A/L Chemistry formulae and reactions

In chemistry, sometimes the difficult part for students is mathematics and the reactions, making them concerned with what to learn and thinking about what is more important in Cambridge and Edexcel exams. In A/L, calculations such as Enthalpy, Entropy and rate of change etc. are more in depth–building upon the previous O/L knowledge. So, what are some more important formulae and reactions for A/Ls? And is simply memorising these formulae and reactions for AL, the right approach to learning?

Gas Law’s

Ideal Gas Equation: PV=nRT;

where:
P – Pressure (pascals or N/m^2)
V – Volume (m^3)
n – Number of moles (mol)
R – Ideal Gas Constant 8.314 (J/mol.K)
T – Temperature (Kelvin)

These describe how pressure, volume, temperature and moles of a gas are all linked, its’s derived from boyle’s law, Charles’ law, and Gay-Lussac’s and assembled into one universal law.
Boyle’s Law – Assume temperature and moles are constant. Then Pressure and Volume will vary.

 

P ∝ 1/V

 

Charles’ Law – Assume pressure and moles are constant. Then Temperature and Volume will vary.

 

V ∝ T

 

Gay Lussac’s Law – Assume volume and moles are constant. Then Temperature and Pressure will vary.

 

P ∝ T

 

But let’s write this in a slightly different manner so we have 1 parameter to connect it all
Avagadro’s Law – Assume Temperature and Pressure are constants. Then Volume and Moles will vary.

 

V ∝ n

 

R is the universal gas constant (8.314 J/mol · K)

If V is proportional to all the simultaneously

 

V ∝ nT/P

 

Finally, when we have a proportionality constant R to make it an equation

PV = nRT

 

R is the amount of energy (in joules) that one mole of gas gains for every one kelvin rise in temperature.

R is the universal gas constant 8.314 (J/mol.K) it’s the same for every gas which is what makes this equation so prominent. And this equation can be further written in different suited ways.

When n is Constant: PV/T = nR; n & R are constants

 

∴ P1V1/T1 = P2V2/T2

 

And when T1=T2

Boyle’s Law – For Calculating Changes in volume and pressure:

 

P1V1 = P2V2

 

This equation then states that in a fixed cylinder (mass is unchanging) where the temperature is constant, pressure increases when volume decreases, vice versa. This equation can also be used in liquids as well.

Click the link for the below formulae explanations

Chemical Energetics

Standard Enthalpy change (Hess’ Law)

ΔHrxn = ∑ΔH°f(Products) + ∑ΔH°f(Reactants)

Bond Enthalpy Approximation

ΔH ≈ ∑(bond energies broken) – ∑(bond energies formed)
*Endothermic to break bonds* and *Exothermic to form bond*

Entropy Change

AS°rxn = ∑S°(Products) – ∑S°(reactants)
Gas has much higher entropy than liquid/solid; more moles of gas → ΔS >0

Gibbs free energy

ΔG = ΔH – TΔS
ΔG < 0: Spontaneous, ΔG = Equilibrium, ΔG > 0: Non-Spontaneous
ΔG and Equilibrium constant
ΔG° = -RT ln K
ΔG = ΔG° + RT ln Q
R=8.314 j/mol·k; When Q = K, ΔG = 0 (at equilibrium)

Conditions for spontaneity

ΔH < 0, ΔS > 0 → always spontaneous
ΔH > 0, ΔS < 0 → never spontaneous
ΔH < 0, ΔS < 0 → spontaneous at low T ΔH > 0, ΔS > 0 → spontaneous at high T

Acids-Bases equilibrium

Expression for Kc

aA +bB ⇌ cC + dD
Kc = [C]c [D]d / [A]a[B]b

Expression for Kp

Kp = P(C)^c · p(D)^d / p(A)^a ·p(B)^b

Relationship between Ka  and Kb

Acid Dissociation constant (Ka)

Base Dissociation constant (Kb)

Ka x Kb = Kw = 1.0 × 10⁻¹⁴ (25°C)
pKa + pKb = pKw = 14
*For a Conjugate acid-base pair at 25°C*
Acid dissociation constant (Ka)
HA ⇌ H⁺ + A⁻
Ka = [H⁺][A⁻] / [HA]
*pKa = -log Ka; lower pka = Stronger acid*

Henderson-Hasselbalch (buffer)

pH = pKa + log([A¯] / [HA])
*Buffer is most effective when pH ≈ pKa (ratio 1:1)*

Ionic product of water Kw

H₂O ⇌ H⁺ + OH⁻
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ (25°C)
At 25°C: [H+] = [OH⁻] = 1.0 x 10-7 mol/dm3 (neutral)

Le Chatelier – effect of changes

↑ concentration → equilibrium shifts away from added species
↑ temperature → favours endothermic direction
↑ pressure → shifts to fewer moles of gas side
Catalyst → no shift; reaches equilibrium faster
*K only changes with temperature; Catalyst does not change K*

Rate of Change (Arrhenius Equation)

Rate = k[A]^a[B]^b ; rate (mol/dm³s)
k = rate constant
A & B = Concentrations in mol/dm3
a = Order of reaction with respect to A
b = Order of reaction with respect to B

k = A e(-Ea/RT)

ln k = ln A – Ea/RT ; k = Rate constant
A = Frequency factor or pre-exponential factor
e = Exponential factor
Ea = Activation Energy
R = Gas Constant
T = Temperature (Kelvin)

 

Kinetics

Half Life (first-order)

t1/2 = 0.693 / k

Integrated rate Laws

Order	Units
0th Order	mol dm−3 s−1
1st Order	s−1
2nd Order	dm3 mol−1 s−1

Key Characteristics of Maxwell-Boltzmann

• Area under curve = total number of molecules
• Ea = minimum energy for reaction
• Catalyst lowers Ea → more molecules exceed reaction Ea
• ↑T Shifts curve right, peak decreases, tail larger

 

Electrochemistry

Standard cell EMF

E°Cell = E°cathode – E°anode
= E°(reduced) – E°(oxidized)

ΔG° from E°cell

ΔG° = -n × F × E°cell
n = moles of electrons; F = 96485 C/mol

For A/L Chemistry reactions

Is memorising Chemistry formulae and reactions the right way?

Memorising the formulae above is not often the right way of learning in general, some students may already know the uses, and others may see a bunch of letters that match with what they are doing but could not actually fit correctly for calculations. That becomes the problem. Learning how to utilise these formulae and reactions become crucial to progressively maintain that knowledge and apply it better without each looking abstract.

So what is the correct way?

“It depends on the formulae / equation, because not all derivations of formulae / equation in the A/Ls are supposed to be known except for few. When it comes to recalling certain formulae which cannot be derived stepwise, what I advise my students is to use photographic memory.”

 

-Prof. Imran Razeek,

Chief Chemistry Educator & Head of Department at Chembase.