The diagram shows a gas being collected in a syringe as zinc reacts with acid. The plunger moves out most quickly at the start. Why does the reaction slow d...
Question 1 Report
The diagram shows a gas being collected in a syringe as zinc reacts with acid. The plunger moves out most quickly at the start. Why does the reaction slow down later?
Answer Details
The plunger moves out fastest at the beginning because that is when the reaction is producing hydrogen most quickly. As time passes, the reactants are steadily consumed.
The reaction slows because the acid is being used up, so its concentration falls. With fewer acid particles in each cubic centimetre, collisions with the zinc surface become less frequent, so the gradient of the volume against time curve decreases.
No extra acid is being added, and the temperature does not keep rising; even the slight warming from this exothermic reaction would speed it up rather than slow it. Zinc is a reactant here, not a catalyst, and it is consumed too, so the exposed surface shrinks as well.
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