Chapter 9 Enthalpy

Question 1

Enthalpy

Answer 1

A measure of the heat energy in a chemical system

Question 2

Chemical system

Answer 2

Refers to the atoms, molecules or ions making up the chemicals

Question 3

Enthalpy change

Answer 3

The difference in the enthalpies of the reactants and products

Question 4

What happens when a chemical reaction involving an enthalpy change takes place?

Answer 4

heat energy is transferred between the system and the surroundings

Question 5

What makes up a system?

Answer 5

the chemicals - the reactants and products

Question 6

What makes up the surroundings

Answer 6

The apparatus, laboratory, and everything that is not in the chemical system

Question 7

What makes up the universe

Answer 7

Both the system and surroundings

Question 8

Directions of energy transfer

Answer 8

• Exothermic change
• Endothermic change

Question 9

Exothermic change

Answer 9

Energy is transferred to the surroundings

Question 10

Endothermic change

Answer 10

Energy is transferred to the system

Question 11

Enthalpy profile diagrams

Answer 11

Shows the:
* relative enthalpies of the reactants and products
* the enthalpy change

Question 12

Exothermic reaction

Answer 12

• The chemical system releases heat energy to the surroundings
• Any energy loss by the chemical system is balanced by the same energy gain by the surroundings (the temperature of the surroundings increases as they gain energy)
• Delta H = negative

Question 13

Endothermic reaction

Answer 13

• The chemical system takes in heat energy from the surroundings
• Any energy gain by the chemical system is balances by the same energy loss by the surroundings (the temperature of the surroundings decreases as they lose energy)

Question 14

Activation energy

Answer 14

• The energy input required to break bonds in the reactants acts as an energy barrier to the reaction
• The minimum energy required for a reaction to take place

Question 15

Standard conditions (⦵)

Answer 15

• Standard pressure = 101 kPa
• Standard temperature = 298K
• Standard concentration = 1 moldm-3
• Standard state = physical state of a substance under standard conditions

Question 16

Standard enthalpy change of reaction

Answer 16

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

Question 17

Standard enthalpy change of formation

Answer 17

The enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 18

Standard enthalpy change of formation for elements

Answer 18

0

Question 19

Standard enthalpy change of combustion

Answer 19

The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard conditions

Question 20

Standard enthalpy change of neutralisation

Answer 20

The energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O, under standard conditions with all reactants and products in their standard states

Question 21

Reaction of neutralisation

Answer 21

Involves the reaction of H+ with OH- to form 1 mole of H2O

Question 22

Symbol for heat energy

Answer 22

q

Question 23

Equation for heat energy

Answer 23

q = mc△T

• m = mass of surroundings
  c = specific heat capacity

Question 24

Specific heat capacity, c

Answer 24

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

Question 25

Why isn’t enthalpy of combustion calculated with spirit burners accurate?

Answer 25

• Heat loss to the surroundings, other than water (e.g. beaker, air)
• Incomplete combustion of methanol (carbon monoxide and carbon produced on the beaker)
• Evaporation of methanol from wick
• Non-standard conditions

Question 26

Experimentally determining the enthalpy of combustion

Answer 26

1. Put a known volume of water in a beaker and record initial temp. of water
2. Weigh a spirit burner containing methanol
3. Light the spirit burner and use it to heat the water and use a thermometer to stir the water in the beaker
4. After 3 minutes extinguish the flame by putting the lid on the wick
5. Re-weigh the spirit burner at the end

Question 27

Experimentally determining the enthalpy change of reaction

Answer 27

Mix the solid + solution or solution + solution in a polystyrene cups (cheap and offer insulation)

Question 28

Cooling curves

Answer 28

x-axis: time
y-axis: temperature

When mixing solutions in the polystrene cup, record the temperature of the solution every 30 seconds

A cooling curve can be used to find the highest temperature that the solution reached, in order to find the temp. difference

Extrapolate the cooling curve section of the graph back to the time when zinc was added

Question 29

Average bond enthalpy

Answer 29

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

Question 30

Limitations of average bond enthalpies

Answer 30

It is calculated from the actual bond enthalpies in different environments -> not under standard conditions

Question 31

Bond breaking

Answer 31

Energy is required to break bonds so is endothermic

Question 32

Bond making

Answer 32

Energy is released when bonds are made so is exothermic

Question 33

Calculating enthalpy changes from average bond enthalpies

Answer 33

• Bond breaking - bond making
• Bond enthalpies in reactants - bond enthalpies in products

Question 34

Why is Hess’ law useful?

Answer 34

The enthalpy changes of many reactions are very difficult to determine directly

Question 35

Hess’ Law

Answer 35

If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route