Controlling reactions (OCR GCSE Combined Science A (Gateway): Chemistry): Flashcards
Exam code: J250
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- FrontMeasuring Rates
Define rate of reaction.
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Define rate of reaction.
The 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 = amount of reactant used or product formed ÷ time. Units are typically g s-1 or cm3 s-1.
True or False?
Measuring the loss in mass of a reaction flask is a suitable method for monitoring the rate of a reaction that produces a gas.
True.
When a gas-producing reaction occurs, the gas escapes and the flask loses mass. Placing the flask on a balance and recording the mass at regular intervals gives multiple data points to plot a rate of reaction graph.
When collecting a gas to measure the rate of reaction, the volume of gas is recorded every few seconds and plotted against ...........
When collecting a gas to measure the rate of reaction, the volume of gas is recorded every few seconds and plotted against time.
Why is the loss in mass method unsuitable for reactions that produce hydrogen gas?
The loss in mass method is unsuitable for hydrogen because hydrogen has a very low relative formula mass, so the change in mass is too small to measure accurately on a balance.
In the disappearing cross experiment, what causes the cross to become invisible?
A yellow precipitate of sulfur forms in the reaction mixture, making it cloudy. The precipitate builds up until the cross can no longer be seen through the flask.
True or False?
The disappearing cross experiment produces several data points that can be used to plot a rate of reaction graph.
False.
The disappearing cross experiment produces only a single data point — the time for the cross to disappear. It does not give multiple readings over time, so it cannot be used to plot a full rate of reaction graph.
Why is cotton wool placed in the mouth of a flask during a loss-in-mass experiment?
A loose plug of cotton wool is placed in the neck of the flask during the marble chips experiment. It prevents acid droplets from escaping the flask while allowing CO2 gas to escape, giving accurate mass readings.
The units of rate of reaction for a gas-producing reaction are .......... s-1, where the numerator is the unit of volume.
The units of rate of reaction for a gas-producing reaction are cm3 s-1, where the numerator is the unit of volume.
What is a tangent in the context of a rate of reaction graph?
A tangent is a straight line drawn to touch a curve at one point without crossing it. The gradient of the tangent gives the instantaneous rate of reaction at that point.
On a rate of reaction graph, what does it mean when the line becomes horizontal and flat?
When the line becomes horizontal and flat, it means the reaction has reached completion. No more product is being formed because at least one reactant has been used up.
To find the mean rate of reaction between two time points, divide the change in .......... by the time interval.
To find the mean rate of reaction between two time points, divide the change in amount of product formed (or reactant used up) by the time interval.
How does the gradient of a rate of reaction graph change as the reaction progresses?
The gradient is steepest at the start of the reaction when the concentration of reactants is highest. As the reaction progresses, reactants are used up, so the frequency of successful collisions decreases and the gradient becomes less steep. When the reaction is complete, the gradient is zero (the line is flat).
True or False?
The rate of reaction is directly proportional to the reaction time.
False.
Rate of reaction is inversely proportional to reaction time. A longer reaction time means a slower rate. The relationship is: rate ∝ 1/time.
How do you calculate the instantaneous rate of reaction at a specific point on a curve?
To find the instantaneous rate at a specific point, draw a tangent to the curve at that point and calculate the gradient of the tangent using: gradient = change in y ÷ change in x.
Instead of plotting reaction time against a variable, reaction times can be converted to rates using .......... and plotted to show trends.
Instead of plotting reaction time against a variable, reaction times can be converted to rates using 1/t and plotted to show trends.
True or False?
On a rate of reaction graph, a steeper initial gradient always means a faster initial rate of reaction.
True.
The rate of reaction at any point on a concentration–time graph is equal to the gradient of the tangent drawn at that point. A steeper gradient indicates a faster rate.
Why should you draw a large triangle when calculating the gradient of a tangent?
Using a large triangle minimises errors of precision. A larger triangle makes it easier to read off accurate values from the axes, reducing the chance of misreading the graph.
Define activation energy.
Activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. Collisions with less than this energy are unsuccessful.
What are the four factors that can increase the rate of a reaction?
The four factors are:
Increasing concentration of reactants in solution (or pressure for gases)
Increasing temperature
Increasing surface area of solid reactants
Using a catalyst
Increasing the concentration of a solution increases the rate because there are more reactant particles in a given .........., leading to more frequent successful collisions.
Increasing the concentration of a solution increases the rate because there are more reactant particles in a given volume, leading to more frequent successful collisions.
Why does increasing temperature increase the rate of reaction?
Increasing temperature gives particles more kinetic energy. More particles exceed the activation energy, so there are more frequent and successful collisions per second, increasing the rate.
True or False?
Smaller pieces of a solid reactant have a greater surface area to volume ratio than larger lumps.
True.
Breaking a solid into smaller pieces increases its surface area to volume ratio. More particles are exposed at the surface, leading to more frequent collisions with reactant particles and a faster rate of reaction.
According to collision theory, under what conditions does a reaction occur?
According to collision theory, a reaction occurs only when particles collide with sufficient energy to exceed the activation energy. Collisions below this energy threshold are unsuccessful.
True or False?
For a gaseous reaction, increasing pressure has the same effect on rate as increasing the concentration of a solution.
True.
Increasing concentration increases the rate of reaction because there are more particles per unit volume, leading to more frequent collisions and a higher probability of successful collisions per second.
For a solid reactant, only particles at the .......... can take part in collisions with other reactants.
For a solid reactant, only particles at the surface can take part in collisions with other reactants.
Why is a higher rate of reaction of economic interest in industrial processes?
A higher rate of reaction means products are made faster, reducing the time and energy costs of production. In industry, this lowers manufacturing costs and increases profit, making it economically important to optimise reaction conditions.
Define catalyst.
A catalyst is a substance that speeds up the rate of a reaction without itself being used up or permanently changed. Its mass is the same at the start and 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. More particles then have enough energy to react, so the rate increases.
Iron is used as a catalyst in the .......... process for the manufacture of ammonia.
Iron is used as a catalyst in the Haber process for the manufacture of ammonia.
True or False?
A catalyst is chemically changed and consumed during the reaction it speeds up.
False.
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is not consumed in the reaction and can be reused.
What are enzymes?
Enzymes are biological catalysts that control reactions in living cells. They speed up chemical reactions in biological systems without being permanently changed themselves.
On an energy profile diagram, what effect does a catalyst have on the curve?
A catalyst lowers the activation energy, shown by a smaller energy hill on the profile. The reactants and products remain at the same energy levels; only the peak is reduced.
True or False?
Enzymes are only important in the digestive system and have no industrial uses.
False.
Enzymes are biological catalysts that speed up chemical reactions in living organisms. Enzymes are specific — each enzyme catalyses a particular reaction due to the shape of its active site.
Vanadium(V) oxide is used as a catalyst in the Contact Process for the production of .......... .
Vanadium(V) oxide is used as a catalyst in the Contact Process for the production of sulfuric acid.
Why are transition metals commonly used as industrial catalysts?
Transition metals often act as catalysts because their ions can have variable oxidation states, allowing them to donate and accept electrons easily during a reaction. Iron in the Haber process and platinum in catalytic converters are examples.
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