CAcT HomePage

# Chemical Kinetics - mechanism and temperature dependence

### The quiz on chemical kinetics covers the following topics

• Calculation of reaction rates from experimental values
• Various factors affecting the rate of reactions
• Differential rate laws
• Calculation of rate constants
• Integrated rate laws
• Calculate rate constants from integrated rate laws
• Calculations related to half-life of a 1st or 2nd order reaction
• Finding order of reactions
• Determination of rate laws from experimental data
• Mathematical and graphic skills for treating experimental data for chemical kinetics
• Calculate activation energy

There are some questions that look similar, but they demands careful work to figure out the answer. See them as a challenge.

Problems in this quiz are demanding. You should be challenged at this time of the term. Be prepared before you try. The practice questions in the DIALOGUE are easier than the problems in Quiz 3.

If you are running the DOS version, press the (Q) key for Quiz or the (Ins) key for some practice.

### Confidence Building Questions

• Assume that the decomposition of CaCO3 in the formation of CaO and CO2 follows the rule of thumb (double the rate at every 10 degree increase in temperature). If the rate at 350 K is 0.012 mol/m, what would the rate be at 400 K?

Hint...
0.012*25 = 0.38 mol/m

Consider...
The rate becomes 0.38 mol/m, 25 (=32) times faster.

• In a reaction the concentration of one of the reactants was monitored. Its values at various times is given below:
```time   0     6    12    18    24    30    36
[A]  3.34  2.34  1.78  1.47  1.24  1.05  0.94 M
```
Find the reaction order with respect to [A]. You will need to do a graph in order to answer this question.

Hint...

Consider...
A lot of work to find the order!

• In a reaction, the concentration of one of the reactants was monitored. Its value at various times is given below:
```time   0     6    12    18    24    30    36
[A]  3.34  2.34  1.78  1.47  1.24  1.05  0.94 M
```
Calculate the reaction rate constant, and enter a value in the form #.#### .

Hint...

Consider...
A rate constant of 0.0208 was used to generate the above data.

• In a first-order dissociation, the concentration of the reactant as a function of time is given below:
```time    0     2     4     6     8     10
[A]   0.70  0.57  0.48  0.40  0.32  0.27 M
```
Calculate the rate constant. (The value is expected to be entered in the form #.####)

Hint...
Calculate a few values, and obtain an average.

Consider...
Calculate the half life.

• In a reaction, the concentration of the only reactant was monitored. Its value at various times is given below:
```time   0     6     12    18    min
[A]  3.34  2.46  1.58  0.69   M
```
What is the order of the reaction. You need to do a graph. Enter a numerical value

Hint...
Plot of [A] vs time gives a straight line. Thus, rate = k

Consider...
There is a zeroth order.

• The reaction rate at 321 K is three times of that at 315 K. Calculate the activation energy in multiples of R, the gas constant.

Hint...
Le r = rate at 315 K, then
ln (3r/r) = -Ea/R (1/321 - 1/315)
Ea = 18514 R

Consider...
Express Ea in terms of R made the calculation simpler.

• For the reaction A + 2 B + C --> D the mechanism is proposed to be:
```step 1   A  +  B  =  X     fast equilibrium
step 2   X  +  C  -->  Y   slow
step 3   Y  +  B  -->  D   very fast
```
Derive the rate law. From your result, what is the order with respect to B?

Hint...
rate = k [A][B][C]
The rate law will change if step 2 is fast, and step 3 is slow.