Oceans | OCR A-Level Chemistry B (Salters) (H433)

Oceans

282 questions
19 subtopics
The ten storylines, examined across all three papers
Component 01, Component 02 and Component 03

The storyline that carries thermodynamics and acid-base equilibria: solubility and the enthalpies of lattice, hydration and solution, entropy and feasibility, the solubility product, strong and weak acids with pH and buffers, and the greenhouse effect and ocean acidification..

It covers solubility in aqueous and non-aqueous solvents, lattice, hydration and solution enthalpies and how they relate, measuring an enthalpy change of solution, charge density and the size of enthalpy terms, entropy: qualitative treatment, total entropy change and the condition for feasibility, the limiting temperature, and why a feasible reaction may not happen, calculating the entropy change of the system, the solubility product expression, its units and calculations, predicting precipitation and the common ion effect, bronsted-Lowry theory of acids and bases, strong acids and strong bases, weak acids, Ka and pKa, pH of strong acids and strong bases, pH of weak acids, dilution and the limits of the approximation, what a buffer is and how it resists a change in pH, buffer calculations, capacity and the natural buffers, how the greenhouse effect works and which gases cause it and why a molecule absorbs, concentrations in ppm, and ocean chemistry.

Sample questions from Oceans

Answer each one closed book first, then open the answer.

Solubility in aqueous and non-aqueous solvents

What does the rule 'like dissolves like' mean in terms of intermolecular bonds?

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A solute dissolves readily when the solute–solvent bonds formed are of a similar type and strength to the solute–solute and solvent–solvent bonds that have to be broken.
Measuring an enthalpy change of solution

What assumption is made about the mass used in q = mcΔT for an experiment on dissolving?

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The mass of water is taken as the mass of the whole solution, and the solution is assumed to have the same specific heat capacity as water.
Entropy: qualitative treatment

Predict the sign of ΔsysS for N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

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It is negative, because four moles of gas become two moles of gas.
The limiting temperature, and why a feasible reaction may not happen

A reaction has a positive ΔH and a negative ΔsysS. What can you say about its feasibility?

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It is not feasible at any temperature, because both contributions to ΔtotS are negative.
The solubility product expression, its units and calculations

What are the units of Ksp for silver chloride?

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They are mol² dm⁻⁶.
Bronsted-Lowry theory of acids and bases

What is meant by a conjugate acid?

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It is the species formed when a base has accepted a proton.
Weak acids, Ka and pKa

How does the value of pKₐ relate to acid strength?

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The smaller the pKₐ, the stronger the acid, because pKₐ runs in the opposite direction to Kₐ.
pH of weak acids, dilution and the limits of the approximation

A 0.250 mol dm⁻³ solution of a weak acid has pH 2.72. Calculate its Kₐ.

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[H⁺] = 10⁻²·⁷² = 1.91 × 10⁻³ mol dm⁻³, so Kₐ = (1.91 × 10⁻³)²/0.250 = 1.45 × 10⁻⁵ mol dm⁻³.

The 19 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Solubility in aqueous and non-aqueous solvents	Recall questions on why like dissolves like, why sodium chloride will not dissolve in hexane, and why alcohol solubility falls with chain length.	20
Lattice, hydration and solution enthalpies and how they relate	Recall questions on the three definitions, why hydration is exothermic, and the cycle relating them.	13
Measuring an enthalpy change of solution	Recall questions on the polystyrene cup method, extrapolating the temperature graph, the assumptions in q = mcΔT, and converting q to a molar value.	13
Charge density and the size of enthalpy terms	Recall questions on why greater charge density makes both lattice and hydration enthalpies more exothermic, and comparing named compounds.	19
Entropy: qualitative treatment	Recall questions on what entropy measures and its units, the order of the three states, and what melting or making more gas does to it.	21
Total entropy change and the condition for feasibility	Recall questions on the entropy change of the surroundings, adding the two contributions, and the sign that makes a reaction feasible.	11
The limiting temperature, and why a feasible reaction may not happen	Recall questions on setting the total entropy change to zero to find a limiting temperature, and why a feasible reaction may still not be observed.	12
Calculating the entropy change of the system	Recall questions on using standard entropies, multiplying by the stoichiometric numbers, and why elements do not have zero standard entropy.	18
The solubility product expression, its units and calculations	Recall questions on writing Ksp for a given salt, deducing its units, and converting between solubility and Ksp.	14
Predicting precipitation and the common ion effect	Recall questions on comparing the ionic product with Ksp, halving concentrations on mixing, and why a common ion reduces solubility.	11
Bronsted-Lowry theory of acids and bases	Recall questions on the proton-donor and proton-acceptor definitions, conjugate pairs, and identifying each species in an equation.	20
Strong acids and strong bases	Recall questions on full ionisation, the equations for the common strong acids and bases, and strong against concentrated.	18
Weak acids, Ka and pKa	Recall questions on partial ionisation and the equilibrium arrow, the Ka expression, pKa, and comparing two acids by their values.	21
pH of strong acids and strong bases	Recall questions on the definition of pH, the ionic product of water, and calculating pH for a strong acid and for a strong base.	10
pH of weak acids, dilution and the limits of the approximation	Recall questions on the weak acid expression and its two approximations, the effect of tenfold dilution, and when the approximation fails.	15
What a buffer is and how it resists a change in pH	Recall questions on the two components, the two ways of preparing one, and what happens when acid or alkali is added.	12
Buffer calculations, capacity and the natural buffers	Recall questions on calculating buffer pH from Ka and the two concentrations, why dilution barely changes it, and the blood and seawater buffers.	11
How the greenhouse effect works and which gases cause it	Recall questions on why the Earth radiates infrared, the infrared window, what happens inside an absorbing molecule, and which gases matter.	11
Why a molecule absorbs, concentrations in ppm, and ocean chemistry	Recall questions on the dipole condition for absorption, why carbon dioxide absorbs though it has no permanent dipole, ppm, and ocean acidification.	12

Oceans is 282 of the 3,430 questions in the guide.Get the guide, £8

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.