Module 5: Energy

Question 1

Define lattice enthalpy.
Give an example using NaCl (s)
Why is it exothermic?

Answer 1

The enthalpy change when 1 mol of an ionic lattice is formed from it constituent gaseous ions under standard conditions.
Na+ (g) + Cl- (g) —> NaCl(s)
It is highly exothermic as there are electrostatic bonds being formed throughout the whole ionic lattice.

Question 2

Is bond breaking exothermic or endothermic?

Answer 2

Endothermic

To break bonds, energy must be absorbed

Question 3

Is bond making exothermic or endothermic?

Answer 3

Exothermic.

Energy is released into the surroundings.

Question 4

Define enthalpy of formation of an ionic solid.
Give an example reaction, using NaCl (s)
Is it exothermic or endothermic?

Answer 4

Enthalpy change when 1 mol of a product is formed from its constituent elements under standard conditions.
Na (s) + 1/2Cl2 —> NaCl
Exothermic-bonds are being formed.

Question 5

Define enthalpy of atomisation.
Give an example reaction using Na (g)
Is it exothermic or endothermic?

Answer 5

Enthalpy change when 1 mol of gaseous atoms is formed from its constituent elements, under standard conditions. Na (s) —> Na (g)
Endothermic - bonds are broken in the solid.

Question 6

Define 1st ionisation energy.
Give an example reaction using Na (g)
Is it exothermic or endothermic.

Answer 6

Enthalpy change when 1 mol of gaseous 1+ ions is formed from its gaseous atoms, under standard conditions.
Na (g) —> Na+ + e-
Endothermic - electrostatic bonds are broken.

Question 7

Define 1st Electron Affinity
Give an example reaction using Cl (g)
Is it exothermic or endothermic.

Answer 7

Enthalpy change when 1 mol of gaseous 1- ions are formed from 1 mol of gaseous atoms.
Cl (g) + e- —> Cl- (g)
Exothermic - bonds are forming between the Cl atom and electron.

Question 8

Why is the 2nd electron affinity endothermic?

Give an example reaction using O (g)

Answer 8

You are trying to force a negative electron onto an already negative ion. They will try to repel eachother, and energy is needed to overcome the repulsive force.
O- (g) + e- —> O^2-

Question 9

How do you calculate Lattice enthalpy on a Born Haber Cycle?

Answer 9

Sum of clockwise enthalpy changes
=
Sum of the anticlockwise enthalpy changes.

Question 10

What 2 factors affect lattice enthalpy?

Answer 10

1. Charge of the ion.
   Higher charge = higher attraction between ions, and stronger electrostatic bonds.
2. Size of the ion
   Smaller ions = closer together and stronger electrostatic bonds.

Question 11

Define enthalpy of hydration.

Give an example reaction using K+ (g)

Answer 11

Enthalpy change when 1 mol of aqueous ions is formed from 1 mol of gaseous ions.
K+ (g) + (aq) —> K+(aq)

Question 12

Why do reactions favour exothermic?

Answer 12

Exothermic reactions are more favoured as they are more stable.

Question 13

What is entropy?

Answer 13

Entropy is a measure of disorder in a system.

Question 14

What does a positive entropy value suggest?

Answer 14

There is more disorder created.

Reaction is more likely to occur.

Question 15

What does a negative entropy value suggest?

Answer 15

Disorder has decreased (become more ordered).

Reaction is less likely to occur.

Question 16

Give 3 examples of when there may be a positive entropy value.

Answer 16

1. Solid —> Liquid (Ice melting)
2. Produce a gas.
3. Separate an ion into separate disordered ions.
   E.g = NaCl(s) —> Na+(aq) + Cl-(aq)

Question 17

Give 2 examples of when there may be a negative entropy value.

Answer 17

1. No. of mols decreases from left to right.

2. Liquid —> solid (Freezing of water)

Question 18

How do you calculate ΔS?

Answer 18

ΔS = ∑S products - ∑S reactants

Question 19

Give the equation linking free energy, entropy and ΔH.

Answer 19

ΔG = ΔH - TΔS
Temperature must be in K
ΔS must be in KJ.

Question 20

Based on ΔG, when is a reaction feasible?

Answer 20

If ΔG = 0 OR -ve, reaction is feasible at that temperature.

If ΔG = +ve, reaction is not feasible at that temperature.

Question 21

How do you convert from celsius to kelvin?

Answer 21

0 degrees = 273K

Question 22

Why may a reaction not take place, even if ΔG is feasible (less than or equal to zero)

Answer 22

1. Could have a high activation energy.

2. Rate of reaction may be too slow.