?

Answer 9

Monitoring the volume of gas produced at regular time intervals using gas collection or monitoring the loss of mass of reactants using a balance

Question 10

What is a catalyst?

Answer 10

A catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself.

Question 11

How does a catalyst increase the rate of a chemical reaction?

Answer 11

A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway of lower activation energy.

Question 12

What is a homogeneous catalyst?

Answer 12

A homogeneous catalyst has the same physical state as the reactants.

Question 13

How does a homogeneous catalyst work?

Answer 13

The catalyst reacts with the reactants to form an intermediate. The intermediate then breaks down to give the product and regenerates the catalyst.

Question 14

What is a heterogeneous catalyst?

Answer 14

A heterogeneous catalyst has a different physical state from the reactants.

Question 15

How does a heterogeneous catalyst work?

Answer 15

Reactant molecules are adsorbed onto the surface of the catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst by desorption.

Question 16

When are heterogeneous catalysts usually used?

Answer 16

Heterogeneous catalysts are usually solids in contact with gaseous reactants or reactants in solution.

Question 17

Why is it beneficial to use a catalyst?

Answer 17

They reduce the activation energy which in turn reduces the temperature needed for the process. If a chemical process requires less energy, less electricity or fossil fuel is used.

Question 18

True or false? The economic advantages of using a catalyst outweigh any costs associated with developing a catalytic process.

Answer 18

True.

Question 19

Give some of the features of the Boltzmann distribution curve.

Answer 19

No molecules have zero energy- the curve starts at the origin. The area under the curve is equal to the total number of molecules. There is no maximum energy for a molecule- the curve does not meet the x-axis at high energy. The curve would need to reach infinite energy to meet the x-axis.

Question 20

If the temperature of a reaction is increased, describe the new position of the Boltzmann distribution curve.

Answer 20

The peak of the graph is lower on the y-axis and further to the right on the x-axis.

Question 21

Where would a catalyst be shown on the Boltzmann distribution curve?

Answer 21

To the left of the activation energy.

Question 22

What are the two directions of a reversible reaction?

Answer 22

‘forward’ and ‘reverse’

Question 23

Fill in the blanks. The rate of the forward reaction is _____ to the rate of the reverse reaction. The ________ of reactants and products do not change.

Answer 23

Equal, concentrations.

Question 24

What does the position of equilibrium indicate?

Answer 24

The extent of a reaction.

Question 25

What does the Chatelier’s principle state?

Answer 25

The Chatelier’s principle states that when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change.

Question 26

Describe the effect of concentration changes on equilibrium.

Answer 26

Changing the concentration of a reactant or a product in an equilibrium system will change the rate of the forward or reverse reaction. The position of equilibrium will shift to the right if concentration of reactants is increased and shift to the left if the concentration of the products is increased.

Question 27

How can you tell if a reaction is exothermic or endothermic?

Answer 27

An exothermic reaction will have a negative delta H whereas an endothermic reaction will have a positive delta H.

Question 28

Describe the effect temperature has on the diirection of equilibrium shift.

Answer 28

An increase in temperature shifts the equilibrium position in the endothermic direction. A decrease in temperature shifts the equilibrium position in the exothermic direction.

Question 29

When can changing the pressure of a system containing gases change the position of equlibrium?

Answer 29

If all the reactants are gases and if there are more gaseous molecues on one side of the equation than the other.

Question 30

Fill in the gaps. If there are fewer ______ moles on the right-hand side of the equilibrium, the position of ______ shifts to the _____ reducing the number of gaseous moles to ______ the increase in pressure.

Answer 30

Gaseous, equilibrium, right, minimise.

Question 31

What afftect does decreasing the pressure have?

Answer 31

Decreasing the pressure shifts the position of equilibrium in the opposite direction, to the side with more gaseous moles on the left.

Question 32

Describe the change in position of equilibrium when adding a catalyst.

Answer 32

There is no change. It speeds up the rates of forward and backward reactions equally.