22- enthalpy and entropy

Question 1

What are the 3 different processes that route 1 requires in a Born-haber cycle?

Answer 1

1. formation of gaseous atoms; changing elements in standard state to gaseous atoms, change is endothermic as it involves bond breaking.
2. formation of gaseous ions; changing the gaseous atoms to + and - gaseous ions, overall this change is endothermic.
3. Lattice formation; changing gaseous ions into the solid ionic lattice- lattice enthalpy -this is exothermic.

Question 1

What is lattice enthalpy?

Answer 1

The enthalpy change that accompanies the formation of one mole of ionic compound from its gaseous ions under standard conditions. Its an exothermic change and the value for enthalpy change will always be negative.

Question 2

What is route 2 of the Born-haber cycle?

Answer 2

Converting the elements in their standard states directly to the ionic lattice. There is just one enthalpy change, its the enthalpy change of formation and it is exothermic.

Question 3

What is the standard enthalpy change of formation?

Answer 3

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

Question 4

What is the standard enthalpy change of atomisation?

Answer 4

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions. Its always exothermic as bonds are broken to form gaseous atoms.

Question 5

What is first electron affinity?

Answer 5

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. ( energy to gain an electron) It is exothermic because the electron being added is attracted towards the nucleus.

Question 6

What is the first ionisation energy?

Answer 6

The enthalpy change requires to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. Its endothermic as energy is required to overcome the attraction between a negative electron and the positive nucleus.

Question 7

What is the standard enthalpy change of solution?

Answer 7

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

Question 8

What experiment could be used to determine the enthalpy change of solution?

Answer 8

1. weigh out a sample of an ionic compound.
2. Using a measuring cylinder, pour 25.0 cm cubed of distilled water into a plastic cup in a beaker. Measure the temp of the water to the nearest 0.5 degrees c.
3. quickly tip all of the ionic compound into the water in the plastic cup. Stir the mixture with a thermometer until the ionic compound is dissolved and the temp no longer changes.
4. use the q=mcΔt equation to work out energy change.

Question 9

What processes take place when a solid ionic compound dissolves in water?

Answer 9

1. The ionic lattice is broken up, forming separate gaseous ions.
2. The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. The energy change involved is called the enthalpy change of hydration.

Question 10

What is the enthalpy change of hydration?

Answer 10

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

Question 11

What are the general properties of an ionic compound?

Answer 11

• high melting and boiling points
• soluble in polar solvents
• conduct electricity when molten or in an aqueous solution.

Question 12

What factors affect lattice enthalpy?

Answer 12

Ionic size and ionic charge.

Question 13

What are the effects of ionic size on lattice enthalpy?

Answer 13

As ionic radius increases, attraction between ions decreases. Lattice energy is less negative so melting point decreases.

Question 14

What are the effects of ionic charge on lattice enthalpy?

Answer 14

As ionic charge increases, attraction between ions increases. Lattice enthalpy becomes more negative, so melting point increases.

Question 15

How does ionic size affect hydration enthalpies?

Answer 15

As ionic radius increases, attraction between ion and water molecules decreases, so hydration energy is less negative.

Question 16

How does ionic charge affect hydration enthalpy?

Answer 16

A charge increases, attraction with water molecules increases, so hydration enthalpy becomes more negative.

Question 17

What can entropy be used to explain?

Answer 17

• gas spreading throughout a room
• heat from a fire spreading through a room
• ice melting in a hot room

Question 18

How can entropy changes be predicted?

Answer 18

• If a system changes to become more dispersed, energy can be spread out more- there will be an entropy change which is positive.
• If a system becomes more arranged, energy becomes more concentrated- entropy change will be more negative.

Question 19

Give an example of a reaction that produces gases as a result of an increase in entropy.

Answer 19

Calcium carbonate reacting with HCl. It produces calcium chloride, water and CO2. The production of a gas increases the disorder of particles. The energy is spread out more and Δs is positive.

Question 20

What entropy change does the reaction of nitrogen gas and hydrogen gas produce?

Answer 20

N2+ 3H2 -> 2NH3
4 moles -> 2 moles of gas.
There is a decrease in the randomness of particles. The energy is less spread out and therefore Δs is negative.

Question 21

What is the standard entropy of a substance?

Answer 21

Its the entropy of one mole of a substance under standard conditions (100kPa and 298K)
They have a unit of JK-1 mol-1 and are always positive.

Question 22

What are the 2 types of energy that make up free energy change- ΔG?

Answer 22

1. The enthalpy change- the heat transfer between the chemical system and the surroundings.
2. The entropy change at the temperature of the reaction TΔS. This is the dispersal of energy within the chemical system itself.

Question 23

What is Gibbs’ equation?

Answer 23

The relationship between the two types of energy, ΔH and TΔS.
ΔG= ΔH - TΔS
where ΔG= free energy change

Question 24

What is the condition for feasability?

Answer 24

It depends on the balance between ΔH and TΔS in the Gibbs’ equation. For a reaction to be feasible, there must be a decrease in free energy;
ΔG<0

Question 25

What are the limitations of predictions made for feasibility?

Answer 25

Some reactions may still have a very large activation energy, resulting in a slow reaction rate, even though they have a negative ΔG. Although the ΔG indicates the thermodynamic feasibility, it takes no account of the kinetics or rate of reaction.