Elements From the Sea

What Elements From the Sea covers

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.

The 22 subtopics

Each one has its own page.

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

What you need to know

Answer each one before you reveal it.

Take Elements From the Sea further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• Oxidising agent vs Reducing agent

  Oxidising agent: a species that takes electrons and is itself reduced. Reducing agent: a species that gives electrons and is itself oxidised.

• Anode vs Cathode

  Anode: the electrode at which oxidation occurs. Cathode: the electrode at which reduction occurs.

• Percentage yield vs Atom economy

  Percentage yield: the mass of product obtained as a proportion of the theoretical mass. Atom economy: the proportion of the reactant mass that ends up in the desired product.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What does the atom economy of a reaction measure?

   Model answer

   It measures the proportion of the mass of the reactants that ends up in the desired product.

2. 2

   State the expression used to calculate atom economy.

   Model answer

   Atom economy = (Mᵣ of the desired product ÷ sum of the Mᵣ values of all the products) × 100, taken from the balanced equation.

3. 3

   Why does it make no difference whether you divide by the total Mᵣ of the products or of the reactants?

   Model answer

   Mass is conserved in a balanced equation, so the two totals are identical.

4. 4

   What is the highest value atom economy can take, and when is it reached?

   Model answer

   It can reach 100%, which happens when every atom supplied by the reactants ends up in the desired product and there is no by-product.

5. 5

   A student calculates an atom economy of 130%. Explain why this answer must be wrong.

   Model answer

   The desired product cannot contain more mass than the reactants supplied, so an atom economy above 100% is impossible and there is an error in the working.

6. 6

   Chlorine is made from hydrogen chloride by the Deacon reaction, 4HCl + O₂ ⇌ 2Cl₂ + 2H₂O. Calculate the atom economy for chlorine. (Aᵣ: H 1.0, O 16.0, Cl 35.5)

   Model answer

   The products total 2 × 71.0 + 2 × 18.0 = 178.0, so the atom economy is (142.0 ÷ 178.0) × 100 = 79.8%.

What is inside Levo

Six ways to work the same material.

Elements From the Sea revision questions

What does Elements From the Sea cover in OCR A-Level Chemistry B?

22 subtopics: 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. Each one has its own page with questions and model answers.

How is Elements From the Sea organised?

By the OCR H433 specification, subtopic by subtopic, so it matches the order you are taught it in.