5. Kinetics

Question 1

State the conditions for when reactions can only occur

Answer 1

• When collisions take place between particles having sufficient energy.

Question 2

Define activation energy

Answer 2

• The minimum energy which particles need to start breaking bonds to start a reaction.

Question 3

Describe the Maxwell-Boltzmann distribution

Answer 3

• Graph that shows the spread of energies that molecules of a gas or liquid have at a particular temperature

Question 4

State the y-axis of the Maxwell Boltzmann distribution

Answer 4

• Number of molecules

Question 5

State the x-axis

Answer 5

• Collision energy

Question 6

Describe what happens in the Maxwell Boltzmann distribution at the origin

Answer 6

• The energy distribution should go through the origin because there are no molecules with no energy

Question 7

Describe what happens in the Maxwell Boltzmann distribution before its peak

Answer 7

• Collision energy and no. molecules both start to increase
• A few have low energies because collisions cause some particles to slow down

Question 8

Describe what happens in the Maxwell Boltzmann distribution at its peak

Answer 8

• Emp - most probable energy

Question 9

Describe what happens in the Maxwell Boltzmann distribution after its peak

Answer 9

• Most molecules have energies between the two extremes but the distribution is not symmetrical

Question 10

Describe what happens in the Maxwell Boltzmann distirubiton past the point of Ea

Answer 10

• Only a few particles have energy greater than Ea
• The energy disturbition should never meet the x axis as there is no maximum energy for molecules

Question 11

What does the area under the curve in a Maxwell Boltzmann distribution graph represent?

Answer 11

• The total number of particles present

Question 12

The mea energy of the particles is not at the peak of the curve

Question 13

How can a reaction go to completion if few partciesl have energy greater than Ea?

Answer 13

• Particles can gain energy through collisions

Question 14

increaasins temp

Answer 14

As the temperature increases the distribution shifts towards having more molecules with higher energies

Question 15

Define rate of reaction

Answer 15

• The change in concentration of a substance in unit time. The usual unit is mol dm-3s-1

Answer 16

When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction.
The initial rate is the rate at the start of the reaction where it is fastest.
Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve (at different times) and calculating the gradient of the tangent.

Answer 17

In the experiment between sodium thiosulfate and hydrochloric acid we usually measure reaction rate as 1/time. The time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur . Na2S2O3 + 2HCl  2NaCl + SO2 + S + H2O
This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of sulfur produced is fixed and constant.

Question 18

rate = gradient tangent to cruve

Question 19

effect of increasing concentration and increasing pressure

Answer 19

At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions.

Answer 20

Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions.

Answer 21

If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change.
They curves will be higher, and the area under the curves will be greater because there are more particles.

Answer 22

More molecules have energy > EA (although not a greater proportion

Question 23

comparing rate curves

Question 24

effect of increasing temperature

Answer 24

At higher temperatures the kinetic energy of the particles increases. The pariticles collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction.
As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases.

Question 25

effect of increasing surface area

Answer 25

Increasing surface area will cause successful collisions to occur more frequently between the reactant particles and this increases the rate of the reaction.

Question 26

Define catalyst

Question 27

effect of catalyst