Zadanie 1b
Wzorowane na: Chemistry SL Paper 1B, May 2026, TZ3, pytanie 1(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 1a-1c.
A student investigated the reaction between aqueous nitric acid, HNO3, and solid magnesium carbonate, MgCO3, using two different methods to follow its rate.
In Method 1, a conical flask containing the aqueous acid stood on a balance and the mass reading was recorded at regular intervals while the gas escaped from the open flask. In Method 2, the flask was closed with a stopper carrying a delivery tube; the gas passed through the tube into an inverted measuring cylinder filled with water and standing in a trough of water, and the volume of gas collected was recorded at regular intervals.
Polecenie
In Method 1 the flask containing 25.0 cm3 of the aqueous acid was put on the balance, which was then set to zero; 0.400 g of MgCO3 was added and the reading was noted every 20 seconds until the reaction had stopped. Each point is the mean of three trials and a smooth curve was drawn through the points; the error bars were smaller than the plotted symbols (the largest uncertainty in a mean mass was ± 0.002 g).
Determine the initial rate of the reaction and show on the graph how you obtained it.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
tangent drawn to the curve at t = 0 s ✔
«e.g. the tangent passes through (0 s, 0.400 g) and (20 s, 0.321 g)»
«rate = −gradient =» 3.9 × 10−3 «g s−1» ✔
Schemat punktowania
M1: a tangent drawn at t = 0 s; do not award M1 for a line that only joins two data points or for a line drawn at a later time. M2: initial rate with a unit; accept values from 3.4 × 10−3 to 4.4 × 10−3 «g s−1». Do not award M2 for a negative rate. Accept the answer in other units, such as 8.9 × 10−5 mol s−1 (range 7.7 × 10−5 to 1.0 × 10−4), provided the unit is given.
Komentarz
The rate is the magnitude of the gradient, so the sign is dropped; a chord between the first two points gives a value that is too low because the curve is steepest at the start.