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

Monitoring and Controlling Chemical Reactions

123 questions
8 subtopics

Monitoring and Controlling Chemical Reactions is examined in Paper 10 (Chemistry), except the mass of a solute and concentration, which the specification places in Topic C3 and examines in Paper 9 (Chemistry).

It covers mass of solute, concentration and dilution calculations, 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.

Mass of solute, concentration and dilution calculations

State the relationship between the mass of solute, the volume of solution and the concentration in g/dm³.

Show the answer
Concentration in g/dm³ equals the mass of solute in grams divided by the volume of the solution in dm³.
Measuring the rate of a reaction, and rate graphs

Give the units in which a rate of reaction is expressed when gas volume is measured, and when mass loss is measured.

Show the answer
Gas volume gives a rate in cm³/s or cm³/min, and mass loss gives a rate in g/s or g/min.
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

A cube of side 3 cm is cut into twenty-seven cubes of side 1 cm. Calculate the new total surface area and the new surface area to volume ratio.

Show the answer
Each small cube has a surface area of 6 cm², so the total is 162 cm² for the same 27 cm³, giving a ratio of 6:1, three times the original.
Catalysts and how they work

Give two reasons why catalysts are used so widely in industry.

Show the answer
They allow a useful rate at a lower temperature or pressure, which saves energy and money, and they increase the amount of product made per day from the same plant.
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 continuously removing a product from an equilibrium mixture as it forms.

Show the answer
The equilibrium shifts to the right to replace it, so more reactant is converted to product.
Activation energy, and enzymes as catalysts

Explain why lowering the activation energy makes a reaction faster.

Show the answer
A greater proportion of the colliding particles then have enough energy to react, so a larger fraction of collisions are successful.

The 8 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Mass of solute, concentration and dilution calculations	How the mass of solute and the volume of solution decide the concentration, the effect of adding water, concentration in g/dm³, and calculating masses, volumes and concentrations in mol/dm³ for sodium and potassium hydroxide solutions, including a dilution.	8
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 123 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.