Chapter 10 - Reaction Rates And Equilibrium (10.1-10.5)

Question 1

How do you work out the rate of a chemical reaction?

Answer 1

Change in concentration/time

Question 2

What are the units for the rate of a chemical reaction?

Answer 2

Moldm^-3s^-1

Question 3

How can you measure a rate of reaction?

Answer 3

By measuring the change in concentration of a reactant or a product in a given time

Question 4

Describe a concentration time graph

Answer 4

1) At the start the rate of reaction is fastest because each reactant has a high concentration
2) Rate of reaction slows as the reaction proceeds, because the reactants are being used up
3) Once the reactants have been completely used up graph plateaus as rate of reaction is 0

Question 5

What 4 factors can effect the rate of reaction?

Answer 5

Concentration, temperature, use of a catalyst, surface area of solid reactants

Question 6

What two conditions have to be met for a collision to be effective?

Answer 6

Particles must collide with the correct orientation and the particles must have sufficient energy to overcome the activation energy

Question 7

How does increasing the concentration or pressure affect the rate of reaction?

Answer 7

When the concentration is increased, the rate of reaction is increased as it increases the number of particles in the same volume and particles are closer together, so the particles will collide more frequently and there will be more effective collisions

Question 8

What two methods can be used to monitor the progress of a reaction that produces a gas?

Answer 8

1) Monitoring the volume of gas produced at regular time intervals using gas collection
2) Monitoring the loss of mass of reactants using a balance

Question 9

How can you get a value for the rate of reaction?

Answer 9

Plot a graph of total volume of gas produced against time. Draw a tangent at the time you need a rate of reaction for and calculate the gradient

Question 10

What are the 3 major features of a catalyst?

Answer 10

1) It is not used up in a chemical reaction
2) The catalyst may react to form an intermediate or may provide a surface a reaction can take place on
3) At the end of the reaction the catalyst is regenerated

Question 11

How does a catalyst increase the rate of reaction?

Answer 11

It provides an alternative reaction pathway of lower activation energy

Question 12

What is a homogeneous catalyst and how do they work?

Answer 12

It has the same physical state as the reactants, it reacts to form an intermediate then the intermediate breaks down to give the product and regenerates the catalyst

Question 13

What is a heterogeneous catalyst and how do they work?

Answer 13

They have a different physical state to the reactants. Reactants are adsorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place then after the reaction the product molecules leave by desorption

Question 14

What state are heterogeneous catalysts used in?

Answer 14

They are usually solids in contact with gaseous reactants or reactants in solution.

Question 15

How are catalysts sustainable?

Answer 15

They lower the activation energy, this reduces energy requirements and temperature of the process. This means less electricity or fossil fuel is used.

Question 16

How are catalysts economically friendly?

Answer 16

It makes the product faster and uses less energy which cuts costs and increases profitability

Economic advantages outweigh costs associated with developing a catalyst

Question 17

What are the 3 features of any Boltzmann distribution?

Answer 17

1) No molecules have zero energy so the curve starts at the origin
2) The area under the curve is equal to the total number of molecules
3) There is no maximum energy for a molecule - the curve does not meet the x axis

Question 18

How does temperature effect the Boltzmann distribution?

Answer 18

As the temperature increase 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 of reaction. Collisions will also be more frequent as particles are moving faster.

Question 19

What is up the y-axis on a Boltzmann distribution curve?

Answer 19

Number of molecules with a given energy

Question 20

What is up the x-axis on a Boltzmann distribution curve?

Answer 20

Energy

Question 21

How does the Boltzmann distribution graph change with a catalyst?

Answer 21

Ec (the activation energy with a catalyst) is further to the left than Ea.

Question 22

What is the effect of a catalyst on the Boltzmann distribution?

Answer 22

A larger proportion of molecules have an energy that is equal to or greater than the lower activation energy.

Question 23

What is a reversible reaction?

Answer 23

It is a reaction that takes place in both the “forward” and “reverse” directions

Question 24

How must a reaction be kept to remain in equilibrium?

Answer 24

It must be in a closed system and isolated from its surroundings, so the temperature, pressure and concentrations of reactants or products are unaffected

Question 25

How is an equilibrium system dynamic?

Answer 25

Both the forward and reverse reactions are taking place, this means that the rate of the forward reaction is equal to the rate of the reverse reaction meaning that the concentrations of reactants and products do not change.

Question 26

What is le Chatelier’s principle?

Answer 26

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

Question 27

What 3 things effect the position of equilibrium?

Answer 27

Temperature, pressure or concentration

Question 28

What happens when you increase the concentration of reactants or decrease the concentration of products?

Answer 28

This sends the position of equilibrium to the right to increase the production of the products and minimise the change in concentration of the reactants and products.

Question 29

What happens when you decrease the concentration of reactants or increase the concentration of products?

Answer 29

This sends the position of equilibrium to the left to increase production of the reactants and minimise the difference in concentration between reactants and products

Question 30

What happens when you increase the temperature of an equilibrium reaction?

Answer 30

It sends the position of equilibrium in the endothermic direction, this minimises the increase in temperature by taking in more heat.

Question 31

What happens when you decrease the temperature in an equilibrium reaction?

Answer 31

It shifts the position of equilibrium in the exothermic direction, to minimise the amount of heat taken in by giving out more heat

Question 32

What does increasing the pressure of a system do?

Answer 32

It shifts the position of equilibrium to the side with fewer moles of gas reducing the number of gaseous miles to minimise the increase in pressure

Question 33

What does decreasing the pressure do for an equilibrium system?

Answer 33

It shifts the position of equilibrium to the side with more miles of gas to increase the number of gaseous miles and increase the pressure.

Question 34

What is a catalysts effect on equilibrium?

Answer 34

It does not change the position of equilibrium it speeds up the rates that the forward and reverse reactions equally, increasing the rate that equilibrium is established.

Question 35

Considering that low temperature and high pressure will push the equilibrium to the right producing more ammonia why are these conditions not used?

Answer 35

Low temperature would produce a high yield but slowly - low rate.

High pressure increases the yield and rate of reaction but it requires a very strong container increasing the cost of the process and safety is a concern as there could be a leak so a compromise between yield and rate and yield and cost/safety is made.

Question 36

If you consider a reaction

aA + bB –> cC + dD how do you work out the equilibrium constant?

Answer 36

C^c x D^d / A^a x B^b

Products / Reactants

Question 37

What does a Kc value of 1 indicate?

Answer 37

That the position of equilibrium is halfway between reactions and products

Question 38

What does a Kc value of more than one indicate?

Answer 38

That the position of equilibrium is towards the products

Question 39

What does a Kc value of less than one indicate?

Answer 39

That the position of equilibrium is towards the reactants

Question 40

equation of Haber process

Answer 40

2N2 (g) + 3H2 (g) –> 2NH3 (g) [ammonia]