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, less frequent collisions
3) Once the reactants have beenh 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

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

Answer 6

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 7

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

Answer 7

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 8

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

Answer 8

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 9

What are the 2 major features of a catalyst?

Answer 9

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

Question 10

How does a catalyst increase the rate of reaction?

Answer 10

It provides an alternative reaction pathway of lower activation energy

Question 11

What is a homogeneous catalyst and how do they work?

Answer 11

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 12

What is a heterogeneous catalyst and how do they work?

Answer 12

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 13

What state are heterogeneous catalysts used in?

Answer 13

usually solids (gaseous or aqueous reactants)

Question 14

How are catalysts sustainable?

Answer 14

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

Question 15

How are catalysts economically friendly?

Answer 15

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 16

What are the 3 features of any Boltzmann distribution?

Answer 16

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 17

Effects of increasing temperature on a reaction?

Answer 17

More molecules have an energy greater than or equal to the activation energy.

Collisions will be more frequent as particles are moving faster so reaction is faster.

More successful collisions

Question 18

labels on x and y axis on a Boltzmann distribution curve?

Answer 18

X = energy 
Y = Number of molecules with a given energy

Question 19

How does the Boltzmann distribution graph change with a catalyst?

Answer 19

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

Question 20

What is the effect of a catalyst on the reaction?

Answer 20

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

Activation energy is lowered with a catalyst

More successful collisions

Question 21

What is a reversible reaction?

Answer 21

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

Question 22

How must a reaction be kept to remain in equilibrium?

Answer 22

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 23

How is an equilibrium system dynamic?

Answer 23

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 24

What is le Chatelier’s principle?

Answer 24

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 25

What 3 things effect the position of equilibrium?

Answer 25

Temperature, pressure or concentration

Question 26

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

Answer 26

This sends the position of equilibrium to the right to increase the production of the products and use up the reactants.

Question 27

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

Answer 27

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 28

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

Answer 28

• It sends the position of equilibrium in the endothermic direction
• State whether the forward or reverse reaction is endothermic
• This takes in heat

Question 29

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

Answer 29

• It shifts the position of equilibrium in the exothermic direction
• State whether the forward or reverse reaction are exothermic
• This gives out heat

Question 30

What does increasing the pressure of a system do?

Answer 30

• It shifts the position of equilibrium to the side with fewer moles of gas
• State whether this is to the left or right
• This uses up gas to minimise the increase in pressure

Question 31

What does decreasing the pressure do for an equilibrium system?

Answer 31

• It shifts the position of equilibrium to the side with more moles of gas
• State whether this is to the left or right
• To produces more the gas to increase the pressure.

Question 32

What is a catalysts effect on equilibrium?

Answer 32

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 33

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

Answer 33

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 34

If you consider a reaction

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

Answer 34

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

Products / Reactants

Question 35

What does a Kc value of 1 indicate?

Answer 35

That the position of equilibrium is halfway between reactions and products

Question 36

What does a Kc value of more than one indicate?

Answer 36

That the position of equilibrium is towards the products

Question 37

What does a Kc value of less than one indicate?

Answer 37

That the position of equilibrium is towards the reactants

Question 38

equation of Haber process

Answer 38

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

Question 39

effect of increasing temperature on Boltzmann distribution curve?

Answer 39

Curve is shallower, peak is further to the right.