3.2 physical chemistry

Question 1

What is chemical energy?

Answer 1

It is a special form of potential energy that lies within chemical bonds.

Question 2

Outline what happens when chemicals react in terms of energy.

Answer 2

When chemicals react together to form new substances bonds break in the reactants and new bonds are formed as products are made.
This process changes the chemical energy of the substance.

Question 3

Define enthalpy.

Answer 3

Thermal energy that is stored in a chemical system.

Question 4

How can we measure enthalpy of reactants/products?

Answer 4

It is impossible to directly measure their enthalpy. However, we can measure the energy absorbed or released to the surroundings during a chemical change.

Question 5

Define, exothermic reaction.

Answer 5

the enthalpy of the products is smaller than that of the reactants and there is heat loss to the surroundings.

Question 6

Define exothermic reaction in terms of bonds broken.l

Answer 6

More energy is released when forming bonds than was needed to break bonds.

Question 7

Define, endothermic reactions.

Answer 7

The enthalpy of the products is greater than that of reactant and there is a heat gain to the chemical system.

Question 8

Define endothermic reactions in terms of bonds.

Answer 8

The energy needed to break existing bonds is greater than energy released when forming bonds

Question 9

Define enthalpy change of reactions.

Answer 9

It is the enthalpy change associated with a given reaction under standard conditions.

Question 10

Define enthalpy change of formation.

Answer 10

Energy change that takes place when 1 mole of a compound is formed from it’s constituent elements in their standard state at standard conditions.

Question 11

Define enthalpy change of combustion.

Answer 11

Energy change that takes place when 1 mole of a substance is completely combusted in oxygen under standard conditions.

Question 12

Define enthalpy change of neutralisation.

Answer 12

Energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions.

Question 13

Define standard conditions.

Answer 13

pressure: 100KPa
temperature : 298k

Question 14

Define bond enthalpy.

Answer 14

How much energy is needed to break bonds

Question 15

Define average bond enthalpy.

Answer 15

Energy needed to break 1 mole of bonds in the gas phase, averaged over many different compounds.

Question 16

Why are average bond enthalpies used in calculations

Answer 16

Because the energy required to break an individual bond can change depending on where it is.

Question 17

How do you calculate enthalpy change?

Answer 17

Total energy absorbed - total energy released
(reactants - products)

Question 18

Describe a bomb calorimeter.

Answer 18

It contains many compartments:
- The outer layer is an air jacket
- Next is a water bath with a thermometer and stirrer
- Lastly is the sample with electrically heated wires and pure oxygen

Question 19

Why are bomb calorimeters so useful.

Answer 19

They are a sophisticated piece of equipment that minimises heat loss as much as possible.

Question 20

What equation can be used to calculate enthalpy change?

Answer 20

Q = MCT
Enthalpy change = mass * specific heat capacity * temperature change

Question 21

What is the specific heat capacity of water?

Answer 21

4.18Kj/mol

Question 22

What are the steps to calculating enthalpy change of combustion?

Answer 22

1- Calculate the amount of heat given out by the fuel using q=mct.
2- Find out how many moles of fuel produced this heat.
3- Divide Q by the moles.

Question 23

How does calculating enthalpy change of combustion differ to that of reaction?

Answer 23

For the enthalpy change of reaction you need to find the enthalpy change for the number of moles shown in the balanced chemical equation, instead of calculating per mole.

Question 24

What does Hess’ law state?

Answer 24

Hess’ law states that:
‘The total enthalpy change of a reaction is always the same no matter, which route is taken’

Question 25

how do you calculate rate of reaction?

Answer 25

rate of reaction = change in concentration / time

Question 26

define activation energy

Answer 26

minimum amount of energy needed in order to react

Question 27

How does temperature affect rate of reaction?

Answer 27

more kinetic energy so there are more collisions

More particles have the minimum activation energy so when particles collide they’re more succesful

Question 28

What is the affect of concentration on the rate of reaction

Question 29

What is the affect of concentration on the rate of reaction

Question 30

What is the affect of concentration on the rate of reaction

Answer 30

there are more molecules in the same volume so they will be close and there will be a greater chance of them colliding with sufficient energy to overcome activation energy

Question 31

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

Answer 31

When the pressure of a gas is increases the molecules are closer together as the same number of molecules occupy a smaller volume and more collisions are likely to occur with sufficient energy to overcome the activation energy

Question 32

What are the two types of catalysts

Answer 32

Homogenous
heterogenous

Question 33

Define homogenous catalyst

Answer 33

catalyst in the same phase as reactants
e.g. enzymes

Question 34

Define heterogenous catalyst

Answer 34

catalyst in a different phase to the reactants
e.g. iron in Haber process

Question 35

What is the Boltzmann distribution?

Answer 35

A graph depicting the change in number of particles with minimum activation energy at a higher temperature in comparison

Question 36

Define dynamic equilibrium

Answer 36

the concentration of reactants and products remains constant and the forward and backwards reactions are the same and there is no observable change

Question 37

What factors affect the position equilibrium

Answer 37

concentrations of reactants and products
pressure in reactions involving gases
temperature

Question 38

Define le Chateliers principle

Answer 38

when a system in dynamic equilibrium is subjected to a change the position of equilibrium will shift to minimise the change

Question 39

What is the effect of concentration on the position of equilibrium

Answer 39

increasing the concentration of reactants will cause the system to oppose the change by decreasing its concentration so the position of equilibrium will move to the right forming more products
same goes for the products

Question 40

What is the effect of pressure on the position of equilibrium

Answer 40

increasing the total pressure of the system causes the position of equilibrium to shift to the side with lower pressure i.e. fewer gas molecules

Question 41

What is the effect of temperature on equilibrium

Answer 41

Increasing the temperature causes the equilibrium to shift to the endothermic direction in order to decrease the temperature and take in heat

decreasing the temperature will cause the equilibrium to shift to the exothermic direction to oppose the change and release heat to increase the temperature

But too low of a temperature can cause the rate of a reaction to slow

Question 42

What is the effect of a catalyst on the position of equilibrium

Answer 42

It doesn’t alter the position of equilibrium but rather speed up the forward and reserve reactions equally and increases the rate at which equilibrium is reached

Question 43

What are the conditions of the Haber process

Answer 43

400-450c
200 atm
iron catalyst
N2 and H2 1:3 by volume

Question 44

What is the purpose of the equilibrium constant Kc

Answer 44

Chemists use it to consider the position of equilibrium and gives a measure of where the equilibrium lies

Question 45

What is the equation for Kc

Answer 45

product/reactants
[D]d [C]c/[A]a[B]b