Oceans A Level Chemistry OCR B

In terms of bond energy, what is required for a substance to dissolve?

New bonds formed must be the same strength or stronger than those broken

What is Standard Lattice Enthalpy?

The enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions

What does a more negative lattice enthalpy value mean?

Stronger bonding in the compound as more energy is required to form those bonds

What is the enthalpy of solution?

The enthalpy change when 1 mole of an ionic substance is dissolved in the minimum amount of solvent

What is the enthalpy of hydration?

enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions

Formula to calculate the enthalpy of solution

ΔhydH CATION + ΔhydH ANION - ΔleH

2 factors affecting the enthalpy of hydration

Charge - larger charge means greater enthalpy of hydration

Size - smaller ions have greater enthalpy of hydration

What is entropy?

A measure of the number of ways particles can be arranged AND the ways energy can be shared between them

What factors affect entropy?

Physical states
Amount of energy the substance has
more particles = more entropy

Units for entropy

J K^-1 mol^-1

How do you calculate ΔsysS

S products - S reactants

What does positive entropy mean?

The reaction is feasible

How do you find Δsurr S

-ΔH/T

How do you calculate Δtot S

Δsys S + Δsurr S

Whats the formula of Ksp for Fe(OH)3 in solution

[Fe3+] x [OH-]^3

Reaction when adding an acid to water?

HA + H2O -> H3O+ + A-

Reaction when adding a base to water?

B + H2O -> OH- +BH+

Rule for [H+] for
1) Strong acids
2) Neutral solutions

1) = [Acid]
2) = [OH-]

How to calculate pH?

pH = -log[H+]

What is Kw?

The ionic product of water
[H+] x [OH-]

If temperature doesn't change what is Kw?

1 x 10^-14

What assumptions do we make when doing Ka

1) Only a small amount dissociates because its a weak acid so... [HA]equilibrium = [HA]start

2) Almost all H+ comes from the acid and not much from water so... [H+] = [A-]

What is the equation for Ka?

Ka = [H+]^2/[HA] when a weak acid
Ka + [A-][H+]/[HA]

when Ka is very small

pKa = -logKa

What happens when we add an acid/base to an acidic buffer ( ethanoic) ?

H+ thats added reacts with CH3COO-, This causes the equilibrium to shift to the left making more salt, removing H+

OH- added reacts with H+ in solution, causing more CH3COOH to be made. Causing equilibrium shift to right to replace H+