C6 - Rate and Extent of Chemical Change

Question 1

What are the two equations for mean rate of reaction

include units

Answer 1

quantity of reactant used/time taken
quantity of product formed/time taken
moles/minut

g/s or cm^3 per second
mol/min dm^3/min

Question 2

What does a graph curving off represent

Answer 2

mass of reactant gets used up less quickly
Running out of reactants

Question 3

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1 i
1 i
1 i
1 i
1i_______________
(product used - time taken)
Describe and explain the shape of the graph
Sodium + HCl = hydrogen
(3 marks)

Answer 3

Hydrogen produced quickly at first because high concentrations of sodium and hydrochloric acid
Rate of production slows because concentrations of sodium and hydrochloric acid decrease
Hydrogen stop being produced / graph plateaus because all the sodium and hydrochloric acid has been used up

Question 4

What is collision theory

Answer 4

For particles to react, they have to collide with each other with sufficient energy (activation energy)

Question 5

Using collision theory what does the rate of reaction depend on

Answer 5

Amount of energy particles have
Frequency of collisions
(Not all collisions reach activation energy - more collisions more chance)

Question 6

What four things does the rate of reaction depend on

Answer 6

Temperature
Concentration of a solution/pressure of gas
Surface area
Use of a catalyst

Question 7

Describe how temperature affects rate of reaction (6 marks

Answer 7

Temp - particles gain more energy - move faster - collide more frequently. Particles gain more energy - collide with more energy - more likely to exceed activation energy
Overall higher rate of successful collisions - Higher rate of reaction

Question 8

Describe how concentration/pressure affects rate of reaction (4 marks)

Answer 8

Effects how many particles in a unit of volume - increasing - more particles per unit of volume - collisions more frequent - more successful collisions - increases rate of reaction

Question 9

Describe how surface area affects rate of reaction (4 marks)

Answer 9

Higher surface area : volume ratio
Much higher area that collisions can take place
Frequency of collisions higher
Increasing rate of reaction

Question 10

Describe how a catalyst affects rate of reaction (3 marks)

Answer 10

Reduces activation energy by providing an alternate reaction pathway - higher proportion of successful collisions

Question 11

A + B ⇌ C + D
Describe how this is a reversible reaction (4 marks)

Answer 11

As the reactants react, their concentrations fall so the forward reaction will slow down
As more products are made and their concentrations rise, the backward reaction will speed up
After a while the forward reaction will be going at the same rate as the backward reaction - equilibruim

Question 12

If there are more products than reactants which way would the equilibrium lie to

Answer 12

Right!

Question 13

What is exothermic/endothermic

Answer 13

Endothermic: exits thermal energy to surroundings
Endothermic: takes in energy from surroundings

Question 14

In a reversible reaction, if the forward reaction is endothermic, what would be the affect of increasing the temperature (3 marks)

Answer 14

Equilibrium will shift to the left to reduce the temperature and counter act the increase in temperature by producing products

Question 15

What is le chateliers principle

Answer 15

Idea that if you change the conditions of a reversible reaction at equilibrium, the system will try counter act that change

Question 16

In a reversible reaction, what happens if you increase the pressure (2 marks)

Answer 16

equilibrium tries to reduce it - moves in the direction where there are fewer molecules of gas (higher total of moles)

Question 17

In a reversible reaction, what happens if you increase the concentration of products (2 marks)

Answer 17

System will no longer be at equilibrium - system responds to bring itself back by making more reactants