Rates of ReactionAQA GCSE Chemistry: Flashcards

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Define rate of reaction.

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Define rate of reaction.: Rate of reaction is the speed at which a chemical reaction proceeds, measured as the amount of reactant used or product formed per unit time.
Write the equation for calculating rate of reaction.: Rate = quantity of reactant used or product formed ÷ time
Name four methods for measuring the rate of a reaction.: Monitoring change in mass (gas given off) Volume of gas collected Colour change or absorbance Precipitation (cross experiment)
Describe the mass loss method for measuring rate of reaction.: A reactant mixture is placed on a balance and connected to an apparatus that allows gas to escape. As the reaction proceeds, gas is released and the total mass decreases. The rate can be calculated from the gradient of the mass–time graph.
Explain the gas volume method for measuring rate of reaction.: A gas is produced during the reaction and collected in a gas syringe or measuring cylinder. The volume of gas collected is recorded at regular time intervals. The rate is calculated from the gradient of the volume–time graph.
What is the cross experiment and how does it measure rate?: The cross experiment measures the formation of a precipitate during a reaction. A cross is drawn on paper beneath a beaker containing reactants. As a white precipitate forms, the cross becomes obscured. The time taken for the cross to disappear indicates the rate—faster disappearance indicates a faster reaction.
Using collision theory, explain how concentration affects the rate of reaction.: Increasing concentration increases the number of particles per unit volume. This increases the frequency of collisions between reactant particles, so more collisions occur per unit time, increasing the rate of reaction.
Using collision theory, explain how temperature affects the rate of reaction.: Increasing temperature increases both the frequency of collisions between particles and the proportion of particles with energy equal to or greater than the activation energy. Both effects increase the rate of reaction.
Explain how surface area affects the rate of reaction using collision theory.: Increasing the surface area of a solid reactant exposes more particles to reaction. This increases the frequency of collisions between the solid and other reactants, increasing the rate of reaction.
How do catalysts increase the rate of reaction?: Catalysts lower the activation energy required for the reaction to proceed. This allows a greater proportion of particles to have sufficient energy to react when they collide, increasing the rate of reaction without being chemically changed themselves.
What two factors does increasing temperature increase that cause the rate to increase?: The frequency of collisions between reactant particles The proportion of particles with energy equal to or greater than the activation energy
Describe what a rate–time graph shows and its key features.: A rate–time graph plots rate of reaction against time. The graph typically shows a high rate initially (steep line), then decreases over time as reactants are used up. Eventually the rate reaches zero when the reaction is complete.
Describe what a concentration–time graph shows.: A concentration–time graph plots concentration of a substance against time. For reactants, the curve starts high and curves downward, levelling off at zero (or a low value) when the reactant is consumed. For products, the curve starts at zero and curves upward, levelling off when the reaction is complete.
Explain how to calculate the rate at a specific time from a concentration–time graph using a tangent.: Draw a tangent line to the curve at the specific time point. Calculate the gradient of the tangent by dividing the change in concentration by the change in time (rise ÷ run). This gradient represents the instantaneous rate of reaction at that time.
What does the gradient of a tangent to a concentration–time or volume–time curve represent?: The gradient represents the instantaneous rate of reaction at a specific point in time during the reaction.