Equilibrium and Rates of Reaction

Question 1

What is Le Chatelier’s principle?

Answer 1

Le Chatelier’s principle states that when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise this change

Question 2

What is meant by a homogenous catalyst?

Answer 2

When a catalyst is in the same phase as the reactants (when a liquid catalyst is used with liquid reactants)

Question 3

What is meant by a heterogenous catalyst?

Answer 3

When a catalyst is in a different phase to the reactants (when a solid catalyst is used with liquid reactants)

Question 4

How does increasing temperature affect the rate of a reaction?

Answer 4

• Increasing the temperature means that more molecules have energy greater than or equal to the activation energy meaning that the frequency of successful collisions is increased and therefore the rate of reaction is also increased
• Molecules have a greater kinetic energy meaning that molecules collide more frequently as they’re moving faster and therefore there is a higher frequency of successful collisions so this also increases the rate of reaction
• The main factor is molecules colliding with an energy greater than or equal to the activation energy

Question 5

How does increasing pressure affect rate of reaction?

Answer 5

When the pressure of a gas is increased, the same number of molecules occupy a smaller volume so the frequency of successful collisions increases and therefore increasing pressure increases the rate of reaction

Question 6

How does increasing concentration affect rate of reaction?

Answer 6

When the concentration of a reactant is increased, there is a greater number of molecules in the same volume. This means that the frequency of successful reaction increases and therefore the rate of reaction increases

Question 7

How do you calculate the rate of reaction from a graph?

Answer 7

• Draw a tangent to the curve and divide the change in y by the change in x
• This is the change in concentration / time and it is measured in moldm-3s-1

Question 8

How do catalysts work?

Answer 8

They provide an alternative route for the reaction with a lower activation energy

Question 9

How do catalysts have an economical and environmental benefit?

Answer 9

They provide an alternative route for the reaction with a lower activation energy meaning that less fossil fuels have to be burned to achieve this amount of energy, this reduces CO2 emissions and also cost

Question 10

What are the key features of a Boltzmann distribution?

Answer 10

• Starts at origin (no molecules have an energy of 0)
• Area under curve is equal to number of molecules
• There is no maximum energy, the curve never touches the energy axis
• Only molecules with an energy greater than or equal to the activation energy react

Question 11

What is the affect of an increase in temperature on the Boltzmann distribution?

Answer 11

• The distribution has a lower peak and is shifted right but the number of molecules is higher as the energy increases
• The increased temperature curve is higher at the activation energy meaning that the area under the curve is greater past the EA line so more molecules have an energy greater than or equal to the activation energy, therefore there is a higher frequency of successful collisions, meaning the rate of reaction is increased

Question 12

How does a catalyst affect the Boltzmann distribution?

Answer 12

• The catalyst provides an alternative route for the reaction with a lower activation energy
• This means that the activation energy line on the distribution is further left when a catalyst is added
• More molecules now have an energy greater than or equal to the activation energy meaning there is a higher frequency of successful collisions and therefore a faster rate of reaction

Question 13

What is the effect of increasing temperature on the position of equilibrium?

Answer 13

The position of equilibrium shifts in the direction of the endothermic reaction (delta H +value)

Question 14

What is the effect of decreasing temperature on the position of equilibrium?

Answer 14

The position of equilibrium shifts in the direction of the exothermic reaction (delta H -value)

Question 15

What is the problem with having a forward exothermic reaction to obtain maximum yield of product?

Answer 15

The conditions for obtaining a large yield of product are low temperature meaning that the rate of reaction will be very slow. This means that a compromise of conditions has to be used, so a slightly higher temperature is used than ideal

Question 16

What is the effect of increasing pressure on the position of equilibrium?

Answer 16

The position of equilibrium shifts in the direction of the side with the fewest moles of gas

Question 17

What is the effect of decreasing pressure on the position of equilibrium?

Answer 17

The position of equilibrium shifts in the direction of the side with the most moles of gas

Question 18

What is the problem with having products with fewer moles of gas?

Answer 18

The ideal conditions for the reaction are high pressure to obtain a high yield of product, high pressure is dangerous and expensive to maintain, this means that a slightly lower pressure than ideal is used

Question 19

How does adding a catalyst affect the position of equilibrium?

Answer 19

• Catalysts increase the rate of the forward and reverse reaction equally
• A catalyst increases the rate at which equilibrium is reached but it doesn’t affect the position of equilibrium

Question 20

What are the current conditions used in an ammonia plant?

Answer 20

• Temperature of around 400-500°, this is to ensure the reaction proceeds at a reasonable rate while also producing a decent yield of NH3
• A pressure of around 200 atm is used to ensure a decent yield of NH3 is produced but also is cost effective and no one in the workplace is put in danger and the environment isn’t threatened
• An iron catalyst is used to speed up the rate of reaction and allow lower temperatures to be used as an alternative pathway for the reaction with a lower activation energy is provided, this reduces cost