For 2X(g) + Y(g) + 3Z(g) -> Products, the rate law is Rate = k [X]¹ [Y]⁰ [Z]². Which statement is correct?

If [Z] >> [X] and 75% of X reacts in 20 s, then 50% of X reacts in 10 s
The rate decreases when the concentration of Y is halved
The half-life of Z increases on increasing its concentration when [X] >> [Z]
On doubling the concentration of each of X, Y and Z, the rate becomes 8 times

Correct answer: If [Z] >> [X] and 75% of X reacts in 20 s, then 50% of X reacts in 10 s

Solution

With [Z] in large excess the kinetics is pseudo-first-order in X (constant half-life), so 75% in 20 s means two half-lives of 10 s each, hence 50% (one half-life) in 10 s.

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