✨Module 3: Enthalpy

Question 1

Exothermic enthalpy change.

Answer 1

Heat energy is transferred from system to surroundings. Products have less energy than the reactants. Temp of the surroundings increases. Delta H is negative.

Question 2

Examples of exothermic processes?

Answer 2

Combustion of fuels, neutralisation, respiration.

Question 3

Endothermic enthalpy change.

Answer 3

Heat energy taken into system from surroundings. Products have higher energy than reactants. Temp of surroundings decrease as they lose energy. Delta H is positive.

Question 4

Examples of endothermic processes?

Answer 4

Evaporation, photosynthesis, thermal decomposition.

Question 5

Delta H (enthalpy change) =

Answer 5

H(products) - H(reactants)

Question 6

Enthalpy cannot be measured but …

Answer 6

Enthalpy changes can.

Question 7

Define enthalpy.

Answer 7

Measure of heat energy in a chemical system.

Question 8

What does it mean when enthalpy change takes place?

Answer 8

Heat energy is transferred between the system and surroundings.

Question 9

What is activation energy?

Answer 9

Minimum energy particles need to collide with each other to start a reaction.

Question 10

Reactions with a small activation energy …

Answer 10

Take place very rapidly as the energy needed to break the bonds is readily available from surroundings.

Question 11

What are the standard conditions for enthalpy change?

Answer 11

100 kPa pressure
298 K (25oC)
Solutions at 1mol dm-3 conc

Question 12

Define enthalpy change of reaction (don’t need to remember).

Answer 12

Energy change when the number of moles of reactants as specified in the
balanced equation react together. If reactant moles are doubled, enthalpy also doubles (if it’s same equation).

Question 13

Define standard enthalpy change of formation.

Answer 13

Energy change when 1 mole of the compound is formed from its elements under STANDARD conditions, all reactants and products being in their STANDARD states.

Question 14

Give 2 equations to show standard enthalpy change of formation.

Answer 14

Mg (s) + Cl2 (g) = MgCl2 (s)
2Fe (s) + 1.5 O2 (g) = Fe2O3 (s)

The enthalpy of formation of an element is 0 kJ mol-1.

Question 15

Define standard enthalpy change of combustion. Give an example equation to show this.

Answer 15

Energy change when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products being in their standard states.
CH4 (g) + 2O2 (g) = CO2 (g) + 2 H2O (l)

Question 16

Incomplete combustion will lead to …

Answer 16

Soot (carbon), carbon monoxide and
water being produced. It will be less exothermic than complete combustion.

Question 17

Define standard enthalpy change of neutralisation.

Answer 17

Energy change when solutions of an
acid and base react under standard conditions to produce 1 mole of water, with all reactants + products in standard states.

Question 18

Give an equation showing standard enthalpy change of neutralisation.

Answer 18

HCL (aq) + NaOH (aq) = NaCl (aq) + H2O (l)

This enthalpy change value is same for all neutralisation reactions.

Question 19

Zero degrees Celcius is equivalent to …

Answer 19

273K

Question 20

Define specific heat capacity ‘c’.
What is the value of ‘c’ for water?

Answer 20

Energy required to raise the temp of 1g of substance by 1K.
For water, c = 4.18 Jg-1 K-1

Question 21

Is specific heat capacity a high or low value for good conductors of heat such as metal?

Answer 21

Small. But for insulators like plastic have large values of ‘c’.

Question 22

How to calculate energy change in solution? What are the units for each?

Answer 22

q = mc delta T

q in Joules
m in grams
delta T in K

Question 23

Errors in calorimetry?

Answer 23

Energy loss to surroundings.
Assumes all solutions have the same heat capacity of water.
Neglecting specific heat capacity of calorimeter - we ignore any energy absorbed by the apparatus.
Incomplete combustion of alcohol which formed soot.

Question 24

Define average bond enthalpy (don’t need to know). List some facts about bond enthalpies.

Answer 24

Energy required to break 1 mole of a specified type of bond in a gaseous molecule.
Bond enthalpies are always endothermic and have a positive enthalpy value.

Question 25

Energy is required to …

Answer 25

BREAK bonds so bond breaking is endothermic.
Delta H is positive.

Question 26

Energy is released when …

Answer 26

MAKING bonds so bond making is exothermic.
Delta H is negative.

Question 27

PAG 3.3: Enthalpy changes of combustion.

Answer 27

1. Measure 100cm3 of water into beaker.
2. Weigh spirit burner containing alcohol.
3. Make sure spirit burner is arranged in a way that can heat the water using a clamp.
4. Take temp of water before and after.
5. Heat water using spirit burner until above 15oC temp rise of water.
6. Stir water thoroughly and reweigh spirit burner.

Question 28

Suggest how the combustion/calorimetry experiment can be improved.

Answer 28

Have a balance close to you so reweigh alcohol quicker before alcohol evaporates off the wick.
Draught excluder to reduce heat loss to surroundings.
Keep stirring water to transfer heat evenly.
Good flow of oxygen to reduce incomplete combustion.

Question 29

Define bond enthalpy.

Answer 29

Tells us how much heat energy we need to break 1 mole of a bond.