Rate Laws and Reaction Kinetics Questions and Answers

Answer: C

Both are true and the reason gives the qualification: a reaction with a very low activation energy hardly responds to temperature while one with a very high energy responds far more steeply. The rule is convenient rather than exact.

Answer: D

The first two are correct. The third is inverted, since a catalyst provides an alternative path of lower activation energy, which is precisely how it speeds the reaction up. The slope of the Arrhenius plot gives the value.

Answer: A

It equals the logarithm of two divided by the rate constant, with no concentration term. This is a useful diagnostic, since a half life that changes with starting concentration rules first order out. The logarithm of two divided by k is the value.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the proof. For first order \( t_{1/2} = \ln 2/k \), with no concentration in it, so every halving takes the same time whatever the starting point. For any other order the half life carries \( C_0 \) - rising with concentration for zero order and falling for second - which is exactly what makes the constancy diagnostic.

Answer: C

The logarithm of two is about zero point six nine three, so dividing by the constant gives ten minutes. The two numbers are chosen so the arithmetic comes out cleanly and the relation is obvious. The relation holds only for first order.

Answer: D

A form has to be proposed before it can be tested, and the constant only has meaning once the form is settled. The integral and differential methods are two ways of carrying out the middle step. The proposal has to come before the test.

Answer: A

Slopes are taken from the concentration against time curve and plotted against concentration. It is flexible but sensitive to noise, since differentiating data amplifies scatter considerably. The integral method is less sensitive to noise.

Answer: B

Correct answer: measuring the heat of reaction calorimetrically

The heat of reaction is a thermodynamic quantity fixed by the initial and final states, so it says nothing about how the rate depends on concentration. The other three are genuine methods: comparing half lives at several starting concentrations, testing trial integrated rate laws for the one that linearises the data, and taking slopes of concentration against time to fit the differential form directly.

Answer: C

Only for an elementary step do the coefficients give the order, and most overall reactions proceed through several steps. Order is an experimental result rather than something read off an equation. Only elementary steps are the exception.

Answer: D

Molecularity follows from the mechanism assumed rather than from any measurement that can be made. The other three are all obtained by fitting a model to experimental data, and each carries an uncertainty that a proposed molecularity does not.