Module 5: Lattice Enthalpy IPS (5.2.1)

Question 1

Measuring Enthalpy Change of Solution

Answer 1

1. Weigh sample of KCl
2. Pour 25cm³ of distilled water into plastic cup in a beaker
3. Measure °C of water to nearest 0.5°C
4. Tip all KCl into water and stir with thermometer until dissolved and °C no longer changes
5. Record value to nearest 0.5°C

Question 2

Calculation

Answer 2

1. Mass of solution = Mass of water + mass of KCl
2. q=mcΔT (c=4.18)
3. Divide energy by 1000 to find kJ
4. Moles of KCl dissolved
5. Energy divided by moles

Question 3

Factors Affecting Lattice Enthalpy

Answer 3

• Ionic Size
1. Radius increases so the attraction between ions decreases.
2. Lattice energy less negative and melting point decreases.
• Ionic Charge
1. Charge increases so there is greater attraction between ions.
2. Lattice energy more negative but melting point increases.

Question 4

Factors Affecting Hydration

Answer 4

• Ionic Size
1. Radius increases so attraction between ion and H₂O decreases.
2. Hydration energy less negative.
• Ionic Charge
1. Charge increases so attraction with H₂O increases.
2. Hydration energy more negative.