Energetics

Question 1

Define Enthalpy Change. (4)

Give the definition, sign and unit.

Answer 1

• Enthalpy change is the amount of heat energy that is either given out or taken in from the surroundings, during a reaction, at a constant pressure.
• The sign for enthalpy change is ΔH.
• It is measured in kJ/mol.

Question 2

Define Exothermic Reaction. (2)

Give the definition and explain.

Answer 2

• Heat energy is transferred/given out from the system to the surroundings.
• Enthalpy change is negative.
• (e.g. combustion of methane = -890kJ/mol)

Question 3

Define Endothermic Reaction. (2)

Give the definition and explain.

Answer 3

• Heat energy is transferred/taken in from the surroundings into the system.
• Enthalpy change is positive.
• (e.g. thermal decomposition of CaCO3 = +890kJ/mol)

Question 4

Define Standard Enthalpy Change of Formation. (3)

Give the basic definition and an example.

Answer 4

• ΔH’f: The enthalpy change when one mole of a compound is formed from all its’ constituent elements, under standard conditions and with all reactants and products in their standard states.
• ( e.g. C (s) + 2H2 (g) —> CH4 (g) )

Question 5

Define Standard Enthalpy Change of Combustion. (2)

Give the basic definition and an example.

Answer 5

• ΔH’c: The enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions.
• ( e.g. CH4 + 2O2 —> CO2 + 2H2O )

Question 6

Define Standard Enthalpy Change of Atomisation. (2)

Give the basic definition and an example.

Answer 6

• ΔH’at: The enthalpy change when 1 mole of gaseous atoms are formed from the element in its’ standard state under standard conditions.
• ( e.g. Na (s) —> Na (g) , or 1/2Cl2 (g) —> Cl (g) )

Question 7

What is enthalpy (H)?

Answer 7

The thermal energy stored in a system.

Question 8

What are standard conditions? - Enthalpy

Answer 8

298 K and 100 kPa

Question 9

What is an exothermic reaction?

Answer 9

A reaction that releases heat to the surroundings (ΔH negative)

Question 10

What is an endothermic reaction?

Answer 10

A reaction that absorbs heat from the surroundings (ΔH positive)

Question 11

What is activation energy?

Answer 11

The minimum energy required for a reaction to occur

Question 12

What is bond enthalpy?

Answer 12

Energy required to break 1 mole of bonds in gaseous molecules

Question 13

What is mean bond enthalpy?

Answer 13

Average bond enthalpy across different compounds

Question 14

What is Hess’ Law?

Answer 14

The total enthalpy change is independent of the route taken.

Question 15

Bond enthalpy equation

Answer 15

ΔH = energy to break bonds − energy released forming bonds

Question 16

What is calorimetry?

Answer 16

Measuring heat energy changes in reactions

Question 17

Calorimetry equation?

Answer 17

q = mcΔT

Question 18

Why are experimental ΔH values inaccurate?

Answer 18

• Heat loss to surroundings
• Incomplete combustion
• Evaporation
• Non-standard conditions