Topic 8: Chemical Energetics

Question 1

What are the 2 components that make up chemical energy

Answer 1

Kinetic energy- measure of the motion of the particles in a substance
Potential energy- measure of how strongly the particles interact (attract and repel)

Question 2

What is heat energy

Answer 2

The portion of potential energy and kinetic energy of a substance that is responsible for the temperature of the substance

Question 3

What is enthalpy

Answer 3

The measure of the total energy of a system

Question 4

Define exothermic

Answer 4

Heat energy is transferred from the system to the surroundings

Question 5

Define endothermic

Answer 5

Heat energy is transferred from the surroundings to the system

Question 6

Define the standard enthalpy change of reaction

Answer 6

Enthalpy change measured at 100 kPa and a stated temp (usually 298K) when the number of moles of substances in the equation react 

Question 7

Define the enthalpy change of combustion (ΔcH⦵)

Answer 7

Change that accompanies the complete combustion of one mole of a substance under standard
conditions in their standard state

Question 8

Explain how you would find the enthalpy change of combustion of a liquid

Answer 8

• The spirit burner containing the liquid and the test is weighed
• A known vol of water is added to a copper can
• The temperature of the water is measured
• The burner is lit
• The mixture is constantly stirred with the thermometer
• When the temp of the water has reached approx 20° above its initial temp the flame is extinguished and the burner is immediately reweighed
• The final temperature is measured

Question 9

Give the equation for the calculation of enthalpy change

Answer 9

q = mCΔT q : heat energy (J)
m: mass (g)
c : specific heat capacity (4.18 unless told otherwise)
ΔT: temp change

Question 10

What are some of the possible sources of error in the reaction measuring the enthalpy change of combustion of a liquid

Answer 10

• Some of the heat energy produced and burning is transferred to the air and not the water
• Some of the heat energy produced in burning is transferred to the copper cannot the water
• The conditions are not standard e.g. water vapour not liquid water is produced

Question 11

Define standard enthalpy change of neutralisation ΔneutHθ

Answer 11

The enthalpy change that accompanies the production of 1 mole of water produced by the neutralisation of an acid with an alkali under standard conditions in their standard states

Question 12

Describe the steps in a reaction you would take to find out the enthalpy change if neutralisation

Answer 12

Pipette 25 cm³ of 1moldm3 acid into a polystyrene cup
Measure the temperature of the acid
Pipette 25 cm³ of the alkali of a concentration slightly greater than 1moldm3 (ensures all the acid is neutralised) into a beaker
Measure the temperature of the alkali
Add the alkali to the acid
Stir with thermometer & measure the maximum temperature reached 

Question 13

Define the standard enthalpy change of formation △fHθ

Answer 13

The enthalpy change that accompanies the formation of one mole of a substance from its elements in their standard states and conditions

Question 14

Define Hess’ law

Answer 14

The enthalpy change for a chemical reaction that is independent of the route taken

Question 15

Define bond enthalpy

Answer 15

Change when one mole of a bond in the gaseous state is broken 

Question 16

Define mean bond enthalpy 

Answer 16

The enthalpy change when one mole of a bond, averaged out over many different molecules, is broken

Question 17

What is the equation to calculate the enthalpy change of reaction using mean bond enthalpies

Answer 17

△rH = Σ(bonds broken) - Σ(bonds made)

When calculating enthalpy change of combustion only use the fuel for bonds broken

Question 18

Why does a calculation of enthalpy change for a reaction from mean bond enthalpies often not equal the same value as the data book value

Answer 18

Use of mean bond enthalpy values rather than actual values for molecules involved
Substances are in the wrong state for bond enthalpy calculations

Question 19

What type of Hess’ cycle should you draw and what does it look like if you are given combustion data

Answer 19

Use a combustion cycle :

Reactants ———-> Products

Arrows pointing downwards

. CO2 + H2O

Question 20

What type of Hess’ cycle should you draw and what does it look like if you are given formation data data

Answer 20

Use a formation cycle

Reactants —————> Products

. Arrows pointing upwards

. Elements