Speca › Topics › GCSE Combined Science

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• Assessment mark schemeWord, editable
• End-of-topic assessmentWord, editable
• HomeworkWord, editable
• Retrieval-practice quizzesWord, editable
• SlidesPowerPoint, editable
• Starter & exit ticketWord, editable
• Tiered worksheetWord, editable
• Worksheet mark schemeWord, editable

Foundation and Higher tier

• Calculate the mean rate of a reaction from the quantity of reactant used or product formed (measured as mass in grams or volume in cm³) over a given time, using mean rate = quantity of reactant used ÷ time taken or mean rate = quantity of product formed ÷ time taken, with units g/s or cm³/s.
• Draw and interpret graphs of reactant used or product formed against time, and draw a tangent to the curve to use its slope as a measure of reaction rate at that point.
• Recall how each of the following affects reaction rate: concentration of reactants in solution, pressure of reacting gases, surface area of solid reactants, temperature, and the presence of a catalyst.
• Explain, using collision theory, that a reaction only occurs when particles collide with at least the activation energy, and that increasing concentration, gas pressure, or solid surface area increases collision frequency (and so rate), while increasing temperature increases both collision frequency and how energetic those collisions are (so has a larger effect on rate); predict and explain rate effects from changes in these conditions, including reasoning about surface-area-to-volume ratio for solid reactants, and apply simple proportional reasoning to collision-theory explanations.
• Explain that a catalyst changes a reaction's rate without being used up, that different reactions need different catalysts (with enzymes as biological catalysts), and that a catalyst works by providing an alternative reaction pathway with a lower activation energy; identify a catalyst from its effect on rate and from its absence in the reaction's own chemical equation, and explain catalytic action in terms of activation energy: knowledge of specific catalyst names beyond those named in the specification is not required.
• Describe a reversible reaction (where products can react to re-form the original reactants, written A + B ⇌ C + D) and explain that changing the conditions can change the direction the reaction favours.
• Explain that if a reversible reaction is exothermic in one direction, it's endothermic in the other, transferring the same amount of energy either way.
• Define equilibrium, in a closed system, as the point where the forward and reverse reactions occur at exactly the same rate.

Higher tier only

• Calculate the gradient of a tangent to a reactant-used/product-formed-against-time graph, as a measure of reaction rate at that specific moment; and use quantities in moles (rate in mol/s), rather than mass or volume, for rate calculations at this tier.
• Explain, via Le Chatelier's Principle, that a system at equilibrium responds to any change in conditions by shifting to counteract that change, and that the relative amounts of reactants and products at equilibrium depend on the reaction conditions; make qualitative predictions about how a change affects a system at equilibrium, given appropriate information.
• Predict, from given data, the effect of a concentration change on a system at equilibrium: increasing a reactant's concentration shifts the system to form more product; decreasing a product's concentration shifts the system to form more product from the remaining reactants, in both cases until equilibrium is re-established.
• Predict, from given data, the effect of a temperature change on a system at equilibrium: raising temperature increases the relative amount of product for an endothermic reaction but decreases it for an exothermic one; lowering temperature has the opposite effect in each case.
• Predict, from given data, the effect of a pressure change on a gaseous system at equilibrium: increasing pressure shifts equilibrium toward the side with fewer molecules (per the reaction's symbol equation); decreasing pressure shifts it toward the side with more molecules.

Required practicals

• Required practical 11: investigate how concentration affects reaction rate, using both a gas-volume-measurement method and a colour-change/turbidity-change method, as a hypothesis-driven investigation.

Where students go wrong

• Believing a catalyst is "used up" or becomes part of the products, rather than understanding it's chemically unchanged at the end of the reaction and works by lowering activation energy, not by being consumed.
• Confusing what raises collision frequency (concentration, pressure, surface area) with what raises both collision frequency and collision energy (temperature): leading to an incomplete explanation of why temperature has a particularly strong effect on rate.
• Assuming a reversible reaction "stops" at equilibrium, rather than understanding both the forward and reverse reactions are still happening continuously, just at equal rates: a dynamic, not static, balance.
• (Higher tier) Applying Le Chatelier's Principle backwards: e.g. assuming increasing a reactant's concentration shifts equilibrium away from forming more product, rather than toward it (the system counteracts the change by using up the extra reactant).
• (Higher tier) Forgetting that pressure changes only meaningfully shift equilibrium when the two sides of a gaseous reaction have different numbers of molecules, if both sides have the same number, pressure changes don't shift the equilibrium position.

How it gets asked in the exam

"Calculate the mean rate of reaction", "Explain, using collision theory, why increasing [factor] increases the rate", "Explain how a catalyst affects the rate of reaction", "Predict the effect of increasing the concentration/temperature/pressure on the equilibrium position" (Higher), "Calculate the gradient of the tangent at time t" (Higher).

Key vocabulary

Rate of reaction, collision theory, activation energy, catalyst, reversible reaction, equilibrium, Le Chatelier's Principle (HT), tangent, gradient (HT).

Assumed prior knowledge

• Confident graph-plotting and gradient-reading, needed throughout this topic's rate-calculation content.
• Activation energy and reaction profiles from Energy Changes (this taxonomy's topic 12): collision theory in this topic builds directly on that concept.
• (Higher tier) Comfort with proportional reasoning, needed for equilibrium-shift predictions.

How Speca scaffolds this topic

• A single, consistently used collision-theory explanation frame ("more/faster collisions → more successful collisions per second → faster reaction") applied to every rate-affecting factor in turn, so students build one transferable explanation structure rather than five separate memorised facts.
• Rate graphs benefit from a fixed visual routine for tangent-drawing (steepest at the start, flattening as the reaction slows, flat once complete) practised on several example curves before gradient calculation is introduced.
• (Higher tier) A single equilibrium "see-saw" or balance-scale visual, consistently reused across concentration, temperature, and pressure examples, reinforces "the system pushes back against whatever change was made" as one repeated idea rather than three separate rules to memorise.
• (Higher tier) A worked-example bank explicitly pairing each equilibrium-shift rule with a real reaction (e.g. the Haber process, referenced elsewhere in the specification) helps ground the abstract Le Chatelier reasoning in a concrete, memorable context.

More GCSE Combined Science topics

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