Rates of Reaction: Question 2

Worked solution

Part (a): What the loss-in-mass data shows

The total mass of the flask and its contents falls steadily, from $270.00\text{ g}$ to $268.77\text{ g}$. Since nothing solid or liquid can leave the flask (the cotton wool plug keeps the dough in), the only way mass can be lost is if a gas escapes. The gas being produced by this reaction is carbon dioxide, so the falling readings confirm $\text{CO}_2$ is being formed and is escaping through the cotton wool plug.

Note that the flask itself, together with its cotton wool plug, has a fixed mass of $120.00\text{ g}$. This is why the very first reading ($270.00\text{ g}$) is bigger than the $150\text{ g}$ of dough alone. This fixed mass never changes, so it is only the fall in the total reading that tells us how much gas has escaped, not the reading itself.

The mass keeps falling throughout the 12 minutes (rather than dropping instantly) because the yeast enzymes keep converting sugar into ethanol and carbon dioxide the whole time, as long as there is still sugar left to react, more $\text{CO}_2$ keeps being made and escaping.

Part (b): Calculating average rates from the table

(i) Between $t=0$ and $t=2$ minutes: $\text{mass lost} = 270.00 - 269.45 = 0.55\text{ g}$ $\text{average rate} = \frac{0.55}{2} = 0.275\text{ g/min}$

(ii) Between $t=8$ and $t=10$ minutes: $\text{mass lost} = 268.82 - 268.78 = 0.04\text{ g}$ $\text{average rate} = \frac{0.04}{2} = 0.02\text{ g/min}$

Part (c): Why the rate falls over time

The rate in the first 2 minutes ($0.275\text{ g/min}$) is more than ten times faster than the rate between 8 and 10 minutes ($0.02\text{ g/min}$). This is because sugar is a reactant that is being used up as the reaction proceeds. As less sugar remains in the dough, its concentration falls, and a lower reactant concentration gives a slower rate, so the reaction slows as time goes on, until the mass readings become almost constant (the reaction has nearly stopped).

Part (d): Comparing with a warmer trial

Because the warmer dough mixture contains the same total amount of sugar as the original, it will eventually produce the same total mass of carbon dioxide once fermentation is complete, so both graphs level off at the same final mass.

However, a higher temperature increases the rate of reaction, so the warmer mixture’s mass–time graph will fall more steeply at the start (losing mass faster in the first few minutes) and will flatten out (reach its final level) sooner than the original graph.

Final answers

• (a) Falling mass confirms $\text{CO}_2$ gas is escaping; it keeps falling because the reaction keeps proceeding while sugar remains. The flask and cotton wool’s own fixed mass ($120.00\text{ g}$) explains why the readings start above $150\text{ g}$.
• (b) (i) $\boxed{0.275}$ g/min; (ii) $\boxed{0.02}$ g/min.
• (c) The rate falls because sugar (a reactant) is used up, lowering its concentration and slowing the reaction.
• (d) Same final mass lost overall; the warmer graph is steeper at the start and levels off sooner.