### Eric Nicholas K's answer to Irene's Secondary 4 Biology Singapore question.

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In my opinion, none of the options will change the graph from X to Y. In X to Y, the speed of reaction does not change, but the amount of gas produced doubles.

(a)

Unlikely. Using double the amount of zinc granules will not increase the volume of gas produced, since zinc is already in excess. It's like saying, if there are 100 people and 5 chairs, only 5 people can sit, and if there are 200 people and 5 chairs, only 5 people can sit.

(b)

Unlikely. To see this, we have to note that 2 moles of HCl will produce 1 mole of H2 while 1 mole of H2SO4 will produce 1 mole of H2. Bear in mind that zinc is in excess.

In Graph X, a total of 0.01 mol HCl is being reacted. The output is 0.005 mol H2.

In Graph Y, a total of 0.02 mol H2SO4 is being reacted. The output is 0.02 mol H2. That's quadruple, not double, the amount of H2 used.

(c)

Unlikely. Using 100 cm3 of 0.1 mol/dm3 H2SO4 results in an output of 0.01 mol H2. That's double the amount of gas produced. However, using a stronger acid of the same concentration does speed up the reaction. This is not evident in the change from X to Y.

(d)

Unlikely. This move both doubles the volume of gas produced and increases the speed of reaction. This is not evident in the change from X to Y.
Date Posted: 10 months ago
Eric Nicholas K
10 months ago
Having said all the above, if I have to pick an option, the closest answer is C though. Zinc does react rather slowly with acids so the increase in the strength of the acid is not too significant in determining the speed of the reaction.