Every card in Rates and equilibrium, Year 11: measuring rate and what changes it
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- Mean rate of reaction = quantity of product formed ÷ ____ taken.
time
HintIt is the quantity read from a stopwatch.
WhyA rate always says how much happens in each second. The same sum works with the quantity of reactant used up in place of the quantity of product formed.
- Marble chips react with dilute acid and give off 48 cm³ of carbon dioxide in the first 60 s. What is the mean rate of reaction over this time, in cm³/s? (number only)
0.8
HintShare the volume of gas equally among the seconds.
WhyMean rate = volume of gas formed ÷ time taken = 48 ÷ 60 = 0.8 cm³/s. The unit follows from the sum: cubic centimetres divided by seconds.
- A flask of marble chips and acid stands on a balance. As carbon dioxide escapes, the reading falls from 152.40 g to 151.50 g in 45 s. What is the mean rate of reaction over this time, in g/s? (number only)
0.02
HintFirst find how much mass has left the flask.
WhyThe mass lost is 152.40 − 151.50 = 0.90 g, which is the mass of gas that escaped. Mean rate = 0.90 ÷ 45 = 0.02 g/s.
- The units of a rate of reaction are ____ when a mass is measured and ____ when a volume of gas is measured.
g/s; cm³/s
HintEach one is the unit of the quantity, followed by 'per second'.
WhyA rate is a quantity divided by a time, so its unit is the quantity's unit divided by seconds. At Higher tier the amount may also be counted in moles, giving mol/s.
- A reaction makes 60 cm³ of gas in 100 s, so its mean rate is 0.6 cm³/s. Why is this called a 'mean' rate rather than simply 'the rate'?
The rate changes during the reaction; this is its average over the 100 s
HintThink about whether the fizzing is equally fast at the start and near the end.
WhyMost reactions are fastest at the start, when the reactants are most concentrated, and slow down as they are used up. Dividing the total gas by the total time gives one average figure that hides this change.
- A graph shows the volume of gas produced against time. The curve is steep at first, becomes less steep, and finally turns into a flat horizontal line. What does the flat part show?
The reaction has finished: no more gas is being made
HintAsk what a line that no longer rises says about the amount of product.
WhyThe slope of the curve shows the rate. A flat line has no slope, so the rate is zero: one of the reactants has been used up.
- The volume of gas made in a reaction is read every 10 s: 0 s, 0 cm³; 10 s, 20 cm³; 20 s, 32 cm³; 30 s, 38 cm³; 40 s, 40 cm³; 50 s, 40 cm³. In which 10 s interval is the reaction fastest?
From 0 s to 10 s
HintCompare how much the reading goes up in each interval.
WhyThe gas made in each interval is 20, 12, 6, 2 and 0 cm³. The biggest rise in a fixed time is the fastest rate, and on a graph it is where the curve is steepest.
- On a curved graph of gas volume against time, a tangent is drawn at 10 s and another at 40 s. The tangent at 10 s is much steeper than the one at 40 s. What does this tell you about the reaction?
It is faster at 10 s than at 40 s
HintThe slope of each line stands for something about that moment.
WhyThe slope of a tangent measures the rate of reaction at the instant where it touches the curve. A steeper tangent means the amount of product is rising more quickly at that moment.
- A tangent is drawn at 20 s to a curve of gas volume against time. The tangent passes through the points (10 s, 14 cm³) and (30 s, 30 cm³). What is the rate of reaction at 20 s, in cm³/s? (number only)
0.8
HintDivide the rise of the line by its run.
WhyGradient = change in volume ÷ change in time = (30 − 14) ÷ (30 − 10) = 16 ÷ 20 = 0.8 cm³/s. The gradient of the tangent is the rate at the moment where the tangent touches the curve.
- 2.4 g of magnesium (relative atomic mass 24) reacts completely with excess acid in 50 s. What is the mean rate at which the magnesium is used up, in mol/s? (number only)
0.002
HintChange the mass into an amount in moles before dividing by the seconds.
WhyMoles of magnesium = 2.4 ÷ 24 = 0.1 mol. Mean rate = 0.1 ÷ 50 = 0.002 mol/s.
- Changing the pressure affects the rate of a reaction only when the reactants are ____.
gases
HintOnly one state of matter can be squeezed into a much smaller space.
WhyRaising the pressure pushes the particles of a gas closer together, which speeds up a reaction between them. Solids and liquids can hardly be compressed, so pressure makes almost no difference to them.
- The same mass of marble is reacted with the same acid twice: once as large lumps and once as powder. Which reacts faster, and which factor has been changed?
The powder; surface area
HintThink about how much of the solid the acid can actually touch.
WhyBreaking a solid into smaller pieces exposes more of it, so the same mass has a larger surface area. A larger surface area of a solid reactant increases the rate.
- Excess marble chips react with 50 cm³ of acid at 20 °C. A graph of gas volume against time rises and levels off at 60 cm³. The experiment is repeated at 40 °C with the same amounts. How does the new curve compare?
Steeper at the start, but it levels off at the same 60 cm³
HintOne thing about the line changes with how quickly gas comes off; another depends only on how much acid there is.
WhyA higher temperature increases the rate, so gas is made more quickly and the curve climbs more steeply. The amount of acid is unchanged, so the same total volume of gas is made in the end; it is just reached sooner.
- In the required practical on rate, the concentration of the acid is changed on purpose and the volume of gas collected in a fixed time is measured. Why must the acid be at the same temperature in every run?
Temperature also changes the rate, so it must be controlled
HintAsk what makes a comparison fair.
WhyConcentration is the independent variable and the volume of gas is the dependent variable. Anything else that affects the rate, such as temperature or the size of the solid pieces, is a control variable: if it changed too, you could not tell which change caused the result.
- The cloudiness of a liquid, which can be used to follow a reaction that makes a fine solid
Turbidity
HintThe word is related to 'turbulent', as in churned-up, muddy water.
WhyIn one method of the required practical, two colourless solutions are mixed and the liquid slowly turns cloudy. The time taken for a cross under the flask to disappear from view is measured: the shorter the time, the faster the reaction.
- In the 'disappearing cross' method, a cross on paper under the flask is watched from above, and the clock is stopped when the cross can no longer be seen. Why should the same person judge this in every run?
The end point is a judgement, and different people judge it differently
HintWould two observers agree on the exact instant?
WhyThere is no sharp moment when the cross vanishes, so deciding it is the main source of error in this method. Keeping the same observer, the same cross and the same depth of liquid makes the runs comparable.
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