Enthalpy And Entropy

Question 1

Enthalpy change of reaction

Answer 1

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states.

Question 2

Enthalpy of combustion

Answer 2

The enthalpy change when one mole of a compound completely reacts with oxygen under standard conditions, with reactants and products in their standard states. - Exo

Question 3

Enthalpy of neutralisation

Answer 3

The enthalpy change when one mole of water is after is formed from an acid by base reaction, under standard conditions.

Question 4

Enthalpy change of formation

Answer 4

Enthalpy change due to 1 mole of a compound forming from its constituent elements under standard conditions. -Exo

Question 5

Lattice enthalpy

Answer 5

Enthalpy change that takes place due to the formation of 1 mole of an ionic lattice from its gaseous ions under standard conditions. (Used to determine strength of ionic bond in ionic compound)
-Exo
Na+ (g) + Cl-(g) —> NaCl (s)

Question 6

First ionisation energy

Answer 6

Energy required to remove one mole of electron from 1 mole of gaseous atom to form one mole of +1 gaseous ions.
- Endo
Na(g) —> Na+(g) + e-

Question 7

Enthalpy of hydration

Answer 7

The enthalpy change that takes place when one mole gaseous ions are dissolved in water to form aqueous ions.
- Exo
Na+ (g) + aq —> Na+ (aq)

Question 8

Enthalpy of solution

Answer 8

Standard enthalpy change when one mole of solute completely dissolved in a solvent and forms aqueous ions.

(goes below LE on LHS side when negative) or
(below hydration when positive on RHS)

NaCl(s) + aq —> Na+ (aq) + Cl- (aq)

Question 9

Enthalpy change of atomisation

Answer 9

• Endothermic
• 1/2Cl2 (g) —> Cl (g)
  One mole of gaseous atom from elements

Question 10

Lattice dissociation enthalpy (reverse of lattice enthalpy)

Answer 10

• Endo
  NaCl (s) —> Na+ (g) + Cl- (g)
• 1 mole of solid ionic compound separates into its gaseous ions

Question 11

First electron affinity (opposite of ionisation)

Answer 11

• Exo
  Cl (g) + e- —> Cl- (g)

Question 12

Mean bond enthalpy

Answer 12

• Endo
• enthalpy change required to break one mole of a specific type of bond within a gaseous molecule across a range of compounds
• CH4 (g) —> C(g) + 4H (g)

Question 13

Properties of ionic compounds

Answer 13

• high melting point
• soluble in polar solvents
• conduct electricity when molten or in aqueous solution

Question 14

What is the effect of increasing ionic size on lattice enthalpy?

Answer 14

the ionic radius increases, weaker attraction between ions, lattice energy gets less exothermic and melting point decreases.

Question 15

What is the effect of increasing ionic charge on lattice enthalpy?

Answer 15

Ionic charge increases, stronger attraction between ions, lattice energy becomes more exothermic and melting point increases.

Question 16

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

Answer 16

Ionic radius increases, attraction between ion and water molecules decrease and hydration energy gets less exothermic.

Question 17

What is the effect of increasing ionic charge on hydration enthalpy?

Answer 17

Attraction between ions and water molecules increases and hydration energy becomes more exothermic.

Question 18

What is entropy?

Answer 18

A measure of the dispersal of energy in a system, which increases as the system becomes more disordered.

Question 19

Why does entropy always have a positive value?

Answer 19

At below 0K there is no energy within a system and every system has energy so entropy has to have a value of 0 JK-1mol-1, must be above 0 for it to have an entropy value.

Question 20

What effect does changing temperature have on entropy?

Answer 20

As temp increases, kinetic energy of particles increase, system becomes more disordered so entropy increases. (More positive)

Question 21

What effect does dissolving ionic solids have on entropy?

Answer 21

As ionic solids dissolve, ions spread out, disorder increases and entropy increases.

Question 22

What effect does the number of gas molecules have on entropy?

Answer 22

As the number of gas molecules increase, entropy increases (vice versa)

Question 23

Entropy Equation

Answer 23

^S = Products - Reactants

Question 24

Units for entropy

Answer 24

JK-1mol-1

Question 25

Feasibility

Answer 25

Describes whether a reaction takes place.

Question 26

Gibbs’s Equation

Answer 26

^G = ^H - T^S

^G = kJmol-1
^H = kJmol-1
^S = JK-1mol-1 /1000 = kJK-1mol-1

Question 27

When is a process feasible?

Answer 27

When the Gibbs’s equation is less than or equal to 0, so it is negative

Question 28

What is a limitation of predictions made for feasibility?

Answer 28

Does not take kinetics or rate of reaction into account. For example if a reaction has a large activation energy it would take a long time to occur even at high temps (requires a catalyst).

Question 29

Lattice enthalpy equation

Answer 29

Route 2=Route 1

Question 30

What happens to entropy when particles are dissolved

Answer 30

Aqeous is more disordered than solid

Question 31

What is feasibility dependent on?

Answer 31

It is dependent on temperature, enthalpy change and entropy change.

Question 32

Why is second ionisation energy more endothermic than first ionisation energy

Answer 32

Ion is smaller than the atom because there is stronger attraction to the nucleus

Question 33

Why is second electron affinity positive?

Answer 33

Ion and electron are both negative so energy is required to overcome the repulsion