Chapter 10 Reaction Rates And Equilibrium

Question 1

What is the simple collision theory?

Answer 1

Two reacting particles must collide for a reaction to occur. Usually only a small proportion of collisions result in a reaction as particles bounce off each other

Question 2

What 2 conditions must be met for a reaction to occur from a collision?

Answer 2

The particles must collide with the right orientation

The particles must have sufficient energy to overcome the activation energy barrier of the reaction

Question 3

How does concentration influence the rate of reaction?

Answer 3

When concentration is increased the rate of reaction is increased.
Increased 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 effective collisions.

Question 4

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

Answer 4

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 5

How do you calculate the reaction rates from mass loss (experiment)? Could be concentration or gas volume too

Answer 5

Step 1.
Plot a graph of mass loss against time
Step 2.
Draw a tangent to the curve at t=0, this is the initial rate of reaction
Step 3.
Calculate the rate from the gradient of the tangent
Step 4.
To calculate the rate at a specific time, the same tangent method is used

Question 6

What is the role of a catalyst?

Answer 6

A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway of a lower activation energy.
This can be shown on an enthalpy profile diagram where the activation energy of the reaction with a catalyst is lower than without a catalyst.
The catalyst is not used up in the reaction, it may react with a reactant to form an intermediate or provide a surface for the reaction to take place on but it is always regenerated

Question 7

What is a homogeneous catalyst? Give an example of a reaction and catalyst

Answer 7

It has the same physical state as the reactants. The catalyst reacts with the reactants to form an intermediate which breaks down to give the product and regenerates the catalyst.
An example would be ozone depletion with a chlorine radical acting as a catalyst. All these are gases

Question 8

What is a heterogeneous catalyst? Give an example of a reaction and catalyst

Answer 8

The catalyst is in a different physical state to the reactants. They are usually solids in contact with gaseous reactants or reactants in solution.
Reactants are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place. After the reaction the product mo,echoes leave the surface of the catalyst by desorption.
An example of this is the Haber process where nitrogen and hydrogen gas are catalysed by solid iron.

Question 9

What economic importance do catalysts have?

Answer 9

Catalysts increase the rate of many industrial chemical reactions by lowering the activation energy which reduces the temperature. Making the product faster and using less energy can cut costs and increase profitability

Question 10

How do catalysts increase the sustainability of industrial chemical production?

Answer 10

The reaction requires less energy, then less electricity or fossil fuel is used increasing the sustainability of fossil fuel consumption which is a finite resource

Question 11

How can catalysts reduce CO2 emission?

Answer 11

Modern focus on sustainability requires industry to operate processes with high atom economies and fewer pollutants.
Also using less fossil fuels will cut CO2 emissions

Question 12

What experiments can be conducted to investigate the rate of reaction?

Answer 12

Monitoring the production of gas over time using gas collection
Monitoring the loss of mass overtime using a mass balance

Question 13

What is the Boltzmann Distribution?

Answer 13

In a gas, liquid, or solution, some molecules move slowly with low energy and some molecules move fast with high energy. Most molecules move close to the average speed and have close to the average energy. This spread of molecular energies is known as the Boltzmann Distribution.

Question 14

What does a boltzmann distribution diagram look like? include activation energy

Answer 14

The graph axis are:
y= number of molecules
x= (kinetic) energy

• 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 of a molecule the curve does not meet the x axis at high energy
• The graph is marked with a (dotted) line, Ea, that represents the activation energy of a reaction. To the right of this line the area under the curve is shaded which symbolises the small proportion of molecules that have the required activation energy to react

Question 15

How does a temperature change influence the Boltzmann Distribution, number of molecules exceeding the activation energy, and therefore the rate of reaction? How will this change the shape of the graph?

Answer 15

At a higher temperature:

• more molecules have an energy greater than or equal to the activation energy
• therefore a greater proportion of collisions will lead to a reaction, increasing the rate
• collisions will also be more frequent as the molecules are moving faster

On the graph:
At higher temperature the peak is lower and to the right

Question 16

How does catalytic behaviour influence the Boltzmann Distribution, number of molecules exceeding the activation energy, and therefore the rate of reaction? How will this change the shape of the graph?

Answer 16

A catalyst provides an alternative reaction route with a lower activation energy therefore a greater proportion of molecules will have energy equal to or greater than the require activation energy meaning more successful collision will take place which increases the rate of reaction.

On the graph:
The Ea (activation energy) line shifts to the left

Question 17

Explain what dynamic equilibrium means.

Answer 17

A dynamic equilibrium exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change

Question 18

What is Le Chateliers principle?

Answer 18

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

Question 19

How does the concentration of a reactant in a system effect the position of the dynamic equilibrium following le Chateliers principle?

Use the equation:
A + B <=> C + D (reversible reaction)

Answer 19

A + B <=> C + D (reversible reaction)

• if you increase the concentration of A, the reaction will shift right to counteract this change
• the same is true in reverse, decrease te concentration of A and the reaction will shift left
• if you remove C as soon as it is formed, the position of the equilibrium will shift right to replace it, if you continued to do this the equilibrium would keep moving right

Question 20

How does the pressure of a system effect the position of the dynamic equilibrium following le Chateliers principle?

Use the equation:
A(g) + 2B(g) <=> C(g) + D(g) (reversible reaction)

Answer 20

A(g) + 2B(g) <=> C(g) + D(g) (reversible reaction)

• this only applies to gases
• if you increase the pressure the reaction will move so that the pressure is reduced again
• pressure is caused by gas molecules hitting the side of the container. More molecules means more pressure
• in this case there are 3 molecules on the left and 2 on the right so an increase in pressure will go to the right where there is less molecules
• if LHS = RHS pressure has no effect on the equilibrium

Question 21

How does the temperature of a system effect the position of the dynamic equilibrium following le Chateliers principle?

Use the equation:
A + B <=> C + D (reversible reaction)
- the forward reaction is exothermic

Answer 21

A + B <=> C + D (reversible reaction)

• in this case the forward reaction is exothermic (delta)H is negative which means that the backwards reaction is positive (delta)H is positive
• if you increase the temperature the reaction will move to reduce the temperature To cool down the reaction needs to absorb the extra heat you have added which means the endothermic reaction will be favoured (as it takes in heat to break bonds) therefore the reaction moves left

Question 22

What effect does a catalyst have on the position of the equilibrium?

Answer 22

A catalyst increases the rate of both the forward and backward reaction therefore the position does not change

Question 23

What experiment can be conducted to investigate the effects of concentration on an equilibrium?

Answer 23

?

Question 24

What experiment can be conducted to investigate the effects of temperature on an equilibrium?

Answer 24

?

Question 25

Why is there often compromise in industrial production using reversible reactions?

E.g. Haber Process, what are the compromise conditions?

Answer 25

Le Chateliers principle can be used to predict the best conditions of temperature and pressure to force the equilibrium to maximise the yield of a product e.g. ammonia in the Haber Process
The best conditions for the Haber process are:
- a low temperature but this is not used because it would create a high yield but very slowly
- a high pressure but this requires a very strong container and a lot of energy which is expensive. It is also dangerous as any faults in the container could lead to hot toxic ammonia leaks.

Therefore a compromise temperature and pressure are used in order to make the reaction as safe and economically viable as possible. 
Condition for the Haber Process:
350-500C
100-200atm
Iron catalyst

Question 26

How do you calculate the equilibrium constant from this reversible reaction?

aA + bB <=> cC + dD

Answer 26

Kc= [C]^c x [D]^d
________________
[A]^a x [B]^b

• [ ] square brackets are shorthand for ‘concentration of’
• a,b,c,d are the balancing numbers in the overall equation
• [A] [B] [C] [D] are the equilibrium concentrations of the reactants and products of this equilibrium

Question 27

What does the value of Kc tell us about the position of the equilibrium?

Answer 27

• Kc = 1 means the equilibrium is in the centre (nearly impossible)
• Kc > 1 more products are formed
• Kc < more reactants formed