C6 rate of reaction

Question 1

In terms of particles: explain how doubling concentration doubles the rate of reaction

Answer 1

The number of reacting particles doubles. This increases the number of particles for a fixed volume which means particles are closer together. This increases the frequency of successful collisions and so the collisions double.

Question 2

In terms of particles and collisions: explain how increasing surface area affects the rate of reaction

Answer 2

Rate of reaction increases. Increasing surface area means a greater area exposed for particles to collide with (larger surface area to volume ratio) so an increased frequency of (successful) collisions.

Question 3

Give 2 reasons why a catalyst reduces costs

Answer 3

1. Reduces energy requirements
2. The catalyst is not used up

Question 4

In terms of particles: how is diffusion of particles affected when increasing temperature and why?

Answer 4

The particles diffuse faster because they have more energy that has been transferred to them

Question 5

When do you start+stop an experiment when seeing the production of carbon dioxide with limewater?

Answer 5

Start when the energy transfer (like heating) has started
Stop once limewater has turned cloudy

Question 6

Why may results of calculating rate of reaction not fit with the other results? Give 3 reasons.

Answer 6

1. The point is anomalous
2. There was a measurement error
3. There was a timing error

Question 7

In terms of particles: how does an increase in temperature increase the rate of reaction?

Answer 7

Particles gain (kinetic) energy transferred to them. This means they move faster and collide more with more energy. This increases the frequency of successful collisions.

Question 8

Reversible reactions: how does increasing the temperature affect yield of products with a forward reaction that is exothermic?

Answer 8

Decreases yield of product. Equilibrium shifts to the left to counteract the increase in temperature and decrease it.

Question 9

Reversible reactions: how does increasing/decreasing pressure affect reactions at equilibrium?

Answer 9

• an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction
• a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

only in gases

Question 10

Why does the Haber process use a temperature of 450 and pressure of 200?

Answer 10

It is a compromise between yield and rate of reaction. If temperature too high, it gives a low yield as the reaction is exothermic, but at low temperatures the rate is too slow.

The pressure favours the product side as there are less particles, producing a greater yield whilst also increasing the rate of reaction as gas particles are closer together per unit volume so the frequency of successful collisions increases.

Question 11

Reversible reactions: if yield of a product is best when temperatures are cooler whilst at equilibrium what does this suggest?

Answer 11

The forward reaction is exothermic.
(If cooler system will want to counteract by increasing yield of products)

Question 12

How do catalysts benefit a manufacturer and the environment?

Answer 12

Lower energy requirements so lower fuel costs and less pollution caused by fossil fuels as less are burned

Question 13

Why do reversible reactions use a compromise temperature and pressure?

Answer 13

At low temperatures rate of reaction is too slow. Too high pressures are dangerous and expensive. So a compromise gives a reasonable yield at a fast rate.

Question 14

Why would hydrogen be safer if filled with another compound when transported?

Answer 14

Hydrogen would react with the compound. Therefore, there is a lower risk of explosion as hydrogen is very flammable.

Question 15

RPA 6: why may readings of gas from a gas syringe be lower than expected result?

Answer 15

Leak from tube
Gas escapes so lost to surroundings

Question 16

RPA 6: why does volume of gas stop increasing at a certain point? How is this shown on a graph?

Answer 16

The reaction has stopped as at least one of the reactants has been used up. Can be shown by a horizontal line.

Question 17

RPA 6: what change to apparatus (Mg+HCl) could improve accuracy of results and why?

Answer 17

Use a gas syringe so the gas will not dissolve in water

Question 17

RPA 6: why would air bubbles collected before CO2 not affect the volume collected?

Answer 17

CO2 left in the flask at the end and has the same volume as the air collected

Question 18

What do you have to write when discussing trends between variables?

Answer 18

1. How x affects y
2. General relationship (how quick it increases/ linear or directly proportional/ correlation)

Question 19

How would you compare a cube with a surface area to volume ratio of 12:1 (cm) to a cube with sides of 5cm?

Answer 19

Ratio would be 150:125 which in the same ratio is 1.2:1 meaning the cube with sides 5cm ratio decreases by a factor of 10

Question 20

How does the use of a catalyst affect the equilibrium yield?

Answer 20

It has no affect, speeds up the rate of the forward and backward reaction equally.

Question 21

Reversible reaction: why does a change in pressure not affect an aqueous reactions equilibrium?

Answer 21

The solutions are aqueous, they are not gases. Changes in pressure only affect gases so the reaction will remain at equilibrium.

Question 22

What is activation energy?

Answer 22

The minimum amount of energy that particles must have to react

Question 23

When is equilibrium reached?

Answer 23

When a reversible reaction occurs in apparatus which prevents the escape of reactants and products (closed system), equilibrium is reached when the forward and reverse reactions occur at exactly the same rate

Question 24

Reversible reactions: how does increasing/decreasing concentration affect the position of equilibrium?

Answer 24

If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again.
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again.
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.

Question 25

Reversible reactions: how does changing temperature affect the position of equilibrium?

Answer 25

• If temperature is increased:
  the relative amount of products at equilibrium increases for an endothermic reaction
  the relative amount of products at equilibrium decreases for an exothermic reaction.
• If temperature is decreased:
  the relative amount of products at equilibrium decreases for an endothermic reaction
  the relative amount of products at equilibrium increases for an exothermic reaction.
  (system shifts to favour)

Question 26

Why does a reaction stop?

Answer 26

Reactant has completely been used up

Question 27

Explain how catalysts speed up the rate of reaction

Answer 27

Provide an alternative reaction pathway with a lower activation energy without being used up so there are more particles with sufficient energy to react, meaning frequency of successful collisions increases

Question 28

RPA: suggest one reason why there may be an anomalous result which is lower than expected for volume collected

Answer 28

Bung not placed quick enough so gas escaped from flask

Question 29

Describe how further experimental work can be done to prove if an explanation is correct

Answer 29

Measure and compare

Question 30

Explain why these conditions are chosen for economical production of ammonia in the
Haber process.

Answer 30

rate
* higher temperature gives higher rate because of more frequent collisions as particles have more energy
* use of catalyst gives higher rate because the activation energy is lowered
equilibrium
* higher pressure shifts the position of equilibrium to the right because more molecules on left-hand side
* use of catalyst has no effect on the position of equilibrium
other factors
* higher temperatures and pressures would use more energy so would increase costs
* use of a catalyst reduces energy costs as not used up
compromise
* the temperature and pressure chosen is a compromise between rate of reaction and position of equilibrium