Enthalpy

Question 1

What is the standard lattice enthalpy, ΔH°latt?

Answer 1

The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions (298K, 101KPa, 1 mold dm-3). Its a measure of ionic bond strength.

Question 2

What is enthalpy change of neutralisation? Is it exothermic or endo thermic?

Why is the enthalpy change of neutralisation for strong acids and alkalis?

Answer 2

The enthalpy change when 1 mole of water is formed by the reaction between an acid and a base under standard conditions. It’s an exothermic process.

For reactions involving strong acids and alkalis, the values are always very closely similar, with values between -57 and -58 kJ mol-1.

Question 3

How do you calculate ΔH?

Answer 3

ΔH = -mcΔT

Question 4

How do you calculate energy change, q?

Answer 4

q = mcΔT

Question 5

What does exothermic mean?

Answer 5

Energy is released during the reaction, it is negative ie -ΔH.

Question 6

What does endothermic mean?

Answer 6

Energy is absorbed, it is positive ie +ΔH.

Question 7

Why do strong acids reacting with strong alkalis give closely similar values of ΔHneutra…?

Answer 7

We make the assumption that strong acids and strong alkalis are fully ionised in solution, and that the ions behave independently of each other. The equation for any strong acid being neutralised by a strong alkali is essentially just a reaction between hydrogen ions and hydroxide ions to make water. The other ions present are just spectator ions, taking no part in the reaction.

Question 8

Why do weak acids or weak alkalis give different values of ΔHneutra…?

Answer 8

Weak bases and acids don’t fully dissociate. That means that the enthalpy change of neutralisation will include other enthalpy terms involved in ionising the acid as well as the reaction between the hydrogen ions and hydroxide ions.

Question 9

What is ionic bonding?

Answer 9

The electrostatic attraction between oppositely charged ions.

Question 10

What two factors affect lattice enthalpy?

Answer 10

The charges on the ions and ionic radii.

Question 11

What affect does the charge on the ion (the ionic charge) have on the lattice enthalpy?

Answer 11

The higher the charge on the ions, the more energy is released so the more negative (exothermic) the lattice enthalpy will be. E.g. the lattice enthalpy of magnesium oxide is much greater than that of sodium chloride. That’s because in magnesium oxide, 2+ ions are attracting 2- ions; in sodium chloride, the attraction is only between 1+ and 1- ions.

Question 12

What affect does the atomic radii have on the lattice enthalpy?

Answer 12

The smaller the ionic radii of the ions involved, the more exothermic (more negative) the lattice enthalpy. Smaller ions attract more strongly because their charge density is higher. Also, the ions are closer together in the lattice, and that increases the strength of the attractions.

Question 13

What is the atomisation enthalpy, ΔH°a?

Answer 13

The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state. Enthalpy change of atomisation is always positive. Unit is KJ mol-1.

Question 14

What does the born-haber cycle look like?

Answer 14

Question 15

What is the enthalpy change of solution?

Answer 15

The enthalpy change of solution is the enthalpy change when 1 mole of solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution.

Question 16

When a solid ionic lattice dissolves in water, what two things happen?

Answer 16

1. The bonds between ions break — this is endothermic. The enthalpy change is the opposite of the lattice enthalpy.
2. Bonds between the ions and water are made — this is exothermic. The enthalpy change here is the enthalpy change of hydration.
3. The enthalpy change of solution is the overall effect on the enthalpy of these two things.

Question 17

Generally, when will substances only dissolve?

Answer 17

When the energy released is roughly the same, or greater than the energy taken in.

Question 18

What is entropy change?

Answer 18

Entropy is the measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles.

Question 19

When is a reaction and state change more likely?

Answer 19

• It’s more likely when there is a positive entropy change.

Question 20

Dissolving normally causes an increase in entropy. However, for small, highly charged ions there may be a decrease. Why?

Answer 20

• For small, highly charged ions there may be a decrease because when water molecules surround the ions, it makes things more orderly.

Question 21

How can you calculate enthalpy change of solution?

Answer 21

• ΔH3 (appears as ΔH in diagram) = Enthalpy change of solution KJ mol-1
• ΔH1= Lattice enthalpy
• ΔH2 = Enthalpy of hydration

enthalpy of hydration = ΔH1 + ΔH2

Question 22

What 2 factors affect the enthalpy of hydration?

Answer 22

1. The charge on the ion
2. The atomic radii/ size of the ion

Question 23

How does the charge on the ion affect the enthalpy of hydration?

Answer 23

• Ions with a greater charge have a greater enthalpy of hydration.
• Ions with a higher charge are better at attracting water molecules than those with lower charges.
• More energy is released when bonds are made giving them a more exothermic enthalpy of hydration.

Question 24

How does the size of an ion affect the enthalpy of hydration?

Answer 24

• Smaller ions have a greater enthalpy of hydration.
• Smaller ions have a higher charge density than bigger ions. They attract the water molecules better and have a more exothermic enthalpy of hydration.

Question 25

What affect does an ion with a high charge and small size have on the enthalpy of hydration?

Answer 25

A high charge and small size create a high charge density. This creates a stronger attraction for the water molecules and gives them a more exothermic enthalpy of hydration.

Question 26

Why do substances like disorder?

Answer 26

It makes them more energetically stable when there’s more disorder. So particles move to try and increase their entropy.

Question 27

What 3 factors affect entropy?

Answer 27

1. Physical state
2. Dissolving
3. More particles

Question 28

Hess’s Energy Cycle: How do you calculate enthalpy of combustion from enthalpy of formation?

Answer 28

∆Hf1 = ∆Hf2 + ∆Hc

∆Hc = ∆Hf1 - ∆Hf2

Question 29

Hess’s Energy Cycle: How do you calculate enthalpy of formation using enthalpy of combustion?

Answer 29

∆Hc1= ∆Hf + ∆Hc2

∆Hf = ∆Hc2 - ∆Hc1