3.4 Rates of Reaction

Question 1

What must happen for a reaction to occur ?

Answer 1

-Collide
-Have collision energy that is same as activation energy.
-Collide with correct orientation.

Question 2

What happens to particles when pressure/concentration increases ?

Answer 2

-More reactant particles per unit volume.
-This increases collision frequency .
-% of successful collisions stays same.
-Applies for gases only.

Question 3

What happens to particles when SA increases ?

Answer 3

-More particles avaliable that have the potential to collide.
-Speeds up the rate of the reaction.
-Increases collision frequency.

Question 4

What happens to particles when temperature increases ?

Answer 4

-Particles have more KE and move faster.
-Increases collision frequency.
-Increases rate of reaction.
-Increases % of collisions that have activation energy for collisions to happen.

Question 5

What happens to particles when a catalyst is added ?

Answer 5

-Lowers activation energy needed for a collision to occur.
-Collision frequency stays same.
-Increases % of collisions that cause a successful reaction.

Question 6

What is the role of a catalyst ?

Answer 6

-Speeds up rate of a reaction but doesn’t undergo any permanent change itself.
-Provides and alternative reaction pathway by lowering activation energy needed for collisions to cause a reaction.

Question 7

What is a homogenous catalyst ?

Answer 7

A catalyst that is in the same physical state as the reactants. Reacts with reactants to form intermediates.
Eg., carboxylic acid +alcohol = ester +water

Question 8

What is a heterogenous catalyst ?

Answer 8

A catalyst in different physical state as the reactants.
Usually solids that react with gaseous/ aqueous reactants.

Question 9

How do heterogenous catalyst work ?

Answer 9

Reactant molecules are absorbed on surface of catalyst. After, product molecules leave catalyst by de-absorption.

Question 10

What are catalytic converters ?

Answer 10

Must be fitted in petrol cars. Contain catalysts made of platinum, palladium and rhodium on a honeycomb mesh that provides a large SA.

Question 11

How do catalytic converters work ?

Answer 11

The hot exhaust gases pass over heterogenous catalysts and harmful gases are converted into less harmful products that are safe to leave the engine.

Question 12

What % of chemical materials are produced using a catalyst ?

Answer 12

90 %

Question 13

Why is using catalysts sustainable ?

Answer 13

If a chemical reaction uses less energy, then less fossil fuels or electricity is need to heat the reaction.

Question 14

Why is using catalysts economic ?

Answer 14

Makes more products faster and cuts costs to increase profitability. Economic advantages outweigh developing catalyst. There are higher atom economies and fewer pollutants.

Question 15

What is the Boltzmann Distribution curve ?

Answer 15

The spread of molecular energies in gases, where you can see the activation energy.
It concerns particles/ molecules of a gas, liquid or solution.

Question 16

Why does the Boltzmann curve start at the origin ?

Answer 16

As there are no particles with no kinetic energy.

Question 17

What is the area under the curve equal to ?

Answer 17

Area under the curve is equal to total number of molecules.

Question 18

Why is there no maximum value for on a Boltzmann distribution curve ?

Answer 18

There is no value for maximum kinetic energy on x axis as curve never reached x axis.

Question 19

How does Boltzmann distribution curve change for temperature ?

Answer 19

The graph move to the right and down and there are more particles that have the kinetic energy same as the activation energy to cause a reaction.

Question 20

How does the Boltzmann distribution curve change when a catalyst is added ?

Answer 20

The graph stays same but activation energy is lower. There is a greater number of molecules that have the amount of energy needed for a reaction.

Question 21

Why is dynamic equilibria described as dynamic ?

Answer 21

Where forward and reverse reaction are ongoing.

Question 22

Why is dynamic equilibria described as equilibria ?

Answer 22

-Rates of forward and reverse reactions are the same.
-Net concentrations remain the same.
-Must be a closed system.

Question 23

How do you model dynamic equilibria ?

Answer 23

Use 2 plastic straws and fill one cullender with water. Transfer water using straws, from one cylinder to the other until dynamic equilibria is reached.

Question 24

What does Le Chateliers Principle state ?

Answer 24

When a system is in dynamic equilibrium, any change in external conditions will cause the equilibrium to adjust itself to oppose the change.

Question 25

How does the equilibrium change if concentration is increased according to Le Chateliers Principle ?

Answer 25

If a concentration of a substance is increased, the equilibrium will shift away from the substance that has been increased, so more of that substances is used up.

Question 26

How does the equilibrium change if concentration is decreased according to Le Chateliers Principle ?

Answer 26

If the concentration of a substance is decreased, the equilibrium swings toward that substance to replace it, so m ore of the substance is made.

Question 27

How does the equilibrium change if temperature is increased according to Le Chateliers Principle ?

Answer 27

If temperature increases, the equilibrium shifts in the endothermic direction as that is when temperature is absorbed and this shift causes temperature to decrease, so increases rate of endothermic reaction.

Question 28

How does the equilibrium change if temperature is decreased according to Le Chateliers Principle ?

Answer 28

If temperature decreases, the equilibrium shift in the exothermic direction as that is when the temperature is released and this shift causes temperature to increase, so increases rate of exothermic reaction.

Question 29

How does the equilibrium change if pressure is increased according to Le Chateliers Principle ?

Answer 29

Increasing pressure causes equilibrium to shift to reduce the pressure. This means the equilibrium shift to the side where there are fewer moles to reduce pressure.

Question 30

How does the equilibrium change if pressure is decreased according to Le Chateliers Principle ?

Answer 30

Decreasing pressure causes equilibrium to shift to increase the pressure. This means the equilibrium shifts to side where there are more molecules.

Question 31

How does adding a catalyst change equilibrium according to Le Chateliers principle ?

Answer 31

Speeds up forward and reverse reaction equally.
No change to position of equilibrium.
Equilibrium is reached faster.
Particularly important in industry.

Question 32

What is the equilibrium constant ?

Answer 32

Kc = [C]y [D]z / [A]w [B]

Question 33

What does a large equilibrium constant value indicate ?

Answer 33

Indicates that the equilibrium lies to the right.

Question 34

What does a small equilibrium constant value indicate ?

Answer 34

Indicates that the equilibrium lies to the left.

Question 35

What does a Kc value of close to 1 indicate ?

Answer 35

Indicates that the equilibrium position lies in the middle.