Kinetics (year 1)

Question 1

Rate of reaction

Answer 1

The change in concentration of a reactant per unit time

Question 2

Why does the rate of the production of H2 gas (for example) decrease with time in the reaction : Mg + H2SO4 —–> MgSO4 + H2

Answer 2

The concentration of the sulfuric acid decreases with time. There are fewer H+ ions/fewer particles in the same volume so the frequency of collisions decreases. As a result, there are fewer successful collisions in a give time.

Question 3

Suggest why pressure has little or no effect on the rate of reaction of magnesium and sulfuric acid

Answer 3

Pressure ONLY affects gaseous reactions

Question 4

Activation energy

Answer 4

The minimum amount of energy needed by the reactant particles to react on collision

Question 5

Why do most collisions not lead to a reaction

Answer 5

Reactions can only occur when collisoins take place between particles having sufficient energy - activation energy

Question 6

Which factors increase the rate of a reaction

Answer 6

1. Surface area
2. Concentration
3. Pressure
4. Temperature
5. Catalyst

Question 7

How to increase the surface area of a solid

Answer 7

Use powdered (solid) or use small pieces of the solid reactant

Question 8

Why does using a solid with a greater surface area increase the rate of reaction

Answer 8

The powder has a larger surface area so more particles are exposed to the acid particles. There is a greater surface area for collisions to take place with the acid particles. There are more frequent successful collisions in a given time

Question 9

How would a graph of volume of product against time differ when you a reactant with a greater surface area

Answer 9

The line for the reaction using a greater surface area will be initially steeper because the rate of reaction is faster.
The final volume will be the same in both experiments because the moles of the acid are unchanged

Question 10

How does an increase in concentration cause an increase in the rate of reaction

Answer 10

When the concentration increases, there are more particles per unit volume. There is a greater chance of the particles colliding so the frequency of collisions increases. As a result, there are more frequent successful collisions in a given time

Question 11

How does an increase in pressure cause an increase in the rate of reaction

Answer 11

When pressure is increased, the particles are pushed closer together so there are more reactant particles per unit volume. The frequency of collisions increases therefore there are more frequency successful collisions per unit time.

Question 12

How to increase the pressure

Answer 12

Reducing the volume of the container
Pumping more reactant gas into the container

Question 13

How does increasing the temperature cause an increase in the rate of reaction

Answer 13

When temperature increases the particles move faster so they collide more frequently. Further, the particles collide with greater energy, so more of the reactant particles have enough energy greater than or equal to the activation energy to react on collision.

At a higher temperature there are more frequency successful collisions per unit time

Question 14

How does a catalyst increase the rate of reaction

Answer 14

A catalyst provides an alternative route for the reaction. A greater proportion of reactant particles have enough energy greater than or equal to the activation energy to react on collision, so there will be more frequency successful collisions per unit time

Question 15

Why must an experiment be repeated 3 times

Answer 15

To improve the reliability of the investigation
To identify anomalous results and discard them
To identify random errors and discard them

Question 16

What does the area to the right of the activation energy under a Maxwell-Boltzmann curve represent

Answer 16

Number of particles with energy greater than activation energy

Question 17

What does the area under a Maxwell - Boltzmann curve represent

Answer 17

Number of particles present (total)

Question 18

Where is the mean energy of the particles found on a Maxwell-Boltzmann curve found

Answer 18

Not at the peak, slightly to the right

Question 19

Why should a Maxwell-Boltzmann curve start at the origin

Answer 19

There are no molecules with no energy. All the particles are moving so have some energy

Question 20

Why should a Maxwell-Boltzmann curve never touch the x axis

Answer 20

There is no maximum energy for the molecules

Question 21

What does a Maxwell-Boltzmann curve look like at a higher temperature

Answer 21

The peak is lower and is more to the right

Question 22

How does the area under a Maxwell-Boltzmann curve change with temperature

Answer 22

It DOES NOT CHANGE. Total area under each curve does not change

Question 23

Why does the reaction between methane and oxygen not take place at room temperature

Answer 23

The mixture of methane and oxygen is kinetically stable because the activation energy is very high. There are virtually no particles with energy greater than or equal to the activation energy at room temperature.

Question 24

Where to find the most probably energy on a Maxwell-Boltzmann curve

Answer 24

At the peak

Question 25

What is a catalyst

Answer 25

A substance that increases the rate of a reaction but remains chemically unchanged

Question 26

How would a Maxwell-Boltzmann curve look like when a catalyst is used

Answer 26

Identical, except the area area to the right of Ea is greater with a catalyst. The Ea will be lower and more to the left for a reaction using a catalyst.

Question 27

What does a homogenous catalyst do

Answer 27

Provide a surface for the reaction to take place

Question 28

What effect does a catalyst have on the energy distribution of gas molecules

Answer 28

No effect because the energies of molecules depends only on the temperature

Question 29

What happens to number of particles per unit volume if you double the concentration

Answer 29

Number of particles per unit volume doubles

Question 30

What happens to a Maxwell-Boltzmann curve if the concentration increases

Answer 30

The shape does not change, the curve will just be higher up as there will be more particles per unit volume