OCR A Level Chemistry B (Salters)
OCR A Level Chemistry B Oceans (O) Exam Questions
Free OCR A Level Chemistry B exam questions for Oceans (O), with a mark scheme for every pack. Covers pH, weak acids, Ka, Kw, buffers, enthalpy of solution, lattice and hydration enthalpies, entropy and carbon dioxide in the oceans.
Oceans (O) exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Chemistry Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About OCR A Level Chemistry B Oceans (O)
Oceans is a Year 2 storyline of OCR A Level Chemistry B (Salters) that uses the chemistry of seawater and the carbon cycle to develop ideas about acids, buffers and energy changes. It covers pH, strong and weak acids, Ka and Kw, buffer solutions, enthalpy changes of solution with lattice and hydration enthalpies, and entropy as a way of predicting whether a change will happen. The storyline links to concerns about increasing carbon dioxide and ocean acidification. Examiners like calculations of pH and buffer composition, enthalpy cycles for dissolving, entropy calculations and explaining how dissolved carbon dioxide affects the oceans.
What you need to know
- pH and Kw pH = -log[H+]. For strong acids [H+] equals the acid concentration; for strong bases use Kw = [H+][OH-] = 1.0 x 10^-14 mol2 dm-6 at 298 K.
- Weak acids Ka = [H+][A-] / [HA] and pKa = -log Ka. For a weak acid alone [H+] = square root of (Ka x [HA]), assuming dissociation is small.
- Buffers A weak acid and its conjugate base resist changes in pH because added H+ or OH- react with large amounts of A- or HA. The hydrogencarbonate system helps buffer the oceans and blood.
- Enthalpy of solution Enthalpy change of solution = -lattice enthalpy + sum of hydration enthalpies of the ions (using the definition of lattice enthalpy as formation of the solid from gaseous ions). Hydration enthalpies are more exothermic for smaller, more highly charged ions.
- Entropy Entropy measures the number of ways particles and energy can be arranged. Delta S total = delta S system + delta S surroundings, with delta S surroundings = -delta H / T. A change is feasible when delta S total is positive.
- Carbon dioxide and the oceans CO2 dissolves to form carbonic acid, which forms hydrogencarbonate and H+ ions. Increased CO2 lowers ocean pH and affects the formation of calcium carbonate shells.
Common mistakes in Oceans (O)
- Forgetting to convert delta H in kJ mol-1 to J mol-1 before finding delta S surroundings.
- Using the strong acid method for a weak acid.
- Getting the sign of lattice enthalpy wrong in solution cycles.
- Explaining buffer action without stating that both HA and A- are present in large amounts.
- Saying an endothermic change can never be feasible. It can be, if the entropy of the system increases enough.
Try three Oceans (O) exam-style questions
1. Calculate the pH of 0.0100 mol dm-3 sodium hydroxide at 298 K. (Kw = 1.0 x 10^-14 mol2 dm-6) (2 marks)
Show the mark scheme answer
[H+] = 1.0 x 10^-14 / 0.0100 = 1.0 x 10^-12 mol dm-3; pH = 12.00.
2. Ammonium nitrate dissolves in water: delta H = +25.7 kJ mol-1 and delta S system = +108.7 J K-1 mol-1. Show that dissolving is feasible at 298 K. (3 marks)
Show the mark scheme answer
Delta S surroundings = -25700 / 298 = -86.2 J K-1 mol-1; delta S total = 108.7 - 86.2 = +22.5 J K-1 mol-1; positive so dissolving is feasible.
3. Explain, with equations, how increasing carbon dioxide in the atmosphere lowers the pH of the oceans. (3 marks)
Show the mark scheme answer
CO2(aq) + H2O(l) and H2CO3(aq); H2CO3 and H+ + HCO3-; more dissolved CO2 shifts these equilibria to the right, increasing [H+] and lowering pH.
Oceans (O) FAQs
What does OCR A Level Chemistry B Oceans (O) cover?
It covers pH of strong and weak acids and bases, Ka and Kw, buffers, enthalpy changes of solution with lattice and hydration enthalpies, entropy and the chemistry of carbon dioxide in the oceans.
Which papers is Oceans examined in?
In the Salters A Level, content from any storyline can be assessed in all three papers: Fundamentals of chemistry, Scientific literacy in chemistry and Practical skills in chemistry.
What equations do I need for Oceans?
You need pH = -log[H+], Kw = [H+][OH-], Ka = [H+][A-]/[HA], buffer [H+] = Ka[HA]/[A-], the enthalpy of solution cycle and delta S total = delta S system - delta H / T.
