Physical Chemistry

Question 1

What is endothermic?

Answer 1

Heat taken in, in a chemical reaction. The products have more energy than the reactants.

Question 2

What is exothermic?

Answer 2

Heat given out, in a chemical reaction. The products have less energy than the reactants.

Question 3

What is activation energy?

Answer 3

Energy needed to start a reaction. Activation energy used to break bonds so reaction can take place and new bonds can form.

Question 4

What is bond enthalpy?

Answer 4

The energy it takes to break one mole of a specific bond type in gas form.

Question 5

How to calculate energy change?

Answer 5

Draw out bonds
Use average bond enthalpy table
Sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds made.
- figure shows exothermic
+ figures shows endothermic

Question 6

What is calorimetry?

Answer 6

Doing an experiment in set conditions so you can measure temperature change. Allows you to measure enthalpy change directly. Gives true value rather than approximate.

Mass of surroundings x specific heat capacity of surrounding x change in temperature of surroundings.
Then usually scale up to one mole.

Surroundings usually = water

Question 7

What is specific heat capacity?

Answer 7

The energy required to raise the temperature of 1g of a substance by 1C.
Units: JK-1g-1
Specific heat capacity of water: 4.18

Question 8

What is enthalpy of reaction?

Answer 8

Overall energy change at constant pressure, if the conditions are standard conditions we call it the standard enthalpy of reaction

Question 9

What are standard conditions?

Answer 9

Pressure of 100kPa
Temperature of 298K (25°C)
Concentration of 1M, for reactions with aqueous solutions
The standard state is the physical state of a substance under standard conditions. E.g. H20 is liquid

Question 10

Standard enthalpy change of reaction definition?

Answer 10

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.

Question 11

Standard enthalpy change of combustion definition?

Answer 11

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
Since it is only one mole there may be fractions of other substances in the equation.

Question 12

Standard enthalpy of formation definition?

Answer 12

The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Question 13

What is Hess’ law?

Answer 13

If a reaction can take place by more than one route and the initial and final concentrations are the same, the total enthalpy change is the same.

Question 14

How to use an enthalpy cycle?

Answer 14

Add if arrow going correct way
Subtract if arrow going wrong way

Combustion reactants-products
Formation products-reactants

Question 15

What is entropy?

Answer 15

A measure of disorder
Solids lower
Gases higher

Question 16

Reaction rate formula?

Answer 16

Change in concentration (mol dm-3)
——————————————
Time for change to occur (seconds)

For the average time

For more specific use a graph and use a tangent to work out the gradient

Question 17

What can increase rate of reaction?

Answer 17

Increase temperature
Increase reactant concentration
Increase reactant pressure
Increase surface area e.g. use a powder
Adding a catalyst

Question 18

Collision theory?

Answer 18

1) particles have to collide
2) have collision energy equal to or greater than the reactions activation energy
3) collide with proper orientation

Question 19

How does a catalyst affect kinetics?

Answer 19

Increases the rate of reaction without being consumed. The reaction follows a different reaction route, usually with a lower activation energy.

Question 20

Types of catalyst?

Answer 20

Homogeneous catalyst- reactant and catalyst in same states

Heterogeneous catalysts- reactant and catalyst in different states

Question 21

Maxwell Boltzmann curve?

Answer 21

Total area under the curve= total number of molecules in the system

Area under the curve and to the right of activation energy= number of molecules with sufficient energetic to react

Question 22

Main characteristics of maxwell Boltzmann distribution?

Answer 22

No particles have 0 energy
Most particles have intermediate energy
A few particles have high energy
The average energy is not the same as the most probable energy

Question 23

What is equilibrium?

Answer 23

When reversible reactions reach a balance point, where the amount of reactants and the amount of products formed remains constants.

Question 24

What is dynamic equilibrium?

Answer 24

The forward and backwards reactions continue at equal rates so the concentrations of reactants and products do not change. On a molecular scale there is constant change, but on the macroscopic scale nothing appears to be happening.

Required conditions:

1) must be a reversible reaction
2) must be in a closed container

Question 25

What is La Chatelier’s principle?

Answer 25

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

Question 26

Factors that affect the position of equilibrium?

Answer 26

1) pressure
2) concentration
3) temperature

Question 27

La chatelier’s principle with a change in concentration

Answer 27

If you increase the concentration of the reactants, the equilibrium will shift to the right
If you decreases the concentration of the reactants, the equilibrium will shift to the left

If a product is removed the equilibrium would shift to the right. If the product was continually removed the reaction would eventually turn into an irreversible reaction.

Question 28

La chatelier’s principle with a change in pressure

Answer 28

Only applies to reactions involving gases

If pressure is increased the equilibrium will move in the direction on the least molecules to try to reduce the pressure.

If pressure is decrease the equilibrium will move in the direction on the most molecules to try to increase the pressure.

If there are the same number of molecules on each side, changing the pressure would have no effect on the position of equilibrium

Question 29

La chatelier’s principle with a change in temperature

Answer 29

Depends if the forward reaction is exothermic to endothermic

If temperature is increased the equilibrium will try to cool down. The equilibrium will move towards the endothermic reaction.

If temperature is decreased the equilibrium will try to heat up. The equilibrium will move towards the exothermic reaction.

Question 30

Catalysts and equilibrium

Answer 30

They have no influence on the position of equilibrium but help it to be reached quicker

Question 31

Industry and la Chateliers principle

Answer 31

Important as can a determine conditions for highest yield but need to consider:
Low temperature and pressure means slow rate of reaction
High temperature and pressure means expensive to run and build suitable equipment
So most use a compromise set of conditions

Question 32

What is the equilibrium constant?

Answer 32

Kc
Different for every reaction
The size of Kc shows how far a chemical reaction has gone. Indicates the position of equilibrium

Question 33

What does Kc value tell us?

Answer 33

If >1 the reaction is product favoured so equilibrium lies to the right
If <1 the reaction is reactant favoured so equilibrium lies to the right

When Kc is 10^10 they are regarded as gone to completion, so no longer reversible
When Kc is 10^-10 the reaction is regarded as not taking place

Question 34

How to work out Kc?

Answer 34

aA + bB cC + dD

Kc = [C]^c [D]^d
——————-
[A]^a [B]^b

So equilibrium concentration of products divided by reactants, raised to the power of their stoichiometry

Question 35

How to work out Kc questions

Answer 35

Find out the initial amounts, changes and therefore equilibrium moles

Find concentrations, so moles/volume

Use Kc expression