Chemical Reactions | OCR GCSE Combined Science Chemistry, Higher tier (J250)

Chemical Reactions

276 questions
17 subtopics
Paper 9 (Chemistry)
Paper 9 (Chemistry)

Chemical Reactions is examined in Paper 9 (Chemistry).

It covers writing formulae of elements and compounds, balanced equations, and deducing formulae from ions, ionic equations, and state symbols, the mole and the Avogadro constant, conservation of mass, and changes of mass in open systems, stoichiometry, and calculating reacting masses, exothermic and endothermic reactions, and reaction profiles, activation energy, and bond energy calculations, oxidation and reduction, by oxygen and by electrons, acids, alkalis and neutralisation, the ionic equation for neutralisation, and reactions of acids, dilute and concentrated, weak and strong, the pH scale, and hydrogen ion concentration, pH as a factor of ten, and measuring pH, products at the electrodes, and molten ionic compounds, electrolysis of aqueous solutions and inert and non-inert electrodes.

Sample questions from Chemical Reactions

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

Writing formulae of elements and compounds

Sodium chloride is written NaCl even though it has no molecules. What does this formula represent?

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The simplest whole-number ratio of the ions in the giant lattice, one sodium ion to one chloride ion.
Ionic equations, and state symbols

Write the ionic equation for magnesium reacting with a dilute acid.

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Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g).
Conservation of mass, and changes of mass in open systems

4 g of hydrogen reacts completely with 32 g of oxygen. What mass of water is formed?

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36 g.
Exothermic and endothermic reactions, and reaction profiles

Why is an instant cold pack for a sports injury based on an endothermic change?

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It takes in energy from its surroundings, so it cools the injury.
Oxidation and reduction, by oxygen and by electrons

In Fe₂O₃ + 3CO → 2Fe + 3CO₂, name the reducing agent and explain your choice.

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Carbon monoxide, because it removes oxygen from the iron oxide and is itself oxidised to carbon dioxide.
The ionic equation for neutralisation, and reactions of acids

Why is a solution neutral only when the amounts of acid and alkali match exactly?

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Only then have all the hydrogen ions and all the hydroxide ions been used up in forming water.
The pH scale, and hydrogen ion concentration

Which is the more acidic solution, one of pH 1 or one of pH 5?

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The one of pH 1, because the lower the pH the more acidic the solution.
Products at the electrodes, and molten ionic compounds

Why does each type of ion travel to a particular electrode?

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Opposite charges attract, so positive ions are drawn to the negative cathode and negative ions to the positive anode.

The 17 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Writing formulae of elements and compounds	Which elements exist as molecules of two atoms and why neon does not, the formulae of water, methane, carbon dioxide, magnesium oxide and carbon monoxide, what NaCl represents, how a symbol differs from a formula, working out a formula from ion charges, the formulae of common acids, sodium hydroxide, calcium carbonate and ammonia, and half equations for aluminium and bromine.	16
Balanced equations, and deducing formulae from ions	Balanced equations for sodium in chlorine, methane burning, potassium in water and calcium carbonate decomposing, why only the large numbers change, and deducing formulae such as Al₂O₃ and Ca(NO₃)₂ from ion charges.	16
Ionic equations, and state symbols	What an ionic equation and a spectator ion are, why charge must balance, ionic equations for the precipitates copper(II) hydroxide, silver chloride and barium sulfate and for magnesium with acid, what the state symbols s, l, g and aq mean, and adding them to equations such as zinc or marble chips with acid.	16
The mole and the Avogadro constant	What the Avogadro constant and a mole are, counting the atoms, molecules and ions in a number of moles, the mass of a single atom or molecule, why chemists use the mole, and converting between mass and moles with relative formula mass for carbon dioxide, calcium carbonate, magnesium, water and sodium hydroxide.	16
Conservation of mass, and changes of mass in open systems	The law of conservation of mass and what it means for atoms, reacting-mass calculations in sealed containers, why a balance reading falls when a carbonate gives off gas and rises when magnesium burns or a nail rusts, showing that mass is conserved, and using y = mx + c and the gradient of a magnesium oxide mass graph, including a line that misses the origin.	21
Stoichiometry, and calculating reacting masses	What stoichiometry shows, deducing an equation from reacting masses, limiting reactants and reactants in excess, and calculating masses of product or reactant from balanced equations for magnesium oxide, calcium oxide, hydrogen, water and sodium chloride.	16
Exothermic and endothermic reactions, and reaction profiles	What exothermic and endothermic reactions are, how a temperature change shows which is happening, examples including combustion, neutralisation, hand warmers and cold packs, drawing and labelling reaction profiles with the activation energy and overall energy change, and what accurate, precise, repeatable and reproducible results and random and systematic errors mean when measuring a temperature change.	22
Activation energy, and bond energy calculations	What activation energy is, why heating, a spark or a flame starts a reaction, why bond breaking is endothermic and bond making exothermic, and calculating energy changes from bond energies.	16
Oxidation and reduction, by oxygen and by electrons	Oxidation and reduction as the gain and loss of oxygen, oxidising and reducing agents in reactions such as copper oxide with carbon and iron oxide with carbon monoxide, why the two always happen together, oxidation and reduction as the loss and gain of electrons, OIL RIG, half equations for magnesium and chlorine, and the reaction of zinc with copper ions.	16
Acids, alkalis and neutralisation	The hydrogen ions that acids release and the hydroxide ions in alkalis, common acids and alkalis with their formulae, what neutralisation, a base and an alkali are, naming salts and writing equations such as sodium chloride and copper(II) sulfate, and making a pure dry salt by adding an insoluble base in excess to warm dilute acid.	21
The ionic equation for neutralisation, and reactions of acids	The reaction of hydrogen ions with hydroxide ions common to every neutralisation, spectator ions, what is left at the end point, the products and equations of acids with carbonates and metals, and the limewater test for carbon dioxide.	18
Dilute and concentrated, weak and strong	What concentrated and dilute mean, what strong and weak mean, hydrochloric, sulfuric and ethanoic acids as examples, why an acid can be strong and dilute, and which solution holds more hydrogen ions.	8
The pH scale, and hydrogen ion concentration	What pH measures, the 0 to 14 scale and its acidic, neutral and alkaline values, how pH relates to hydrogen ion concentration, and the shape of a pH curve as alkali is added to acid.	16
pH as a factor of ten, and measuring pH	How pH changes by one unit for each tenfold change in hydrogen ion concentration, calculating those factors between pH values, what universal indicator and litmus show and their colours, using, rinsing and calibrating a pH meter and its advantage over indicator, and what the resolution of a meter means.	18
Products at the electrodes, and molten ionic compounds	What cations and anions are and which electrode each moves to, what forms at the cathode and the anode, inert electrodes, why a compound must be molten or dissolved, and predicting the products of electrolysing molten sodium chloride, lead bromide, magnesium oxide and potassium iodide, with the difficulties of electrolysing molten sodium chloride.	16
Electrolysis of aqueous solutions	The hydrogen and hydroxide ions from water, the rules for what forms at each electrode, the products from sodium chloride and copper(II) sulfate solutions, and the half equations for hydrogen, chlorine, oxygen and copper.	16
Inert and non-inert electrodes	Graphite and platinum as inert electrodes, what a non-inert electrode is, how copper electrodes change in copper(II) sulfate solution, purifying copper, setting up the cell, and why direct current is used.	8

Chemical Reactions is 276 of the 916 questions in the guide.Get the guide, £7

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.