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OCR GCSE Chemistry A · J248
OCR J248 · CM5.3iii Check the specification (PDF) (opens in a new tab)
A reaction can be monitored by measuring how much product has formed at different times. For example, a gas syringe can measure the volume of gas produced in cm³, while a stopwatch records time in seconds. Plotting gas volume against time shows how the reaction progresses. : a steeper upward curve means gas is being produced faster.
Gradient is calculated by dividing the change on the vertical axis by the change on the horizontal axis:
If volume is in cm³ and time is in s, the rate is in cm³/s. The units change with what is measured: a graph of mass against time could give a gradient in g/s.
A reaction graph is often curved because its rate changes as it proceeds. The gradient of a straight line joining two points on the curve gives the average rate between those times. To find the rate at one particular moment, draw a tangent to a smooth curve at that time. Then choose two clear points on the tangent—not necessarily on the curve—and calculate its gradient.
Smooth reaction curve with a true tangent at 20 s. Two points on the tangent, (0 s, 10 cm³) and (40 s, 30 cm³), give a gradient of 0.5 cm³/s at that moment.
In this graph, the tangent at 20 s has a gradient of . This means the gas volume is increasing at about 0.5 cm³ each second at that moment; it does not mean the rate stays at 0.5 cm³/s throughout the reaction.
A graph of product formed rises, so its gradient is positive. A graph of reactant remaining falls as reactant is used up, so its gradient is negative. The rate at which that reactant is used up is the size of the negative gradient, expressed as a positive rate. A horizontal line has zero gradient: the measured quantity is not changing.
In a closed system at dynamic equilibrium, reactant and product concentrations remain constant, so their concentration–time graphs are horizontal. This does not mean both reactions have stopped: the forward and backward reactions continue at equal rates, producing no overall change in concentration.
. On a quantity–time graph, this gives change in quantity per unit time; use units such as cm³/s or g/s.
A line between two points on a curve gives an average rate over that interval. A tangent gives the rate at one chosen time. A steeper gradient means a faster change. For a falling reactant graph, use the magnitude of its negative gradient for the rate of use.
At dynamic equilibrium, concentration–time gradients are zero because there is no net change; the forward and backward reactions still occur at equal rates.
For the rate at a particular time, draw a tangent at that time. A line joining two points on the curve gives an average rate over an interval instead.
Calculate the gradient using two well-separated points on the tangent, and include units of quantity per unit time.
A reactant-amount graph slopes downwards: its negative gradient shows reactant being used up, while the rate of use is reported as a positive value.
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Gradient
The change in the quantity on a graph’s vertical axis divided by the change in the quantity on its horizontal axis.
Tangent
A straight line drawn to touch a curve at a chosen point, following the curve’s direction there.
Rate of reaction
The change in the amount of reactant used or product formed per unit time.
Put your knowledge into practice — try past paper questions for Chemistry A
Gradient
The change in the quantity on a graph’s vertical axis divided by the change in the quantity on its horizontal axis.
Tangent
A straight line drawn to touch a curve at a chosen point, following the curve’s direction there.
Rate of reaction
The change in the amount of reactant used or product formed per unit time.