# A Level Chemistry Exam – Equations and Key Information

Welcome back to Beyond’s Science Blog! This A Level Chemistry blog post covers the key information and equations you’ll need for your AS Level Chemistry or A Level Chemistry exams. Use the links below to navigate to the different sections:

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The Periodic Table of the Elements

Formulas of Common Ions

The charges of some common ions can be deduced from the position of the elements in the periodic table, as shown in Table 1. However, the charges of compound ions and some transition metal ions are not as easily deduced. The charges of some of these positive ions are shown in Table 2 and negative ions in Table 3.

Table 1

Table 2 – Positive Ions

Table 2 – Negative Ions

Constants

These values are provided in exams.

Units

Orders of Magnitude

Equations for AS and A Level Chemistry

Relative atomic mass (Ar)

$\text{A}_{\text{r}} =\frac{\sum\text{(isotope}\times\text{isotope abundance)}}{\text{total abundance}}$

Relative molecular mass (Mr)

$\text{M}_{\text{r}}=\sum\text{(A}_{\text{r}}\times\text{number of each atom)}$

Number of moles

$\text{number of moles} = \frac{\text{mass of substance}}{\text{M}_{\text{r}}\text{ of substance}}$

Concentration

$\text{concentration}=\frac{\text{moles}}{\text{volume}}$

Number of particles

$\text{number of particles}=\text{number of moles} \times L$

L = the Avogadro constant (6.022 × 1023mol-1)

Kinetic energy (KE) *provided in exams

$KE = \frac{1}{2}mv^2$

KE = kinetic energy (J)
m = mass (kg)
v = velocity (ms-1)

Velocity (V) *provided in exams

$V = \frac{d}{t}$

v = velocity (ms-1)
pV = nRTd = distance (m)
t = time (s)

Ideal gas law

$pV = nRT$

p = pressure (Pa)
V = volume (m3)
n = number of moles (mol)
R = the gas constant (8.31 JK-1mol-1)
T = temperature (K)

Percentage atom economy

$\text{percentage atom economy}=\frac{\text{M}_{\text{r}} \text{ of desired product}}{\sum\text{M}_{\text{r}} \text{ of all reactants}} \times 100$

Percentage yield

$\text{percentage yield}=\frac{\text{actual yield of desired product}}{\text{maximum theoretical yield of desired product}}\times 100$

Heat capacity

$q=mc\delta T$

q = heat change (J)
m = mass of the substance (g)
c = specific heat capacity of the
substance (JK-1g-1)
ΔT = change in temperature

Enthalpy change

$\Delta H = \frac{q}{n}$

ΔH = enthalpy change of reaction (kJ mol-1)
q = heat change (J)
n = number of moles

Hess’s Law

$\Delta H^{\theta} = \sum H^{\theta}_{\text{products}}-\sum H^{\theta}_{\text{reactants}}$

ΔH = enthalpy change of reaction (kJ mol-1)

Rate of reaction

$\text{rate of reaction}=\frac{\text{amount of reactant used or product formed}}{\text{time}}$

Equations for A Level Chemistry Only

Entropy change (ΔS)

$\Delta S^{\theta} = \sum S^{\theta}_{\text{products}} - \sum S^{\theta}_{\text{reactants}}$

ΔS = entropy change of reaction (J K-1 mol-1)

Gibb’s Free Energy (ΔG)

$\Delta G = \Delta H - T \Delta S$

ΔG = Gibb’s free energy change (kJ mol-1)
ΔH = enthalpy change (kJ mol-1)
T = temperature (K)
ΔS = entropy change (J K-1 mol-1)

Arrhenius equation *provided in exams

$k = Ae^{-\frac{E_a}{RT}}$

rearranged form:

$\ln k = -\frac{E_a}{RT}+\ln A$

k = rate constant
A = Arrhenius constant
Ea = activation energy (J mol-1)
R = the gas constant (8.31J K-1 mol-1)
T = temperature (K)

Mole fraction of a gas

$\text{mole fraction of a gas} = \frac{\text{number of moles of gas}}{\text{total number of moles of all gases in the mixture}}$

Partial pressure of a gas

$\text{partial pressure of a gas in a mixture} = \text{mole fraction} \times \text{total pressure}$

pressure (Pa)

Electrochemical cell reactions

$\text{EMF} = E^{\theta}_{\text{species being reduced}} - E^{\theta}_{\text{species being oxidised}}$

or (if given a cell diagram)

$\text{EMF} = E^{\theta}_{\text{right hand side}} - E^{\theta}_{\text{left hand side}}$

EMF (also known as Ecell) = electromotive force (V)

pH

$\text{pH} = -\log_{10}[H^{+}]$

$[H^{+}]=10^{-\text{pH}}$

Dissociation constant

$\text{pK}_{a} = -\log_10{K_a}$

$K_a = 10^{-\text{pK}_a}$

Planck’s equation

$\Delta E = hv = \frac{c}{\lambda}$

ΔE = energy difference between the ground state and excited state of an electron (J)
h = Planck’s constant (6.63 × 10-34Js)
v = frequency of light absorbed (Hz)
c = speed of light (3.00 × 108m s-1)
λ = wavelength of light absorbed (m)

Rf value

$\text{R}_{\text{f}} \text{ value} = \frac{\text{distance travelled by sample}}{\text{distance travelled by solvent}}$

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