Enthalpy and Entropy A2

Question 1

What is the definition of Enthalpy of Formation? (ΔfH) Exo or endothermic?

Answer 1

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.

Exothermic for most substances.

Question 2

What is the definition of Enthalpy of Combustion? (ΔcH) Exo or endothermic?

Answer 2

Enthalpy change when one mole of substance undergoes complete combustion in oxygen with all substances in standard states.

Exothermic.

Question 3

What is the definition of Enthalpy of Neutralisation?(ΔneutH) Exo or endothermic?

Answer 3

Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions.

Exothermic.

Question 4

What are the definitions of Ionisation enthalpy? (ΔIEH) Exo or endothermic?

Answer 4

First ionisation energy = enthalpy change when each atom from one mole of atoms loses an electron to form one mole of gaseous 1+ ions.

Second ionisation energy = enthalpy change when each ion from one mole of 1+ ions loses an electron to form one mole of gaseous 2+ ions.

Endothermic

Question 5

What are the definitions of Electron Affinity (ΔEAH) Exo or endothermic?

Answer 5

First electron affinity = enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

Exothermic for many non-metals.

Second electron affinity = enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions.

Endothermic as adding -ve electron to -ve ion = +ve

Question 6

What is the definition of Enthalpy of Atomisation? (ΔatH) Exo or endothermic?

Answer 6

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.

e.g. I2 (s) –> I (g)

Endothermic

Question 7

What is the definition of Hydration enthalpy? (ΔhydH) Exo or endothermic?

Answer 7

Enthalpy change when one more of gaseous ions becomes hydrated.

e.g. Mg2+(g) + aq –> Mg2+(aq)

Exothermic

Question 8

What is the definition of Enthalpy of Solution? (ΔsolH) Exo or endothermic?

Answer 8

Enthalpy change when one mole of ionic solid dissolves in a solvent.

e.g. MgCl2(s) + aq –> Mg2+(aq) + 2 Cl-(aq)

Varies

Question 9

What is the definition of Bond Dissociation enthalpy? (ΔdisH) Exo or endothermic?

Answer 9

Enthalpy change when one mole of covalent bonds are broken in gaseous state.

Endothermic.

Question 10

What is the definition of lattice enthalpy? (ΔLEH) Exo or endothermic?

Answer 10

Enthalpy change when one moles of a solid ionic compound is formed from its constituent gaseous ions.

Exothermic

Question 11

What is the definition of Enthalpy of vaporisation? (ΔvapH) Exo or endothermic?

Answer 11

Enthalpy change when one mole of a liquid is turned into a gas.

Endothermic

Question 12

What is the definition of Enthalpy of Fusion? (ΔfusH) Exo or endothermic?

Answer 12

Enthalpy when one mole of a solid is turned into a liquid.

Endothermic

Question 13

What type of enthalpy cycle relates lattice enthalpy and enthalpy of formation?

Answer 13

Born-Haber cycle

Question 14

What two processes make up dissolving a substance in water?

Answer 14

Breaking ionic lattice (reverse lattice enthalpy) and water molecules surrounding (hydrating) the ions.

Enthalpy of solution = reverse lattice enthalpy* + hydration enthalpy

Question 15

How does ionic size/radius and ionic charge affect lattice enthalpy?

Answer 15

IONIC RADIUS:
- The larger the ionic radius, the further apart and the ions are and so the forces of attraction are weaker. (So less exothermic)
- The smaller the ionic radius, the more closely packed together the ions are so greater forces of attraction. (So lattice enthalpy more exothermic).

IONIC CHARGE:
As ionic charge increases, attraction between ions increases and latice enthalpy becomes more negative/exothermic.

Question 16

How does ionic radius and ionic charge affect hydration enthalpy?

Answer 16

IONIC RADIUS:
As ionic radius increases, attraction between water molecules and ions decreases and hydration enthalpy becomes less negative/exothermic.
(smaller molecules have a greater attraction between water molecules).

IONIC CHARGE:
As ionic charge increases, attraction between water molecules and ions increases and hydration enthalpy becomes more negative/exothermic.

Question 17

What is the definition of Entropy? What is the symbol to represent entropy? Units?

Answer 17

A measure of THE DISPERSAL OF ENERGY IN A SYSTEM, which increases as the system gets more disordered.

ΔS - J / K / mol

(higher the entropy, greater the chaoticness!)

Question 18

Does lowering the entropy make the value more or less negative?

Answer 18

Decreasing entropy = more negative

Question 19

Why are the values for enthalpy negative?

Answer 19

Energy is lost (to the surroundings).

Question 20

Put the 3 states of matter in order of increasing entropy.

Answer 20

Solid –> liquid –> gas

Question 21

When…
- a liquid boils to become a gas
- a lquid freezes to become a solid

is ΔS postive or negative?

Answer 21

Evaporation = Positive

Freezing = negative

Question 22

If there are more moles of products than reactants, does entropy increase or decrease?

Answer 22

Increase

Question 23

What is the equation to work out standard entropy? What is standard entropy?

Answer 23

ΔS = ΣS (products) - ΣS (reactants)

The entropy of one mole of a substance under standard conditions (always positive values).

Question 24

If there are the same number of moles of products and reactants and they are in the same state, what happens to entropy?

Answer 24

Entropy change is very small so you can’t tell a difference (increase/decrease) without calculations.

Question 25

In an equation, when gas is a product but not a reactant, is ΔS positive or negative?

Answer 25

Positive

e.g. CaCO3 + HCl –> CaCl2 + H2O + CO2(g)

Question 26

What is free energy change? What is the symbol?

Answer 26

ΔG
The overall energy change caused by the:
- heat transfer between the system and surroundings (ΔH)
- dispersal of energy within the system at the temperature of the reaction (TΔS)

Internal and external energy change

Question 27

What is the Gibbs equation?

Answer 27

ΔG = ΔH - (T x ΔS)

Free energy change = enthalpy change - (Temperature (K) x entropy change)

Question 28

When is a reaction feasible, in relation to free energy?

Answer 28

When ΔG < 0 (negative value)

Question 29

If ΔH is positive and ΔS is negative, can the reaction ever be feasible?

Answer 29

No - two negatives make a positive

Question 30

How do you arrange the Gibbs equation to calculate the minimum temperature that the reaction is feasible?

Answer 30

ΔG = ΔH - (T x ΔS)

Set ΔG to 0.

Then rearrange to T = ΔH / ΔS

Question 31

How can endothermic reactions be made feasible?

Answer 31

If ΔS is positive enough and T really high, then then -TΔS may be negative enough to outweigh ΔH.

Question 32

Why might a feasible reaction not take place?

Answer 32

May have a very high activation energy, so is very slow at normal conditions.

Question 33

What is the thing I keep forgetting to do?

Answer 33

Use a negative sign for exothermic reaction!!!! ΔH

Ask yourself for everything ΔH reaction - is this endo or exothermic?