Energy

Question 1

what is lattice enthalpy?

∆H sign?

Answer 1

the amount of energy released when a mole of ionic solid is formed from its gaseous ions.
Used as a measure of ionic bond strength.
-∆H due to the strong ionic bonds that form when the lattice is created.

Question 2

What factors affect the size of lattice enthalpy?

Answer 2

The same as those that affect the strength of an ionic bond: charge and size of ions.
The higher the charge density of cation and anion the stronger the electrostatic attraction and the more exothermic the lattice enthalpy.

Question 3

What does the calculation of lattice enthalpy assume - why might it be different from the experimental value determined from Born Haber cycles?

Answer 3

Assumes perfect ionic bonding within the compound - differences are due to the covalent character of the bond that may arise as a result of the cation polarising the anion.

Question 4

explain the term enthalpy change of hydration

∆H sign?

Answer 4

the enthalpy change when 1 mol of gaseous ions is dissolved in water to form an infinitely dilute solution.
exothermic, as ions form bonds with polar water molecules, the strength of which depends on size and charge of ion. The higher the charge density of the ion the stronger its bond with water

Question 5

explain the term enthalpy change of solution

∆H sign?

Answer 5

the enthalpy change when 1 mole of an ionic solid is completely dissolved in water.
∆Hsolution = -∆Hlattice - ∆Hhydration
sign of enthalpy change determines the likely solubility of the compound: only soluble if exothermic or 0

Question 6

explain the trend in solubility for group 2 hydroxides?

Answer 6

They get more soluble down the group because the cation increases in size down the group makes both ∆Hlattice and ∆Hhydration decrease - small size of OH- ion means decrease in ∆Hlattice is greater than that of ∆Hhydration so ∆Hsolution is a more negative value. (∆Hsol = -∆Hlatt - ∆Hhyd)
more exothermic = more soluble

Question 7

what is meant by the term enthalpy of formation?

Answer 7

the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states in standard conditions of 298K, 1atm and 1moldm-3

Question 8

what is meant by the term enthalpy of atomisation?

∆H sign?

Answer 8

the enthalpy change when 1 mole of gaseous atoms are formed from the element in its standard state under standard conditions.
endothermic, as they involve bond breaking

Question 9

what is meant by the term first ionisation energy?

∆H sign?

Answer 9

the energy required to remove one electron from each atom in 1 mole of gaseous atoms.
Always endothermic as the nuclear force of attraction upon the outer electrons has to be overcome

Question 10

why is 2nd ionisation energy more endothermic than 1st?

Answer 10

increased ratio of protons:electrons so force of attraction requires more energy to overcome and remove an electron.

Question 11

what i meant by the term first electron affinity?
∆H sign?
∆H sign for 2nd electron affinity?

Answer 11

the enthalpy change when each atom in one mole of gaseous atoms accepts an electron.
Exothermic as the nucleus of the neutral atom attracts the first electron.
However the 2nd electron affinity is endothermic as the second electron experiences repulsion from the negatively charged ion

Question 12

what is entropy?

Answer 12

entropy, S, is a measure of the disorder of a system. A system becomes energetically more stable when it becomes more disordered and so a reaction that involves an increase in the disorder of the system and its surroundings will be spontaneous.

Question 13

explain the difference in magnitude of entropy of a solid and a gas

Answer 13

increase in entropy as particles are arranged uniformly in a solid and randomly in a gas.

Question 14

explain the difference in magnitude of entropy when a solid lattice dissolves

Answer 14

increase - dissolving of solids changes the very ordered arrangement of particles in the solid to one that is more andom in solution

Question 15

explain the difference in magnitude of entropy for a reaction in which there is a change in the number of gaseous molecules

Answer 15

increase in moles of gas means there are more ways of arranging the particles of the gases leading to an increased disorder.

Question 16

how is the ∆S for a reaction calculated?

units of entropy?

Answer 16

∆S = Sproducts - Sreactants

units Jmol-1K-1

Question 17

What does feasibility depend on?

Gibbs free energy

Answer 17

temperature, T, the entropy change in the system, ∆S, and the enthalpy change, ∆H, with the surroundings
∆G = ∆H - T∆S

Question 18

What does the sign of ∆G tell you about a reaction?

Answer 18

whether it is feasible or not at the given temp.

For a reaction to be spontaneous ∆G must be NEGATIVE

Question 19

explain, in terms of enthalpy and entropy, how endothermic reactions are able to take place spontaneously

Answer 19

A endothermic reaction may be spontaneous if T∆S is more positive than ∆H. So an endothermic process that involves a large increase in entropy e.g. dissolving a solid may still take place and becomes more likely as T is increased.

Question 20

What is the limitation of ∆G in predicting the feasibility of a reaction?

Answer 20

only gives an indication of the likelihood of chemical change not the rate of the change, which is dependent on the activation energy for the reaction.

Question 21

Describe some changes that involve an increase in entropy?

Answer 21

1. A change in state from solid to liquid to gas involves particles becoming more and more randomly arranged
2. An increase in the number of moles of gas

Question 22

what are the units of entropy?

Answer 22

JK-1mol-1

Question 23

∆S = ?

Answer 23

Sproducts - Sreactants

Question 24

∆G = ?

Answer 24

∆H - T∆S