Controlling Reactions (OCR GCSE Chemistry A (Gateway)): Flashcards
Exam code: J248
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- FrontMeasuring Rates
Define rate of reaction.
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Define rate of reaction.
Rate of reaction is a measure of how quickly a reactant is used up or a product is formed over time.
What is the formula for calculating the rate of reaction?
Rate of reaction = quantity of reactant used (or product formed) ÷ time taken.
Units are typically g/s or cm³/s.
In the mass loss method, a flask is placed on a .......... and the loss in .......... is recorded over time as a gaseous product escapes.
In the mass loss method, a flask is placed on a balance and the loss in mass is recorded over time as a gaseous product escapes.
True or False?
Measuring the volume of gas produced is a suitable method for tracking the rate of the magnesium and hydrochloric acid reaction.
True.
The reaction produces hydrogen gas, which can be collected in a gas syringe and its volume measured over time.
Why is cotton wool placed in the mouth of a flask during a mass loss experiment?
Cotton wool allows gas to escape while preventing solid particles from being ejected from the flask, especially during vigorous reactions.
This keeps the experiment safe without blocking the gas.
How is the disappearing cross method used to measure rate of reaction?
A flask is placed over a paper cross. As a cloudy precipitate forms, the cross becomes harder to see.
The time taken for the cross to disappear is recorded — a shorter time indicates a faster rate.
The units of rate of reaction when measuring volume of gas are .......... or .......... per second.
The units of rate of reaction when measuring volume of gas are cm³ or dm³ per second.
Why is the mass loss method not suitable for measuring the rate of reactions that produce hydrogen gas?
Hydrogen has a very small relative formula mass (Mr).
The loss in mass from the flask is too small to be measured accurately on a balance.
True or False?
1 ÷ time is directly proportional to the rate of reaction.
True.
Rate of reaction is inversely proportional to reaction time, so 1/time increases as rate increases — they are directly proportional to each other.
Define mean rate of reaction (from a graph).
The mean rate of reaction is the total change in quantity of reactant or product divided by the total time taken for that change.
On a typical rate of reaction graph, what is plotted on the x-axis and y-axis?
The x-axis shows time and the y-axis shows either the volume of gas produced or the amount of reactant used up.
This allows rate to be calculated from the gradient.
The mean rate of reaction between two time points equals the change in .......... divided by the change in .......... between those points.
The mean rate of reaction between two time points equals the change in quantity divided by the change in time between those points.
How do you find the rate of reaction at a specific point on a curved rate graph?
Draw a tangent to the curve at that point, then calculate the gradient of the tangent.
Gradient = change in y ÷ change in x.
True or False?
A steeper gradient at the start of a rate graph indicates a faster rate of reaction.
True.
A steeper gradient means a greater change in quantity per unit time, which corresponds to a faster rate of reaction.
What does it mean when a rate of reaction graph becomes horizontal (flat)?
The reaction has stopped — the flat line shows no further change in the amount of product or reactant.
This happens when at least one reactant is used up.
Instead of plotting reaction time, the results can be converted using .......... and plotted to observe trends in rate.
Instead of plotting reaction time, the results can be converted using 1/time and plotted to observe trends in rate.
How does the concentration of reactant change over the course of a reaction, and how is this shown on a rate graph?
Reactant concentration decreases over time as it is used up.
On a rate graph, this is shown by a curve that falls and eventually becomes flat when the reactant is fully consumed.
Define activation energy.
Activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur.
What are the four factors that can increase the rate of a chemical reaction?
Increasing concentration of reactants in solution (or pressure for gases)
Increasing temperature
Increasing surface area of solid reactants
Using a catalyst
Collision theory states that reactions only occur when particles collide with sufficient .......... to overcome the .......... energy.
Collision theory states that reactions only occur when particles collide with sufficient energy to overcome the activation energy.
True or False?
Doubling the concentration of a reactant in solution doubles the number of collisions per second.
True.
The number of collisions is proportional to the number of particles present. Double the particles in the same volume means double the collisions.
Why does increasing temperature increase the rate of a reaction?
Particles gain more kinetic energy, so they move faster and collide more frequently.
More collisions have enough energy to exceed the activation energy, increasing the number of successful collisions.
Why does increasing the surface area of a solid reactant increase the rate of reaction?
Only particles on the surface of a solid can collide with other reactants.
Breaking a solid into smaller pieces exposes more surface, increasing the number of collisions per second and therefore the rate.
A powder has a .......... surface area to volume ratio than lumps of the same solid, so it reacts .......... .
A powder has a greater surface area to volume ratio than lumps of the same solid, so it reacts faster.
True or False?
For aqueous reactions, a 10°C rise in temperature approximately doubles the rate of reaction.
True.
This is a rule of thumb for aqueous and gaseous systems. A small rise in temperature causes a large increase in rate because many more particles exceed the activation energy.
Why does increasing the pressure of a gaseous reaction increase the rate?
Higher pressure means the same number of gas particles occupy a smaller volume, increasing their concentration.
This leads to more frequent collisions and a higher rate of reaction.
Define collision theory.
Collision theory states that chemical reactions only occur when reactant particles collide with sufficient energy — at least equal to the activation energy.
Define catalyst.
A catalyst is a substance that speeds up the rate of a reaction without being used up or chemically altered at the end of the reaction.
How does a catalyst increase the rate of a reaction?
A catalyst provides an alternative reaction pathway with a lower activation energy.
This means more particles have enough energy to react, so the rate of reaction increases.
The mass of a catalyst at the .......... of a reaction is the same as at the .......... , because it is not .......... .
The mass of a catalyst at the beginning of a reaction is the same as at the end, because it is not consumed.
True or False?
A catalyst is consumed during a chemical reaction.
False.
A catalyst speeds up the reaction but is not altered or used up. Its mass remains the same before and after the reaction.
Name three industrial catalysts and the processes they are used in.
Iron — the Haber process (making ammonia)
Vanadium(V) oxide — the Contact process (making sulfuric acid)
Manganese(IV) oxide — decomposition of hydrogen peroxide
Define enzyme.
An enzyme is a biological catalyst that speeds up chemical reactions in living organisms without being used up.
Why are enzymes valuable in industrial processes?
Enzymes allow reactions to occur at lower temperatures and pressures than normally required.
This saves energy and reduces production costs.
On an energy profile diagram, a catalyst lowers the .......... energy, providing an alternative .......... for the reaction.
On an energy profile diagram, a catalyst lowers the activation energy, providing an alternative pathway for the reaction.
What does the enzyme catalase do?
Catalase is found in liver cells and catalyses the decomposition of hydrogen peroxide.
This is important because hydrogen peroxide can damage tissues if allowed to accumulate.
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