Elements from the Sea | OCR A-Level Chemistry B (Salters) (H433)

Elements from the Sea

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

The storyline that carries redox and equilibrium: atom economy, extracting the halogens from seawater, electrolysis, oxidation states and balancing redox equations, the chemistry of the halogens and hydrogen halides, and dynamic equilibrium with Kc..

It covers atom economy, extraction of the halogens from sea minerals, electrolysis of molten salts and writing electrode half-equations, electrolysis of aqueous solutions and the electrolysis of brine, redox as electron transfer, the rules for assigning oxidation states, oxidation state calculations, and defining oxidation and reduction, balancing a redox equation with oxidation states, the iodine-thiosulfate titration and its calculations, systematic nomenclature of inorganic compounds, physical properties of the halogens, halogen reactivity and the displacement equations, observing a displacement, and using an organic solvent or starch, silver halide precipitation and tests, preparation of the hydrogen halides, properties of the hydrogen halides, chlorine: risks, benefits and uses, dynamic equilibrium, writing a Kc expression and calculating Kc, what the magnitude of Kc tells you, and what changes it, using the Kc expression to explain a shift in position and temperature, pressure and catalysts on an equilibrium.

Sample questions from Elements from the Sea

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

Atom economy

In the Deacon reaction the only substance formed alongside chlorine is water. Why does this soften the impact of the atom economy being below 100%?

Show the answer
The mass that does not become chlorine leaves as water, which is harmless, so little is genuinely wasted.
Electrolysis of molten salts and writing electrode half-equations

Write the anode half-equation for the electrolysis of molten aluminium oxide.

Show the answer
2O²⁻ → O₂ + 4e⁻.
The rules for assigning oxidation states

What is the usual oxidation state of hydrogen in its compounds, and when does it differ?

Show the answer
It is +1, except in metal hydrides such as NaH, where it is −1.
Balancing a redox equation with oxidation states

Write the equation for iodine reacting with thiosulfate ions.

Show the answer
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻.
Physical properties of the halogens

Describe the trend in boiling point down Group 7.

Show the answer
Boiling point rises steadily from fluorine through to iodine.
Observing a displacement, and using an organic solvent or starch

How could starch be used to tell whether a displaced halogen is bromine or iodine?

Show the answer
Starch turns blue-black with iodine but is unchanged by bromine.
Properties of the hydrogen halides

What type of reaction takes place between a hydrogen halide and ammonia?

Show the answer
It is an acid–base reaction, with the hydrogen halide donating a proton to the ammonia.
Dynamic equilibrium

What does it mean to say that an equilibrium lies to the left?

Show the answer
The mixture contains mostly reactants.

The 22 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Atom economy	Recall questions on the atom economy expression, why it differs from percentage yield, and why a reaction that goes to completion can still waste mass.	21
Extraction of the halogens from sea minerals	Recall questions on the abundance of the halides in seawater, releasing bromine with chlorine, and the equations for the extraction.	20
Electrolysis of molten salts and writing electrode half-equations	Recall questions on what happens at each electrode, why a direct current is used, and writing and checking half-equations.	12
Electrolysis of aqueous solutions and the electrolysis of brine	Recall questions on why two reactions compete at each electrode, the rules for the products, and the three products of electrolysing brine.	14
Redox as electron transfer	Recall questions on oxidation and reduction in terms of electrons, oxidising and reducing agents, and writing half-equations.	22
The rules for assigning oxidation states	Recall questions on the oxidation state of an uncombined element, of oxygen, hydrogen and fluorine, and what the totals must come to.	10
Oxidation state calculations, and defining oxidation and reduction	Recall questions on solving for an unknown oxidation state, why one can be fractional, and defining oxidation and reduction by the change.	16
Balancing a redox equation with oxidation states	Recall questions on using oxidation state changes to balance an equation, and on checking that charge balances as well as atoms.	10
The iodine-thiosulfate titration and its calculations	Recall questions on the equation and mole ratio, generating the iodine, the starch indicator and when to add it, and the calculation.	16
Systematic nomenclature of inorganic compounds	Recall questions on Roman numerals in a name, the chlorate and nitrate ions, and naming a compound from its formula.	22
Physical properties of the halogens	Recall questions on the diatomic molecules, the trend in boiling point, and how polarisability and London forces explain it.	21
Halogen reactivity and the displacement equations	Recall questions on the order of oxidising power, why a halogen displaces one below it, and the ionic equations for each displacement.	15
Observing a displacement, and using an organic solvent or starch	Recall questions on the colours in water and in cyclohexane, using starch to identify iodine, and why fluorine cannot be used in water.	10
Silver halide precipitation and tests	Recall questions on the ionic equations with silver ions, the three precipitate colours, and how ammonia separates them.	19
Preparation of the hydrogen halides	Recall questions on making hydrogen chloride from a chloride and concentrated sulfuric acid, and why a different acid is needed for the bromide and iodide.	20
Properties of the hydrogen halides	Recall questions on the trend in thermal stability and bond enthalpy, the reactions with ammonia and with water, and why dry gas leaves litmus unchanged.	22
Chlorine: risks, benefits and uses	Recall questions on hazard against risk, chlorine with water, the case for and against chlorinating drinking water, and making bleach.	20
Dynamic equilibrium	Recall questions on reversible reactions and closed systems, what dynamic equilibrium means, and what the position of equilibrium describes.	19
Writing a Kc expression and calculating Kc	Recall questions on the general form of the expression, writing it for a given equation, and substituting equilibrium concentrations.	14
What the magnitude of Kc tells you, and what changes it	Recall questions on what a Kc much greater than, close to, or much less than 1 means, and that only temperature changes its value.	10
Using the Kc expression to explain a shift in position	Recall questions on why adding a reactant or removing a product shifts the position while Kc stays fixed, and on the reaction quotient.	10
Temperature, pressure and catalysts on an equilibrium	Recall questions on the effect of temperature on an exothermic and an endothermic equilibrium, when pressure matters, and why a catalyst does not move the position.	14

Elements from the Sea is 357 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.