# Suppose that the reaction: A → products follows a zero order rate law.

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#1
Which of the following statements is true?
A.) The reactant must also be a catalyst

B.) [A] decreases by the same amount every second

C.) [A] always stays the same

D.) [A] decreases by the same proportion every second

E.) The half life is constant
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5 months ago
#2
(Original post by Me_Me_Big_B0y)
Which of the following statements is true?
A.) The reactant must also be a catalyst

B.) [A] decreases by the same amount every second

C.) [A] always stays the same

D.) [A] decreases by the same proportion every second

E.) The half life is constant
First of all, when asking questions which you have trouble with it is best to show that you've atleast tried to work on the problem because this post could easily become banned for showing a lack of effort as people on fora generally do not want to waste their time on solving problems for people who didn't invest a Joule of effort in solving it for themselves. Since I have a short break at the moment I can spare some time to answer this problem for you since I also want to test my chemistry knowledge because it has become rusty over the years.

When the rate of your chemical reaction follows a zero order rate law you have to think what it means. Zero Order Rate laws refer to a situation wherein changing the concentration of reactants does not affect the rate of the reaction in anyway or the slope k (the rate constant) when the concentration of a reactant is plotted against time.

The rate equation for Zero Order Rate reactions is rate = -k. Because in such a situation the concentration of reactants is obviously decreasing over time as products are being produced (changing the concentration of reactants still leads to the same slope) if the reactant consisted of only a catalyst then there would be nothing to catalyse therefore A.) is likely to be false unless the catalyst is an autocatalyst.

With B.) it would be true since we are only dealing with a constant slope here. If you take the gradient between two points on a graph which represents a Zero Order Rate reaction and compare the gradient to that of another set of two points of equal length then they would be the same.

[A] does not always stay the same otherwise there would be no chemical reaction and therefore it would be pointless to apply a rate law. When we are talking about rate laws with no other information given we can assume that a chemical reaction is going on. Therefore C.) is false.

D.) would be true, similar reasoning as in the case for B.).

E.) would be false since the half-life in a Zero Order Rate reaction decreases with decreasing concentration since a Zero Order Rate reaction has a negative slope according to formal definitions given by many chemistry resources.
Last edited by SebastianSk; 5 months ago
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