Chapter 22- Enthalpy and entropy

Question 1

What is lattice enthalpy?

Answer 1

a. Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
b. Lattice enthalpy involves ionic bond formation from separate ions. It is an exothermic change and the value for the enthalpy change will always be negative.

Question 2

What is the standard enthalpy change of formation?

Answer 2

The standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants and products in their standard states.

Question 3

What is the standard enthalpy change of atomisation?

Answer 3

The standard enthalpy change of atomisation is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the elements in its standard states under standard conditions.

Question 4

What is the first ionisation energy?

Answer 4

The first ionisation energy is the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Question 5

What is the first electron affinity?

Answer 5

The first electron affinity is the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Question 6

Why are second electron affinities endothermic?

Answer 6

Second electron affinities are endothermic because a second electron is being gained by a negative ion, which repels the electron away. So, energy must be put in to force the negatively charged electron onto the negative ion.

Question 7

What is the standard enthalpy change of solution?

Answer 7

a. The standard enthalpy change of solution is the enthalpy change that takes place when one mole of a solute dissolves in a solvent.
b. If the solvent is water, the ions from the ionic lattice finish up surrounded with water molecules as aqueous ions.

Question 8

What mass is used in enthalpy calculations?

Answer 8

a. When using q=mcΔT to calculate the energy change, m is the mass that is changing temperature (where the thermometer is). The thermometer is in the solution that is changing temperature and so the mass should be the mass of the solution and not the mass of water.
b. Whenever you carry out an enthalpy experiment, you should consider what is changing temperature before using q=mcΔT.

Question 9

When a solid ionic lattice dissolves in water what two processes take place?

Answer 9

a. The ionic lattice is broken up forming separate gaseous ions.
i. This is the opposite energy change from lattice energy, which forms the ionic lattice from gaseous ions.

b. The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. The energy change involved is the enthalpy change of hydration.

Question 10

What is the enthalpy change of hydration?

Answer 10

The enthalpy change of hydration is the enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Question 11

How are cold packs made and how do they work?

Answer 11

a. Cold packs are used for sporting injuries to reduce swelling to muscles and sprains.
b. They are made of a pouch containing two compartments – one containing a chemical, the other water.
c. The cold pack is activated by squeezing the pack to break the seal between the two compartments. The pack is then shaken vigorously. The chemical dissolves in the water, cooling the pack and the area around the injury.
d. Various chemicals can be used, the most common being the ionic compound ammonium nitrate, NH4NO3.

Question 12

What general properties do ionic compounds tend to have?

Answer 12

a. High melting and boiling points.
b. Soluble in polar solvents.
c. Conduct electricity when molten or in aqueous solution.

Question 13

What is the effect of ionic size on lattice enthalpy?

Answer 13

a. As the ionic radius increases
b. attraction between ions decreases
c. lattice energy is less negative
d. melting point decreases
e. Therefore, lattice enthalpy decreases down the group.
f. Across period 3, the ionic size of the cations Na+, Mg2+, and Al3+ decreases as more protons attracting the same number of electrons.

Question 14

What is the effect of ionic charge on lattice enthalpy?

Answer 14

a. As the ionic charge increases
b. Attraction between ions increases
c. Lattice energy becomes more negative
d. Melting point increases

Question 15

How does lattice enthalpies relate to melting point?

Answer 15

The more negative the lattice enthalpy is, the greater the melting point of the ionic compound.

Question 16

What are ionic liquids?

Answer 16

Ionic compounds that are liquid at room temperature are known as ionic liquids.

Question 17

What is the effect of ionic size on the hydration enthalpy?

Answer 17

a. As the ionic radius increases
b. the attraction between ions and water molecules decreases
c. Therefore, the hydration energy is less negative.

Question 18

What is the effect of ionic charge on hydration enthalpy?

Answer 18

a. As the ionic charge increases
b. the attraction with water molecules increases
c. Therefore, the hydration energy becomes more negative.

Question 19

What attractions need to be overcome when dissolving an ionic compound in water?

Answer 19

To dissolve an ionic compound in water, the attraction between the ions in the ionic lattice must be overcome. This requires a quantity of energy equal to the lattice enthalpy. Water molecules are attracted to the positive and negative ions, surrounding them and releasing energy equal to hydration enthalpy.

Question 20

If the sum of the hydration enthalpies is greater than the magnitude of the lattice enthalpy what does it mean?

Answer 20

a. If the sum of the hydration enthalpies is greater than the magnitude of the lattice enthalpy, the overall enthalpy change (the enthalpy change of solution) will be exothermic and the compound should dissolve.
b. However, many compounds with endothermic enthalpy changes of solution are soluble so this does not provide the total picture.

Question 21

What does the structure of an ionic liquid entail?

Answer 21

In an ionic liquid, the ions can have irregular shapes and can be very different sizes. This means that they are unable to pack together into a lattice and can be liquids at room temperature.

Question 22

What are the key properties of an ionic liquid?

Answer 22

a. Ionic liquids conduct electricity
b. Do not vaporise easily
c. Can dissolve an amazing range of substances.

Question 23

How can ionic liquids be synthesised?

Answer 23

Ionic liquids can be made from biomass and are seen as potential green solvents in the future to replace oil-derived solvents.

Question 24

What is meant by the term entropy?

Answer 24

a. Entropy is the dispersal of energy within the chemicals making up the chemical system.
b. The greater the entropy value, the greater that energy is spread out per kelvin per mole.
c. The units of entropy are JK-1mol-1.

Question 25

What is the standard entropy of a substance?

Answer 25

a. The standard entropy of a substance is the entropy of one mole of a substance, under standard conditions (100kPa and 298K)
b. Standard entropies have units of JK-1mol-1.
c. Standard entropies are always positive.

Question 26

When is a reaction feasible and what does the term feasibility refer to?

Answer 26

a. A reaction can happen if the products have a lower overall energy than the reactants. In other words, ΔG < 0.
b. The term feasibility is used to describe whether a reaction is able to happen and is energetically feasible (spontaneous).

Question 27

What is meant by Gibbs’ free energy change and what is it made up of?

Answer 27

a. The overall change in energy during a chemical reaction is called the “free energy change”.

b. It is made up of two type of energy.
i. The enthalpy change ΔH. This is the heat transfer between the chemical system and the surroundings.

ii. The entropy change at the temperature of the reaction TΔS. This is the dispersal of energy within the chemical system itself.
c. ΔG = ΔH – TΔS. When calculating ΔG the value and units of ΔS must be changed to kJK-1mol-1 by dividing by 1000.

Question 28

What are the limitations for the predictions made for feasibility using free energy change?

Answer 28

a. Many reactions have negative ΔG and do not seem to take place.
b. This could be due to a very large activation energy resulting in a very slow rate. If the reaction is left for long enough, it would eventually take place.
c. It does not take into account kinetics or the rate of a reaction.