Chapter 5 Enthalpy and Entropy

Question 1

Lattice Enthalpy:

Answer 1

the energy change that accompanies the formation of one mole of an ionic compound from it’s gaseous ions under standard conditions.

Question 2

Why is First Electron affinity exothermic but Second Electron Affinity endothermic?

Answer 2

After the first electron has been added the atom becomes a negative ion, repelling the electron away. Therefore energy must be used to force the negatively-charged electron onto the negative ion.

Question 3

Standard Enthalpy of Solution (ΔsolH⦵):

Answer 3

The enthalpy change when one mole of a solute dissolves in a solvent. Under standard conditions:

Question 4

Enthalpy Change of Hydration:

Answer 4

Enthalpy change when one mole of aqueous ions are formed by dissolving gaseous ions.

Question 5

Key Equation for enthalpy equations:

Answer 5

q = mass * specific heat capacity (usually of water) * delta T(change in temp)

Question 6

Effect of ionic size on Lattice enthalpy:

Answer 6

As ionic radius increases, the attraction between ions decreases, the lattice energy becomes less negative and melting point decreases.

Question 7

Effect of ionic charge on Lattice Enthalpy:

Answer 7

Ionic charge increases, Attraction between ions increases, Lattice enthalpy becomes more negative and melting point increases.

Question 8

Standard Enthalpy of Atomisation (ΔₐₜH⦵):

Answer 8

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.

Question 9

Standard Enthalpy of formation ( ΔfH⦵):

Answer 9

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

Question 10

Why is the Standard enthalpy of atomisation always endothermic?

Answer 10

Intermolecular forces are overcome and sometimes bonds are broken.

Question 11

What determines the ΔₐₜH?

Answer 11

bond enthalpy of the bond being broken.

Question 12

What is first electron affinity?

Answer 12

The energy change when one mole of electrons are added to one more of gaseous atoms to from 1- ions.

Question 13

What is first ionisation energy?

Answer 13

The enthalpy change required to remove one mole of electrons from one mole of gaseous atom to from one mole of gaseous 1+ ions

Question 14

Why is second electron affinity endothermic?

Answer 14

The electron is being added to a 1- ion which repels it and-so energy must be used to overcome the forces of repulsion and add the electron to the ion.

Question 15

What does the “⦵” symbol mean?

Answer 15

Standard conditions and states.

Question 16

Predicting solubility:

Answer 16

If the sum of hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy of solution should be exothermic and the compound should dissolve. However some endothermic enthalpy changes are soluble due to feasibility equations.

Question 17

General properties of ionic compounds:

Answer 17

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

Question 18

Predicting the melting points of ionic compounds:

Answer 18

More exothermic lattice enthalpies suggest they’re very stable compounds with high melting points. However, the packing of ions should also be considered.

Question 19

Entropy:

Answer 19

A measure for the tendency of the dispersal of energy within a chemical system.

Question 20

Solids have the _____ entropies:

Answer 20

Smallest

Question 21

Liquids have _____ entropies:

Answer 21

Greater (than solids)

Question 22

Gases have the _____ entropies:

Answer 22

Greatest

Question 23

If the number of molecules is greater in the products than the reactants, what does this suggest?

Answer 23

There is an increases in enthalpy.

Question 24

Calculating the entropy change:

Answer 24

ΔS⦵ = ΣS⦵ (products) - ΣS⦵ (reactants)

Question 25

Gibb’s equation:

Answer 25

ΔG=ΔH−TΔS

Question 26

What is the Gibb’s equation used for?

Answer 26

Predicting the feasibility of a reaction.

Question 27

What is feasibility?

Answer 27

Feasibility is used to describe whether or not a reaction is able to happen and is energetically feasible (spontaneous).

Question 28

What must ΔG be for a reaction to be feasible?

Answer 28

ΔG < 0 - There must be a DECREASE in free energy.

Question 29

What two factors effect ΔG?

Answer 29

ΔH - the enthalpy change
ΔS - The dispersal of energy
T- The temperature

Question 30

Limitations of using Gibbs to predict feasibility:

Answer 30

Some reactions with -ve ΔG don’t spontaneously take place. This could be due to the reaction having a high activation energy. The negative ΔG indicates thermodynamic feasibility but with no account of kinetics or the rate of reaction.

Question 31

what factors effect enthalpy change of solution

Answer 31

hydration enthalpy and lattice enthalpy

Question 32

how does ionic size effect enthalpy change of hydration

Answer 32

the smaller the size of the ion the greater the attraction to water so would be more exothermic