AQA Chemistry A Level Mark scheme Study Guide 2025

Define the term relative atomic mass (2 marks). - Correct Ans-. 1. The average mass of an atom of an element (average mass of all isotopes of an element) 2. Compared to 1/12th the mass of an atom of carbon-12 Describe how molecules are ionised using electrospray ionisation (4 marks). - Correct Ans-1. Sample dissolved (in a volatile solvent) (allow named solvent eg water or methanol) 2. (injected through) a needle or nozzle or capillary and at high voltage/4000 volts or high potential (allow needle is positively charged) 3. Gains a proton / H+ 4. X + H+ → XH+ Outline how the TOF mass spectrometer works (4 marks). - Correct Ans-1. Positive ions are accelerated by an electric field (allow (ions) accelerated to a negative plate) 2. To a constant kinetic energy 3. Ions create a current when hitting the detector 4. The size of the current is proportional to the abundance of ions hitting the plate and gaining an electron The student rinses the burette with deionised water before filling with sodium hydroxide solution. State and explain the effect, if any, that this rinsing will have on the value of the titre (2 marks). - Correct Ans-1. Titre value would increase / larger value 2. Because the sodium hydroxide solution would be more dilute The student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask. Explain why this improved the accuracy of the titration (1 mark). - Correct Ans-1. Idea that it ensures all ethanedioic acid / acid / sodium hydroxide / alkali / reactants are in the mixture / solution / reaction OR the idea that some of the ethanedioic acid / acid / sodium hydroxide / alkali / reactants would be on the sides of the flask CHEMISTRY CHEMISTRY Explain why pentan-2-ol has a higher boiling point than pent-1-ene (3 marks). - Correct Ans-1. Pentan-2-ol has stronger intermolecular forces 2. Pent-1-ene has van der Waals' forces (only) 3. Pentan-2-ol (also) has hydrogen bonds Explain why a fluoride ion is larger than a sodium ion (2 marks). - Correct Ans-1. Fluoride ion has (two) fewer protons/lower nuclear charge 2. Weaker attraction between nucleus and (outer) electrons Explain, in terms of structure and bonding, why the melting point of sodium fluoride is high (2 marks). - Correct Ans-1. (Electrostatic) forces of attraction between oppositely charged ions/Na+ and F- 2. Lots of energy needed to overcome/break forces Explain how the lone pairs of electrons on the oxygen atom influence the bond angle in oxygen difluoride (2 marks). - Correct Ans-1. Lone pairs repel more than bond pairs 2. Bond angle will be lower than regular tetrahedral angle Explain how permanent dipole-dipole forces arise between hydrogen chloride molecules (2 marks). - Correct Ans-1. Difference in electronegativity leads to bond polarity 2. And there is an attraction between delta positive on one molecule and delta negative on another Explain how the intermolecular forces arise between molecules of chlorine (3 marks). - Correct Ans-1. (Random) movement of electrons in one molecule (creates a dipole)/ a (temporary) dipole is formed in one molecule 2. Induces a dipole a neighbouring molecule 3. (These) temporary dipoles attract/temporary attraction between delta positive and delta negative Explain, in terms of crystal structure and bonding, why silicon (IV) oxide has a higher melting point than phosphorus (V) oxide (4 marks). - Correct Ans-1. SiO2 is macromolecular 2. Strong covalent bonds (between atoms) 3. P4O10 is molecular or simple covalent 4. Weak van der Waals forces between molecules Define the term enthalpy change (1 mark). - Correct Ans-1. Heat (energy) change at constant pressure Define the term mean bond enthalpy (2 marks). - Correct Ans-1. Average enthalpy change when one mole of gaseous molecules 2. Breaks a covalent bond to form two free radicals CHEMISTRY CHEMISTRY State the meaning of the term enthalpy of formation (2 marks). - Correct Ans-1. Enthalpy change when one mole of substance is formed from its constituent elements 2. With all substances in their standard states State the meaning of the term standard enthalpy of combustion (2 marks). - Correct Ans-1. Enthalpy change when one mole of a substance burns completely in oxygen 2. With all substances in their standard states Define the term first ionisation energy (2 marks). - Correct Ans-1. Standard enthalpy change when one mole of gaseous atoms 2. Is converted into a mole of gaseous +1 ions Define the term first electron affinity (2 marks). - Correct Ans-1. Standard enthalpy change when a mole of gaseous atoms 2. Is converted to a mole of gaseous -1 ions Define the term enthalpy of lattice formation (2 marks). - Correct Ans-1. Enthalpy change or heat change when 1 mol of solid ionic compound/substance 1 mol of ionic lattice 2. Is formed from its gaseous ions Define the term enthalpy of lattice disassociation (2 marks). - Correct Ans-1. Enthalpy change or heat change when 1 mol of solid ionic compound/substance 1 mol of ionic lattice 2. Dissociates into its gaseous ions Define the term enthalpy of hydration (2 marks). - Correct Ans-1. Standard enthalpy change when water molecules surround 2. One mole of gaseous ions Define the term enthalpy of solution (2 marks). - Correct Ans-1. Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent 2. To form a solution in which the molecules or ions are far enough apart to not interact with each other State Hess' law (1 mark). - Correct Ans-1. The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products State, in terms of electrons, the meaning of the term oxidising agent (1 mark). - Correct Ans-1. Electron acceptor State the meaning of the electrochemical series (1 mark). - Correct Ans-1. List of electrode potentials in (numerical) order State the substances and conditions needed in a standard hydrogen cell (3 marks). - Correct Ans-1. H2(g) AND 1000kPa CHEMISTRY CHEMISTRY 2. 1 mol dm-3 AND HCl 3. Pt electrode AND 298K Explain the function of the salt bridge (2 marks). - Correct Ans-1. The ions in the ionic substance in the salt bridge move through the salt bridge 2. To maintain charge balance/complete the circuit Things to remember about electrode potentials: - Correct Ans-1. Reduction occurs at the positive electrode which is on the right hand side (most positive Ecell value) 2. Oxidation occurs at the negative electrode which is on the left hand side (most negative Ecell value) 3. EMF = E(RHS) - E(LHS) OR EMF = E(reduction) - E(oxidation) 4. Conventional representation: reduced | oxidised || oxidised | reduced Explain why [H2O] is not shown in the Kw expression (1 mark). - Correct Ans-1. [H2O] is (almost) constant Suggest why the pH probe is washed with distilled water between each of the calibration measurements. (1 mark). - Correct Ans