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

time

提示It is the quantity read from a stopwatch.

为什么A 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

提示Share the volume of gas equally among the seconds.

为什么Mean 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

提示First find how much mass has left the flask.

为什么The 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

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

为什么A 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

提示Think about whether the fizzing is equally fast at the start and near the end.

为什么Most 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

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

为什么The 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

提示Compare how much the reading goes up in each interval.

为什么The 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

提示The slope of each line stands for something about that moment.

为什么The 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

提示Divide the rise of the line by its run.

为什么Gradient = 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

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

为什么Moles of magnesium = 2.4 ÷ 24 = 0.1 mol. Mean rate = 0.1 ÷ 50 = 0.002 mol/s.

gases

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

为什么Raising 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

提示Think about how much of the solid the acid can actually touch.

为什么Breaking 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

提示One thing about the line changes with how quickly gas comes off; another depends only on how much acid there is.

为什么A 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

提示Ask what makes a comparison fair.

为什么Concentration 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

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

为什么In 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

提示Would two observers agree on the exact instant?

为什么There 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.