Rate and Extent of Chemical Change
Rate and extent of chemical change covers two linked ideas: how quickly a reaction happens (its rate), measured as the quantity of reactant used up or product formed divided by the time taken, and explained using collision theory and the factors that affect it (temperature, concentration, pressure, surface area and catalysts); and reversible reactions, which can go both forwards and backwards and, in a closed system, settle into a dynamic equilibrium where the forward and reverse reactions continue at exactly the same rate.
Method
- Calculate mean rate of reaction using: mean rate = quantity of reactant used or product formed / time taken, giving units of g/s if measuring a mass, or cm3/s if measuring a gas volume.
- On a graph of the quantity of product formed against time, read a steeper section of the line as a faster rate, and the line becoming flat (horizontal) as the point at which the reaction has finished, because a reactant has been fully used up.
- Use collision theory to explain the four factors that increase rate: increasing temperature, concentration (or pressure, for gases) and surface area all increase how often reacting particles collide, and increasing temperature also increases the energy of those collisions, so more collisions succeed in overcoming the activation energy.
- Learn that a catalyst speeds up a reaction without being permanently used up itself, by providing an alternative reaction pathway with a lower activation energy; a catalyst is not included in the balanced chemical equation for the reaction.
- For the required practical using the gas-volume method, for example calcium carbonate chips reacting with dilute hydrochloric acid, the apparatus is a conical flask connected by a bung and delivery tube to a gas syringe; a common independent variable is the concentration of the acid, the dependent variable is the volume of gas collected at set time intervals, and the main source of error is gas escaping before the bung is fully in place, which is reduced by working quickly and consistently.
- Recognise a reversible reaction from the special symbol showing two arrows pointing in opposite directions, and know that if the forward reaction of a reversible reaction is exothermic, the reverse reaction is endothermic by exactly the same amount of energy, and vice versa.
- For a reversible reaction in a closed system, describe dynamic equilibrium as the point at which the forward and reverse reactions are still both happening, but at exactly the same rate as each other, so the amounts of reactants and products present stay constant.
Worked example
In an experiment, 2.4 g of magnesium ribbon reacts completely with excess dilute hydrochloric acid in 48 seconds. Calculate the mean rate of this reaction, in g/s.
- Identify the quantity of reactant used and the time taken: 2.4 g of magnesium, in 48 seconds.
- Write the equation: mean rate = quantity of reactant used / time taken.
- Substitute the values: mean rate = 2.4 / 48.
- Calculate: 2.4 / 48 = 0.05.
- Final answer: the mean rate of reaction is 0.05 g/s.
Practice questions
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Q1State the equation for the mean rate of a reaction in terms of the quantity of reactant used.Show answer
Answer: Mean rate = quantity of reactant used / time taken.
Q2State two units that the rate of a reaction can be measured in.Show answer
Answer: g/s (grams per second) and cm3/s (cubic centimetres per second).
Q3State the four factors that can increase the rate of a chemical reaction.Show answer
Answer: Increasing the temperature, increasing the concentration (or pressure, for a gas), increasing the surface area of a solid reactant, and adding a catalyst.
Q4Explain, using collision theory, why increasing the concentration of a reactant in solution increases the rate of a reaction.Show answer
Answer: A more concentrated solution has more particles of the reactant in the same volume, so particles collide with each other more frequently, increasing the frequency of successful collisions and so the rate of reaction.
Q5A reaction between calcium carbonate and hydrochloric acid produces 72 cm3 of carbon dioxide gas in 30 seconds. Calculate the mean rate of reaction.Show answer
Answer: 2.4 cm3/s (72 / 30 = 2.4).
Q6State what is meant by a catalyst.Show answer
Answer: A substance that speeds up a reaction, without being used up itself, by providing a reaction pathway with a lower activation energy.
Q7State what is shown on a rate graph when the line for the volume of gas produced becomes horizontal (flat).Show answer
Answer: The reaction has finished, because one of the reactants has been completely used up.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student adds a small piece of magnesium ribbon to a test tube of dilute hydrochloric acid, and an identical piece of magnesium ribbon to an identical test tube containing the same volume of a more concentrated hydrochloric acid. State which test tube produces bubbles of gas faster, and explain your answer using collision theory.
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A student investigates the reaction between marble chips (calcium carbonate) and excess dilute hydrochloric acid, measuring the volume of carbon dioxide gas produced over time using a gas syringe. The reaction produces 90 cm3 of gas in total, and the reaction is complete after 60 seconds. (a) Calculate the mean rate of reaction over the whole 60 seconds. (b) State and explain how the rate of reaction in the first 10 seconds compares with the rate of reaction in the last 10 seconds.
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Solid hydrated copper sulfate is blue. When heated strongly, it breaks down in a reversible reaction to form white anhydrous copper sulfate and water vapour. Explain what is meant by a reversible reaction, describe how you could show that this reaction is reversible, and explain what is happening to the forward and reverse reactions if this reaction reaches dynamic equilibrium in a closed system.
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