Monitoring and Controlling Chemical Reactions | OCR GCSE Chemistry A, Higher tier (J248)

Monitoring and Controlling Chemical Reactions

202 questions
12 subtopics
Paper 4
Paper 4

Monitoring and Controlling Chemical Reactions is examined in Chemistry A Paper 4.

It covers concentration in mol/dm³, and titration, titration calculations, and the mass of a solute, molar gas volume and the volumes of gases, theoretical mass and percentage yield, atom economy, and choosing a reaction pathway, measuring the rate of a reaction, and rate graphs, factors affecting rate, and collision theory, surface area and rate, catalysts and how they work, reversible reactions and dynamic equilibrium, changing the conditions of an equilibrium and activation energy, and enzymes as catalysts.

Sample questions from Monitoring and Controlling Chemical Reactions

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

Concentration in mol/dm³, and titration

Calculate the mass of sodium hydroxide, relative formula mass 40, in 100 cm³ of a solution of concentration 2.00 mol/dm³.

Show the answer
2.00 × 0.100 = 0.200 mol, and 0.200 × 40 = 8.00 g, given to 3 significant figures.
Titration calculations, and the mass of a solute

Calculate the mass of potassium hydroxide, relative formula mass 56, in 50.0 cm³ of solution of concentration 0.100 mol/dm³.

Show the answer
0.100 × 0.0500 = 0.00500 mol, and 0.00500 × 56 = 0.280 g, given to 3 significant figures.
Theoretical mass and percentage yield

How is the percentage yield of a reaction calculated?

Show the answer
Divide the actual mass of product obtained by the theoretical mass and multiply by 100.
Atom economy, and choosing a reaction pathway

Give two reasons why a route with a lower percentage yield might still be chosen over one with a higher yield.

Show the answer
It may run much faster or at milder conditions and so cost less energy, or its by-products may be saleable while the other route's are not.
Factors affecting rate, and collision theory

Which of the factors that change rate applies only to reactions involving gases?

Show the answer
Pressure, because only gases are compressed enough for pressure to change how closely the particles are packed.
Surface area and rate

Repeat runs with large marble chips take 82 s, 85 s, 110 s and 83 s. Calculate a suitable mean time.

Show the answer
110 s is anomalous and is left out, so the mean is (82 + 85 + 83) ÷ 3 = 250 ÷ 3 = 83 s, to 2 significant figures.
Reversible reactions and dynamic equilibrium

Describe what happens when ammonium chloride is heated and then allowed to cool.

Show the answer
On heating it breaks down into ammonia and hydrogen chloride gases, and on cooling the two gases recombine to form solid ammonium chloride again.
Changing the conditions of an equilibrium

Predict the effect of increasing the pressure on an equilibrium between gases.

Show the answer
It shifts towards the side with the smaller number of gas molecules, because that reduces the pressure.

The 12 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Concentration in mol/dm³, and titration	What concentration in mol/dm³ means and how it relates to moles and volume, converting cm³ to dm³, calculating concentrations and masses and converting from g/dm³, the effect of dilution, the pipette, burette, white tile and indicators used in a titration, what the end point is, reading a burette, rough titrations and concordant titres.	16
Titration calculations, and the mass of a solute	The three steps from titration results to an unknown concentration, worked titration calculations including a 1:2 ratio, averaging concordant titres, converting mol/dm³ to g/dm³, and mass, volume and concentration calculations.	16
Molar gas volume and the volumes of gases	The 24 dm³ molar gas volume at room temperature and pressure, why every gas occupies the same volume, converting between moles and volume, and calculating gas volumes from reacting masses and equations.	16
Theoretical mass and percentage yield	What a theoretical mass is and the three steps to calculate it, worked examples from calcium carbonate, magnesium, nitrogen and hydrogen, the limiting reactant, percentage yield calculations, and why yields fall short of 100%.	17
Atom economy, and choosing a reaction pathway	What atom economy is and how it is calculated, when it reaches 100%, why manufacturers want it high and how it differs from percentage yield, what happens to by-products, worked calculations for calcium oxide, iron, ammonia, hydrogen, oxygen and magnesium chloride, and choosing a reaction pathway by weighing atom economy, yield, rate, equilibrium position and useful by-products.	22
Measuring the rate of a reaction, and rate graphs	What rate means, following a reaction with a gas syringe, a balance, a disappearing cross, a colorimeter or a pH probe, the units of rate, and reading the gradient of a rate graph.	22
Factors affecting rate, and collision theory	How temperature, concentration, pressure and surface area affect rate, why concentration changes the speed but not the amount of product, the rough doubling for each 10 °C rise, how rate changes during a reaction, the two conditions for a successful collision, explaining each factor with collision theory and activation energy, reading rate ∝ 1/t, and planning, evaluating and improving the sodium thiosulfate investigation.	21
Surface area and rate	How breaking a solid up raises its surface area to volume ratio, why powders react faster, the cube calculations, the flour dust hazard, and comparing the graphs for powder and lumps.	16
Catalysts and how they work	What a catalyst is, why its mass is unchanged and it sits above the arrow, why catalysts are specific and used in industry, and the catalysts for hydrogen peroxide, ammonia and sulfuric acid.	15
Reversible reactions and dynamic equilibrium	What a reversible reaction is and its symbol, why the reverse of an exothermic reaction is endothermic, heating hydrated copper(II) sulfate and testing for water, heating ammonium chloride, the nitrogen and hydrogen that form ammonia, what dynamic equilibrium and a closed system are, what the position of equilibrium means, and how the forward and reverse rates change until equilibrium is reached.	16
Changing the conditions of an equilibrium	Le Chatelier’s principle, and the effect on the position of equilibrium of changing concentration, removing a product, raising or lowering the temperature, and changing the pressure, with the compromise conditions for ammonia and sulfur trioxide.	10
Activation energy, and enzymes as catalysts	How a catalyst gives a pathway with a lower activation energy, reaction profiles for catalysed reactions, why lowering the activation energy speeds a reaction, calculating activation energies and energy changes from profiles, what enzymes are and why organisms need them, denaturing by heat and pH, enzyme specificity, uses in brewing and washing powders, and how enzymes compare with metal catalysts.	15

Monitoring and Controlling Chemical Reactions is 202 of the 1,265 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.