Kinetics

Question 1

What is collision theory?

Answer 1

States that reactions only occurs when collisions take place between particles with sufficient energy- this minimum energy is called the activation energy.

Question 2

Activation energy

Answer 2

This is the minimum amount of energy reacting particles must have for a successful reaction to take place, means that most collisions do not lead to the reaction- only a small proportion of particles collide with energy that’s greater than the activation energy

Question 3

How does concentration affect the rate of reaction?

Answer 3

If the concentration of the reactants is INCREASED, MORE particles of that reactant are present in a given volume, leading to MORE successful collisions between reactant particles in a given period of time, rate of reaction INCREASES.
CONCENTRATION INCREASES = RATE OF REACTION INCREASES

Question 4

What is the effect of pressure on the rate of reaction?

Answer 4

If the pressure of a gaseous reaction is INCREASED, particles are forced closer together, leads to MORE successful collisions between reactant particles in a given period of time, causes the rate of reaction INCREASES
PRESSURE INCREASES = RATE OF REACTION INCREASES

Question 5

What is the effect of temperature on the rate of reaction?

Answer 5

If the temperature GOES UP, gain more KE and move FASTER, more successful collisions between reactant particles in a given period of time, rate of reactions to increase.
TEMPERATURE INCREASES = RATE OF REACTION INCREASES

Question 6

What is the effect of surface area of solid reactants on the rate of reaction?

Answer 6

If the surface area of solid reactants increases, exposed surface of the reactants increases to, number of successful collisions in a given period of time, increases the rate of reaction.
SURFACE AREA UP = RATE OF REACTION UP

Question 7

What is a catalyst?

Answer 7

A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount, catalyst works by providing an alternative reaction pathway or lower the activation energy.

Question 8

What is a Maxwell-Boltzmann distribution ?

Answer 8

A plot of the number of gaseous molecules against the energy they have at a fixed temperature - a single point on the graph shows the distribution of molecular energies at a constant temperature

Question 9

Explain the Maxwell - Boltzmann distribution graph?

Answer 9

1. The curve starts at (0,0) because no molecules have 0 energy
2. Increases - a few molecules are moving slowly
3. The peak of the curve represents the most likely energy of any single molecule
4. The mean (average) energy of all the molecules is a bit to the right of the peak
5. Most molecules are moving at a moderate speed so their energies are in the middle
6. Some molecules have more than the activation energy, these are the only ones that can react

Question 10

Effect of temperature of the Maxwell-Boltzmann distribution graph?

Answer 10

When temperature of the reactant molecules is increased, this will increase the energy of the gaseous reactant molecules- changes the shape of the distribution.
LOWER TEMPERATURE- distribution moved to the left, peak is higher
HIGHER TEMPERATURE- distribution moved to the right, peak is lower
A small increase in temperature can cause a large increase in the rate of reaction as there is a significant increase in the number of molecules with enough energy to undergo a successful collisions.

Question 11

Effect of concentration on maxwell- Boltzmann distributions?

Answer 11

The curve will retain the same basic shape, the most probable energy of the molecules remains the same so the peak would be higher but at the same energy value on the horizontal axis, more reactant molecules at the same temp, overall area under the curve increases, this increases the number of reactant molecules which have enough energy to undergo a successful reaction, leads to an increase in the rate of reaction.

Question 12

Maxwell-distribution with a catalyst present?

Answer 12

Catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, adding a catalyst to a reaction mixture lowers the activation energy, the area above the activation energy with a catalyst is greater there are more molecules with sufficient energy to undergo a successful collisions. Increases the rate of reaction.
Catalyst does not affect the shape of the distribution as long as the temperature and total number of molecules are not changed.

Question 13

Equation for rate of reaction:

Answer 13

Amount of reactant used or product formed / time

Question 14

Measuring rate of reactions: timing how long it takes for a precipitate to form

Answer 14

1.measure fixed volumes of sodium thiosulphate and hydrochloric acid using a measuring cylinder
2. Use a water bath to heat both to a desired temperature before you mix them
3. Mix solutions in a conical flask, place flask over black cross (which can originally be seen), measure the time taken for the cross to disappear

Question 15

How do reversible reactions reach dynamic equilibria?

Answer 15

Lots of chemical reactions are reversible, as the reactants get used up, forward reaction slows down and as more product is formed, the reverse reaction speeds up, after a while the forward and backward reaction would be the exact same rate. Known as DYNAMIC EQUILIBRIA (can only take place in a closed system)

Question 16

Le chatelier’s principle

Answer 16

If a reaction at equilibria is subjected to a change in concentration, pressure or temperature the position of equilibria will move to counteract the change

Question 17

Le Chatelier’s principle: effect of concentration on equilibrium

Answer 17

Increasing the concentration of a reactant, the equilibrium tries to get rid of the extra reactant = makes more product = equilibrium shifts to the right. Decreasing the concentration has the opposite effect.

Question 18

Le Chatelier’s principle: effect of pressure on equilibrium

Answer 18

Changing this only has an effect on gases, increasing the pressure shifts the equilibrium to the side with fewer gas molecules = this decreases the pressure.

Question 19

Le chatelier’s principle: effect of temperature on equilibrium

Answer 19

Increasing the temperature means adding heat, equilibrium shifts in the endothermic (positive ΔH) direction to absorb this heat.
H2O2(l) → H2O(l) + 1/2 O2(g)
ΔH = -98.2 kJ
exothermic in the forward direction, if you increase the temperature, equilibrium shifts to the left to absorb the extra heat.

Question 20

Le chatelier’s principle: effect of catalyst on equilibrium

Answer 20

Catalysts have no effect on the position of equilibrium, they can’t increase yield- but it does mean equilibrium is reached faster.

Question 21

What are the reaction conditions in industry?

Answer 21

-temperature : 300 degrees celcius
-pressure : 60 - 70 atmospheres
-with a phosphoric acid catalyst