In this experiment you compare the rate of the same reaction at two concentrations of acid. Record the temperature of the dilute hydrochloric acid before ea...

This experiment compares reaction rate at two acid concentrations using \(\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\). Halving the acid concentration (25 cm³ acid plus 25 cm³ water in Experiment 2) means fewer acid particles per unit volume, fewer collisions per second, and so a slower reaction. Because the same length of magnesium is used each time, the total volume of hydrogen is the same once all the metal has reacted.

(a) [4] Fill the Experiment 1 column with a reading at every time, starting at 0 cm³ at 0 s, rising and then becoming constant, read to the nearest cm³. Illustrative values: 0, 34, 55, 68, 74, 76, 76 cm³. Marks: a reading for every time [1]; values that increase then level off [1]; readings to the nearest cm³ [1]; 0 cm³ recorded at 0 s [1].

(b) [3] The Experiment 2 column is the same shape but consistently lower at each time and still climbing near the end, for example 0, 18, 33, 47, 58, 67, 73 cm³. Marks: a reading for every time [1]; values consistently lower than Experiment 1 at the same times [1]; still rising at 180 s or reaching a similar final volume later [1].

(c) [4] Plot both sets on the same axes and label the curves A (Experiment 1) and B (Experiment 2):

Marks: axes labelled with quantity and unit [1]; sensible scales using more than half the grid [1]; both sets of points plotted correctly [1]; two smooth curves drawn and labelled A and B [1].

(d) [2] Any two differences: curve A is steeper at the start than curve B; curve A becomes horizontal sooner (at a shorter time); and at any given time before the end curve A lies above curve B. Two correct comparisons score the two marks.

(e) [2] Experiment 1, the undiluted acid, used the more concentrated acid [1]. Its curve is steeper at the start, so gas is produced faster, which means a greater initial rate of reaction [1].

(f) [1] The same length of magnesium reacts in each experiment, so the same amount of hydrogen is produced, and both curves level off at about the same final volume [1].

(g) [1] Read the final (constant) volume from each curve; both are about the same, for example 76 cm³ for Experiment 1 and roughly 73 to 76 cm³ for Experiment 2 once it is complete [1].