Chapter 9 - Enthalpy

Question 1

What is the law of the conservation of energy?

Answer 1

Energy cannot be created or destroyed, just transferred from one form to another.

Question 2

Define enthalpy

Answer 2

The heat content that is stored in a chemical system.

Question 3

Define enthalpy change

Answer 3

The difference between the enthalpy of the products and the reactants.

Question 4

Define exothermic

Answer 4

A chemical system where energy is transferred from the system to the surrounding

Question 5

Define endothermic

Answer 5

A chemical system where energy is transferred to the system from the surrounding

Question 6

Define activation energy

Answer 6

The minimum energy required for a reaction to take place.

Question 7

What are the standard conditions?

Answer 7

Standard pressure - 101kPa
Standard temperature - 298K
Standard concentration - 1mol/dm^3
Standard state - The physical state of a substance under standard conditions

Question 8

Define standard enthalpy change of formation

Answer 8

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

Question 9

Define standard enthalpy change of combustion

Answer 9

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 10

What is the equation used to measure an energy change?

Answer 10

Q=mcΔT
Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)

Question 11

How is enthalpy change worked out from the energy change?

Answer 11

ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)

Question 12

Define standard enthalpy change of neutralisation

Answer 12

The enthalpy change when an acid and an alkali react together under standard conditions give one mole of water(H20) , with all reactants and products in their standard states.

Question 13

What are the causes for less energy being transferred than expected when working out ΔH(c)

Answer 13

Heat loss to the surroundings
Incomplete combustion of ‘alcohol’
Evaporation of ‘alcohol’ from the wick
Non-standard conditions

Question 14

How can heat loss be accounted for using a graph of temperature against time?

Answer 14

Extrapolate the cooling curve back to when it was added.

Question 15

Enthalpy change of neutralisation always has the same value, what is it?

Answer 15

-57.5 kJ/mol

Question 16

Define average bond enthalpy

Answer 16

The energy required to break one mole of a specified type of bond in a gaseous molecule.

Question 17

What are some properties of bond enthalpies?

Answer 17

Energy is always required to break bonds
Bond enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value

Question 18

What kind of energy and enthalpy changes are bond formation?

Answer 18

Exothermic and releases energy

Question 19

Define the specific heat capacity

Answer 19

is the energy required to raise the temperature of 1g of a substance by 1K

Question 20

Determination of an enthalpy change of combustion:

Describe the experiment for spirit burner

Answer 20

1) using a measuring cylinder, measure out 150cm^3 of water. Then pour it into a beaker and record the initial temperature to the nearest 0.5C.
2) add methanol to the spirit burner. Then weigh it.
3) place the spirit burner under the beaker. Then light it. Then stir the water whilst burning with a thermometer.
4) after 3 minutes, extinguish the flame. Immediately, record the maximum temperature reached by the water.
5) re weigh the spirit burner containing the methanol. Assume that the wick has not been burnt

Question 21

what is endothermic in terms of bonds

Answer 21

where energy is required to break bonds

Question 22

what is exothermic in terms of bonds

Answer 22

where energy is released when bonds are formed

Question 23

Formula for calculating enthalpy changes from average bond enthalpies

Answer 23

Delta r H = the sum of bond enthalpies in reactants - the sum of bond enthalpies in products

Question 24

The enthalpy change for any element in its standard state is

Answer 24

0

Question 25

Limitation of calculating the enthalpy change of reaction from average bond enthalpies

Answer 25

As you are using average bond enthalpies, the actual energy involved in breaking and making bonds would be slightly different.

Also calculations need all species to be in gaseous molecules. For example if ur product produces H20(l) then you it would have to be H20(g). This means your calculated enthalpy change of reaction is not a standard enthalpy change

Question 26

Determination of an enthalpy change of reaction, the experiment.

Answer 26

• Determined by using plastic cups made of polystyrene foam (which are cheap,waterproof and light weight), which offer some insulation against heat lost to the surroundings.
• Record temperature change using thermometer
• Heat energy calculated using mcΔT

Question 27

Define the standard enthalpy change of reaction?

Answer 27

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

Question 28

Methods for reducing heat loss in the determination of an enthalpy change of a reaction?

Answer 28

Adding a lid to the polystyrene cup

Adding insulation around the polystyrene cup

Question 29

Methods for reducing heat loss in the determination of an enthalpy change of combustion?

Answer 29

Adding a lid to the beaker

Using draft shields around the apparatus