Marble reacts with hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Marble reacts with hydrochloric acid: . (a) Describe two ways to follow the rate of this reaction in the laboratory. (b) A graph of the total volume of gas against time starts steep and then flattens. What does this say about the rate? (c) How would the graph change if the same mass of marble were used as a powder?
Need a reminder? The key ideaShow
The rate of a reaction is or , in . For , divide each rate by its coefficient to get the rate of the reaction. The rate at one moment is the slope of the tangent.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) Two ways. First, collect the carbon dioxide in a gas syringe and read the volume at fixed times. Second, put the flask on a balance. The carbon dioxide escapes, so the mass falls, and the loss of mass at each time gives the amount of gas made. A third way is to take small samples of the acid at fixed times and titrate them.
(b) The shape. The slope of the volume graph is the rate. A steep start means a high rate at first. A flat end means the rate has fallen almost to zero, because the acid or the marble is used up.
(c) A powder. Powder has a much larger surface area, so more of the marble meets the acid at once. The curve starts steeper and flattens sooner. The final volume is the same, since the same amount of marble is used.
Answer: gas volume, or loss of mass, against time. The rate is the slope of the graph and it falls to zero. A powder gives a steeper start and the same final volume.
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