Unit 1: Section 5 - Kinetics, Equilibria and Redox Reactions

Question 1

What is the equation for reaction rate?

Answer 1

Reaction rate = Amount of reactant used or product formed / Time

Question 2

What is collision theory?

Answer 2

A reaction between 2 particles won’t take place unless they collide in the correct orientation and with at least the minimum amount of kinetic energy required

Question 3

What is the activation energy?

Answer 3

The minimum amount of kinetic energy that particles need to react

Question 4

What does a Maxwell-Boltzmann distribution curve show?

Answer 4

The area under the graph - the total number of molecules
The curve starts at (0,0) - no molecules have zero energy
The peak - the most probable energy
To the right of the peak - mean energy of all the molecules
Molecules that can react - to the right of the activation energy

Question 5

How does increasing the temperature affect the rate of reactions?

Answer 5

1. If you increase the temperature of a reaction, the particles will on average have more kinetic energy and will move faster therefore the rate of reaction will be faster
2. A greater number of particles will have at least the activation energy and be able to react
   3 .This pushes the Maxwell-Boltzmann to the right, but there are the same number of particles so the area has to stay the same
3. as the molecules are moving faster, they will also collide faster
4. As you get 2 effects happening, small increases in temperature will led to large increases in reaction rate

Question 6

How does increasing the concentration affect the rate of reactions?

Answer 6

1. The particles will on average be closer together
2. If they’re closer, they will collide more often
3. If collisions happen more frequently, they’ll have more chances to react
4. Therefore the rate of reaction increases

Question 7

How does increasing the pressure affect the rate of reactions?

Answer 7

1. More of the gas particles are closer together
2. So they are more likely to collide so collisions take place more frequently
3. Therefore the reaction rate increases

Question 8

What is a catalyst?

Answer 8

A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy, the catalyst is chemically unchanged at the end of the reaction

Question 9

How do catalysts affect graphs?

Answer 9

1. In an energy profile, the activation energy will be lowered
2. In a Maxwell-Boltzmann distribution curve, the activation energy line is shifted the the left

Question 10

How to measure reaction rates by timing how long it takes for a precipitate to form?

Answer 10

1. Used when the product is a precipitate which clouds the solution
2. Watch a cross through the solution and time how long it takes to disappear
3. You need to have the same observer every time, and the same cross because roughly the same amount of precipitate will be produced when the cross is obscured
4. This is a very subjective method though

Question 11

How to measure reaction rates by measuring a decreases in mass?

Answer 11

1. Used when the product is a gas
2. As gas is produced, the mass of the reaction mixture decreases
3. The method is accurate, but as releases gas into the atmosphere, it is usually done in a fume cupboard

Question 12

How to measure reaction rates by measuring the volume of gas given off?

Answer 12

1. Use a gas syringe to measure the volume of gas being produced
2. Gas syringe usually gives the volume to the nearest 0.1cm^3 so is usually an accurate method

Question 13

What is dynamic equilibrium?

Answer 13

When the forward reaction is going at the same rate as the backward reaction so the concentrations of reactants and products stays constant
This can only happen in a closed system

Question 14

What is Le Chatelier’s principle?

Answer 14

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

Question 15

What happens if you increase/decrease the concentration of a reactant/product in a reaction at equilibrium?

Answer 15

1. If you increase the concentration of a reactant, the equilibrium will shift to the right to make more product
2. If you increase the concentration of a product, the equilibrium will shift to the left to make more reactant
3. Decreasing the concentration will have the opposite effect

Question 16

What happens if you increases/decrease the pressure in a reaction at equilibrium?

Answer 16

1. Increasing the pressure will shift the equilibrium to the side with fewer gas molecules, this will reduce the pressure
2. Decreasing the pressure will shift the equilibrium to the side with more gas molecules, this will increase the pressure

Question 17

What happens if you increases/decrease the temperature in a reaction at equilibrium?

Answer 17

1. If you increase the temperature, the equilibrium position will shift in the endothermic direction
2. If you decrease the temperature, the equilibrium position will shift in the exothermic direction

Question 18

How do catalysts affect the equilibrium position?

Answer 18

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

Question 19

What is Kc?

Answer 19

The equilibrium constant which can be used to find concentrations in equilibrium mixtures

Question 20

What affects Kc?

Answer 20

1. If you change the temperature of the system, you also will change the equilibrium concentrations of the products and reactants so Kc will change - if change in temp=more product=Kc rises but if change in temp=less product=Kc decreases
2. Changing concentrations or adding a catalyst doesn’t affect Kc

Question 21

What is a redox reaction?

Answer 21

When electrons are transferred
1. A loss of electrons is called oxidation
2. A gain of electrons is called reduction

Question 22

What is the difference between an oxidising reagent and a reducing agent?

Answer 22

Oxidising agent - Accepts electrons and gets reduced
Reducing agent - Donates electrons and gets oxidised

Question 23

What are the oxidation state rules?

Answer 23

1. Uncombined elements/diatomic - 0
2. Monoatomic atom - same as its charge
3. Compound ions - ion charge
4. Neutral compound - 0
5. Combined oxygen - nearly always -2 but in peroxides -1
6. Combined hydrogen - +1 except in metal hydrides where it’s -1

Question 24

How to form half equations

Answer 24

1. Split into the reactant getting oxidised and reduced, and their products
2. Balance the 2 equations, adding H2O to balance oxygens and H+ to balance hydrogens
3. Add electrons to balance charges