Module 3 - enthalpy Changes

Question 1

What is enthalpy (H)?

Answer 1

Enthalpy is the total heat content of a system at constant pressure.

Question 2

What is enthalpy change (ΔH)?

Answer 2

The heat energy transferred in a reaction at constant pressure, measured in kJ mol⁻¹.

Question 3

What are the standard conditions for measuring enthalpy changes?

Answer 3

• 100 kPa pressure
• 298 K (25°C) temperature
• Solutions at 1.0 mol dm⁻³ concentration

Question 4

What is an exothermic reaction?

Answer 4

• A reaction that releases energy to the surroundings.
• ΔH is negative (-)

Question 5

What is an endothermic reaction?

Answer 5

• A reaction that absorbs energy from the surroundings.
• ΔH is positive (+)

Question 6

Define standard enthalpy change of reaction (ΔH⁰r)

Answer 6

The enthalpy change when a reaction occurs in molar quantities as written in the equation, under standard conditions.

Question 7

Define standard enthalpy change of formation (ΔH⁰f)

Answer 7

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 8

Define standard enthalpy change of combustion (ΔH⁰c)

Answer 8

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.

Question 9

Define standard enthalpy change of neutralisation (ΔH⁰neut)

Answer 9

The enthalpy change when an acid and a base react to form 1 mole of water, under standard conditions.

Question 10

What is the equation used to calculate enthalpy change using heat energy?

Answer 10

• q = mcΔT
• q = heat energy (J)
• m = mass of substance (g)
• c = specific heat capacity (J g⁻¹ K⁻¹, usually 4.18 for water)
• ΔT = temperature change (K or °C)

Question 11

How do you calculate enthalpy change per mole (ΔH)?

Answer 11

• ΔH = q / n
• q = heat energy (kJ)
• n = moles of substance reacting

Question 12

What is Hess’s Law?

Answer 12

The total enthalpy change of a reaction is the same, regardless of the route taken.

Question 13

What is the bond enthalpy?

Answer 13

The energy required to break 1 mole of a bond in gaseous molecules.

Question 14

How do you calculate enthalpy change using bond enthalpies?

Answer 14

ΔH = ∑(bond breaking) - ∑(bond making)

Question 15

What is the mean bond enthalpy?

Answer 15

• The average bond enthalpy of a particular bond in different compounds.
• used because bond enthalpies vary in different environments

Question 16

What is the equation for heat energy change?

Answer 16

q = mcΔT

• q = heat energy (J)
• m = mass of substance (g)
• c = specific heat capacity (J g⁻¹ K⁻¹)
• ΔT = temperature change (K)

Question 17

What is the activation energy (Ea)?

Answer 17

The minimum energy required for a reaction to occur

Question 18

Exothermic reaction profile

Answer 18

Reactants have more energy than products

Question 19

Endothermic reaction profile

Answer 19

Products have more energy than reactants

Question 20

Catalyst effect on reaction profile

Answer 20

Lowers activation energy without affecting ΔH

Question 21

Calorimeter experiment (measuring enthalpy changes)

Answer 21

Used to determine enthalpy changes by measuring temp. Changes In a known mass of substance (usually water)

Question 22

Explain the calorimeter experiment:
Measuring combustion enthalpy (using a spirit burner)

Answer 22

• method used to determine enthalpy of combustion (ΔH_c°).
• Experimental Procedure:

1) Measure, Record Initial Mass of Spirit Burner
→ The fuel (e.g., ethanol, methanol) is in the burner.
2) Add Fixed Volume of Water to Calorimeter
→ copper calorimeter (metal container) often used, conducts heat well.
3) Record Initial Temperature of Water
4) Light the Spirit Burner and Stir Continuously
→ Ensures heat distributed evenly in water.
5) Extinguish the Flame After a Measured Time/Temperature Rise
6) Record Final Water Temperature
7) Measure Final Mass of Spirit Burner
→ difference gives mass of fuel burned.
8) Use Formula q = mcΔT
→ Calculate heat energy (q) absorbed by water.
→ Then use ΔH = q / n to find enthalpy change per mole of fuel burned.

Question 23

Explain the calorimeter experiment:
b) Measuring Neutralisation Enthalpy (Polystyrene Cup Method)

Answer 23

-This method is used to determine enthalpy of neutralisation (ΔH_neut°).
Experimental Procedure:

1) Measure Fixed Volumes of Acid and Alkali Using a Pipette
→ Typically 25 cm³ of 1 mol dm⁻³ HCl and 1 mol dm⁻³ NaOH.
2) Place Solutions in Polystyrene Cup (Calorimeter)
→ lid used reduce heat loss.
3) Measure and Record the Initial Temperature
→ Use digital thermometer for accuracy.
4) Mix Acid and Alkali and Stir Continuously
5) Record Maximum Temperature Reached
→ Important to record immediately, avoid cooling effects.
6) Use the Formula q = mcΔT
→ Calculate heat released.
→ Then use ΔH = q / n (where n is the moles of water formed).

Question 24

Explain the calorimeter experiment:
c) Measuring Enthalpy of Solution

Answer 24

This method determines enthalpy of solution (ΔH_sol°), which is the enthalpy change when 1 mole of a substance dissolves in water.