AQA AS LEVEL Chemistry Paper 2 2025
Mark Scheme
Relative atomic mass (Ar) - Average mass of an atom compared with one twelth of the mass of an atom
of carbon 12
Electronegativity - Power of an atom to attract the pair of electrons in a covalent bond
Standard enthalpy of combustion - Enthalpy change when 1 mole of a substance burns completely in
oxygen under standard conditions with all reactants and products being in their standard state
Standard enthalpy of formation - Enthalpy change when 1 mole of a compound is formed from it's
elements under standard conditions in their standard states
Mean bond enthalpy - The average energy needed to break a certain type of bond over a range of
compounds
Activation energy - Minimum energy required for colliding molecules to react
Maxwell-Boltzmann distribution x and y axes - x: Energy
y: fraction/number of molecules
What happens to the Maxwell-Boltzmann curve when temperature increases - Moves down and to the
right
Rate of reaction - Rate at which product formed/reactant used up
Order of reaction - Power to which the concentration of a species is raised in the rate equation
rearrange k=Ae^-Ea/Rt - lnk=-Ea/Rt+lnA
, Conditions for thermal cracking - High temperature and high pressure
Products of thermal cracking - High % of alkenes
Conditions for catalytic cracking - High temperature ,slight pressure and in the presence of a zeolite
catalyst
Products of catalytic cracking - Motor fuels and aromatic hydrocarbons
Why do we crack alkanes? - Smaller alkanes/alkenes generally more useful than longer chain ones
Equation for complete combustion of alkanes - alkane + oxygen > carbon dioxide + water
Equations for incomplete combustion of alkanes - alkane + oxygen > carbon monoxide/carbon + water
Pollutants produced by combustion engines - NOx, CO, C and sunburned hydrocarbons
Reaction of methane with chlorine - I: Cl2 > 2Cl•
P: CH4 + Cl• > CH3• + Chi
CH3• + Cl2 > CH3Cl + Cl•
T: CH3• + Cl• > CH3Cl
2 equations showing decomposition of the ozone - Cl• + O3 > ClO• + O2
ClO• + O3 > 2O2 + Cl•
What types of mechanism do haloalkanes undergo? - -Nucleophilic substitution by aqueous hydroxide
ions, cyanide ions or ammonia
-Elimination with ethanolic hydroxide ions
Mark Scheme
Relative atomic mass (Ar) - Average mass of an atom compared with one twelth of the mass of an atom
of carbon 12
Electronegativity - Power of an atom to attract the pair of electrons in a covalent bond
Standard enthalpy of combustion - Enthalpy change when 1 mole of a substance burns completely in
oxygen under standard conditions with all reactants and products being in their standard state
Standard enthalpy of formation - Enthalpy change when 1 mole of a compound is formed from it's
elements under standard conditions in their standard states
Mean bond enthalpy - The average energy needed to break a certain type of bond over a range of
compounds
Activation energy - Minimum energy required for colliding molecules to react
Maxwell-Boltzmann distribution x and y axes - x: Energy
y: fraction/number of molecules
What happens to the Maxwell-Boltzmann curve when temperature increases - Moves down and to the
right
Rate of reaction - Rate at which product formed/reactant used up
Order of reaction - Power to which the concentration of a species is raised in the rate equation
rearrange k=Ae^-Ea/Rt - lnk=-Ea/Rt+lnA
, Conditions for thermal cracking - High temperature and high pressure
Products of thermal cracking - High % of alkenes
Conditions for catalytic cracking - High temperature ,slight pressure and in the presence of a zeolite
catalyst
Products of catalytic cracking - Motor fuels and aromatic hydrocarbons
Why do we crack alkanes? - Smaller alkanes/alkenes generally more useful than longer chain ones
Equation for complete combustion of alkanes - alkane + oxygen > carbon dioxide + water
Equations for incomplete combustion of alkanes - alkane + oxygen > carbon monoxide/carbon + water
Pollutants produced by combustion engines - NOx, CO, C and sunburned hydrocarbons
Reaction of methane with chlorine - I: Cl2 > 2Cl•
P: CH4 + Cl• > CH3• + Chi
CH3• + Cl2 > CH3Cl + Cl•
T: CH3• + Cl• > CH3Cl
2 equations showing decomposition of the ozone - Cl• + O3 > ClO• + O2
ClO• + O3 > 2O2 + Cl•
What types of mechanism do haloalkanes undergo? - -Nucleophilic substitution by aqueous hydroxide
ions, cyanide ions or ammonia
-Elimination with ethanolic hydroxide ions
