Rates and equilibrium, Year 11: measuring rate and what changes it

time

HintIt is the quantity read from a stopwatch.

The 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.

0.8

HintShare the volume of gas equally among the seconds.

The 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.

0.02

HintFirst find how much mass has left the flask.

The 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.

g/s; cm³/s

HintEach one is the unit of the quantity, followed by 'per second'.

The 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.

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.

The 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.

The reaction has finished: no more gas is being made

HintAsk what a line that no longer rises says about the amount of product.

The 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.

From 0 s to 10 s

HintCompare how much the reading goes up in each interval.

The 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.

It is faster at 10 s than at 40 s

HintThe slope of each line stands for something about that moment.

The 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.

0.8

HintDivide the rise of the line by its run.

The 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.

0.002

HintChange the mass into an amount in moles before dividing by the seconds.

The whyMoles of magnesium = 2.4 ÷ 24 = 0.1 mol. Mean rate = 0.1 ÷ 50 = 0.002 mol/s.

gases

HintOnly one state of matter can be squeezed into a much smaller space.

The 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 powder; surface area

HintThink about how much of the solid the acid can actually touch.

The 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.

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.

The 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.

Temperature also changes the rate, so it must be controlled

HintAsk what makes a comparison fair.

The 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.

Turbidity

HintThe word is related to 'turbulent', as in churned-up, muddy water.

The 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.

The end point is a judgement, and different people judge it differently

HintWould two observers agree on the exact instant?

The 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.