Zadanie 3a, Paper 1B, maj 2025, TZ2: Chemia IB SL

Zadania w arkuszu

Informacja do zadań 3a-3b.

The reaction between powdered calcium carbonate, $\ce{CaCO3(s)}$

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, and dilute nitric acid,

\ce{HNO3(aq)}

, was followed by measuring the loss of mass. In each run the same mass of powder was added to the same volume of acid; three acid concentrations were used. The flask stood on a balance connected to a data logger, which recorded the mass of the reaction mixture every 0.2 seconds for 40 seconds.

$\ce{CaCO3(s) + 2HNO3(aq) -> Ca(NO3)2(aq) + CO2(g) + H2O(l)}$

The results were plotted on the graph.

line graph of the mass of the reaction mixture in g against time in s, three curves for 0.40, 0.80 and 1.20 mol dm-3 nitric acid, each falling steeply in the first few seconds and then levelling off

(i) Determine, using a suitable construction drawn on the graph, the initial rate at which $\ce{CO2(g)}$ is released, in $\mathrm{g\,s^{-1}}$ , in the reaction with 1.20 $\mathrm{mol\,dm^{-3}}$ $\ce{HNO3(aq)}$ .

(ii) Determine the average rate of release of $\ce{CO2(g)}$ , in $\mathrm{g\,s^{-1}}$

Sprawdzane umiejętności

Measuring and calculating rate of reaction o tej umiejętności

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Odpowiedź z klucza

(i) tangent drawn to the 1.20 $\mathrm{mol\,dm^{-3}}$ curve at t = 0 ✔

gradient of the tangent calculated and the rate quoted as a positive value: «for example $\dfrac{104.7 - 102.9}{1.5 - 0} =$ » 1.2 « $\mathrm{g\,s^{-1}}$ » ✔

(ii) « $\left|104.90 - 104.22\right| / 20 =$ » 0.034 « $\mathrm{g\,s^{-1}}$ » ✔

Schemat punktowania

(i) M1: a tangent at t = 0 on the correct curve; M1 is required for M2. M2: gradient from two points on the tangent, given as a positive rate; accept 1.0 to 1.4 g s-1. A gradient taken between two data points on the curve, or a negative value, does not score M2. (ii) [1] for 0.034 g s−1 (accept 0.030 to 0.040 g s−1), from the masses at t = 0 and t = 20 s on the 0.40 mol dm−3 curve; the sign must be positive.

Komentarz

The mass of the mixture decreases, but the rate of release of the gas is reported as a positive number equal to the magnitude of the gradient.

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, in the reaction with 0.40

\mathrm{mol\,dm^{-3}}

\ce{HNO3(aq)}

over the first 20 seconds.