Rates of reaction

Question 1

Definition of rate of reaction?

Answer 1

The change in concentration of a reactant or product in unit time

Question 2

What units is rate of reaction measured in?

Answer 2

mol dm ^-3.s^-1

Question 3

What does increasing pressure and increasing concentration do to the rate of reaction?

Answer 3

Increases it as more collisions

Question 4

Examples of how the progress of a chemical reaction might be monitored?

Answer 4

Colour change, mass change, volume of gas produced, pH, conductivity

Question 5

What does rate of reaction =?

Answer 5

Change in concentration/change in time

Question 6

How to calculate rate of reaction from a concentration time graph?

Answer 6

At the given time draw a tangent at that point and calculate it’s gradient (change in y / change in x)

Question 7

Catalyst definition?

Answer 7

Substance that increases the rate of a chemical reaction without being used up in the process, providing an alternative route for the reaction with lower activation energy

Question 8

What’s a heterogeneous catalyst?

Answer 8

It’s in a different physical state from the reactants, which is most frequently gaseous reactants and a solid catalyst

Question 9

What’s a homogenous catalyst?

Answer 9

Catalyst in the same physical state as reactants, most frequently aqueous or gaseous state

Question 10

Ways catalysts affect chemical industry?

Answer 10

Reduce energy consumption, positive effect on environment
Increase atom economy, reduce waste
Enzyme use in industry
Economic benefits

Question 11

Describe a boltzman distribution graph?

Answer 11

x axis = energy
y axis = number of molecules with a given energy
Shape- straight diagonal line which then slopes down
Activation energy line- near tail

Question 12

What does the area underneath a boltzman distribution represent?

Answer 12

Total number of particles in the system

Question 13

What happens to the boltzman distribution when temperature is increased?

Answer 13

The peak gets lower, and moves to a higher energy. Meaning a higher proportion of particles have more energy than the activation energy

Question 14

Reasons for why an increase in temperature increases rate of reaction

Answer 14

More particle collisions per unit time, and more particles collide have the activation energy or more

Question 15

Effect of a catalyst on a boltzman distribution?

Answer 15

Activation energy lower, so proportion of particles with more energy than activation energy increases

Question 16

Definition of dynamic equilibrium?

Answer 16

The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and concentration do not change

Question 17

What is meant when a system is closed?

Answer 17

Isolated from surroundings
No material added or removed
Conditions (temp/pressure) stay constant

Question 18

Le Chatelier’s principle?

Answer 18

When a system in dynamic equilibrium is subjected to an external change, the system readjusts it self to minimise the effect of change and restore equillibrium

Question 19

Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1

Adding H2 change?

Answer 19

More ammonia produced

Question 20

Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1

Change if take away ammonia?

Answer 20

More ammonia produced

Question 21

Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Change if temperature increased?

Answer 21

More Nitrogen and Hydrogen produced as reaction is exothermic

Question 22

Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Change if pressure increased?

Answer 22

More ammonia produced, as their are more moles of gas of N2 and H2

Question 23

Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Adding a catalyst?

Answer 23

No effect on equilibrium position

Question 24

Formula for equilibrium constant, Kc

Answer 24

aA + bB ⇌ cC + dD

Kc= ([C]^c [D]^d)/([A]^a [B]^b)

Question 25

How to know the units for Kc

Answer 25

Cancel out the amount of moles on top and bottom, then do Mol^-1.dm^3 to that power. Eg. if it was 3 would be Mol^-3.dm^9

Question 26

What does increasing the pressure do to the rate?

Answer 26

Increases rate

particles closer together so more frequent collisions per unit time

Question 27

What does increasing concentration of reactant do to the rate of reaction?

Answer 27

Increases rate

More particles in a given volume, so more collisions per unit time

Question 28

What does adding a catalyst do to the rate of reaction?

Answer 28

Increases rate
Catalyst provides an alternative route with a lower activation energy, therefore a greater proportion of particles have energy equal to or greater than the activation energy so a greater proportion of collisions are succesful