DGP Kinetics

Question 1

Rate is always _

Answer 1

POSITIVE

Question 2

What is the only way the rate law can be determined?

Answer 2

EXPERIMENTALLY

Question 3

In a first order reaction, if you halve the reactant concentration, the rate _

Answer 3

HALVES
In a first order reaction the rate is directly proportional to the concentration of the reactant
i.e. rate ∝ [A]

Question 4

In a second order reaction, if you halve the reactant concentration, the rate _

Answer 4

QUARTERS
This is because: rate ∝ [A]^2

Question 5

Differential rate law provides rate at a _

Answer 5

particular instant where the reaction mixture has a particular composition

Question 6

Integrated rate law is an _

Answer 6

expression that gives the concentration of a species as a function of time. It can also be used to establish the order of a reaction and then it’s rate constant, k.

Question 7

What are the two methods of determining the rate law from experimental data?

Answer 7

• Using the initial rate method (differential rate law, i.e. rate vs. concentration data)
• Using the method of integration (integrated rate law, i.e. concentration vs. time data)

Question 8

Steps in method of integration for finding the rate law

Answer 8

1. Postulate an order of the reaction (first, second, …)
2. Derive an expression for the variation of concentration vs. time (i.e. integrated rate law)
3. Manipulate expression to linear form (i.e. linearise integrated rate law using ln)
4. Plot data to check if original postulate is correct (know the concentration vs. time graph shapes for zero, first, and second order reactions_
5. If correct => derive k

(If not correct return to step 1)

Question 9

What method do you use to find the rate law when you have a reaction in the form A + B -> products?

Answer 9

Method of isolation:
Essentially use two repeats of the experiment where each is alternatively in excess to determine a “pseudo-order of the reaction for one of the reactants. Then you plot the ln[A] graph for each to confirm).

Question 10

What does the “A” mean in the Arrhenius equation?

Answer 10

A = Arrhenius factor or pre-exponential factor. It has the same units as k (the rate constant) and is a measure of frequency of collisions in the correct orientation for reaction.

Question 11

A high Ea (activation energy) VALUE corresponds to a reaction is very _ to _

Answer 11

very SENSITIVE to CHANGES OF TEMPERATURE

Question 12

Steps to determine Ea and A for a reaction

Answer 12

1) Convert T from celsius to Kelvin (+273.15)
2) Calculate ln(k)
3) Calculate 1/T from absolute T
4) Plot lnk vs. 1/T
5) Calculate slope = -Ea/R
6) Calculate A from y-intercept (0, lnA) or using Ea determined from data and two sets of (K, T) data points: