Rates of reaction

Question 1

Rate of reaction equation

Answer 1

change in concentration / time (mol/dm^3)

Question 2

Zero order

Answer 2

Concentration of the reactant has no effect on the rate.

Question 3

First order

Answer 3

The rate of reaction depends on reactant’s concentration raised to the power of 1.

Question 4

Second order

Answer 4

The rate of reaction depends on reactant’s concentration raised to the power of 2.

Question 5

Rate equation

Answer 5

rate = k [A]^m [B]^n, 
where
k = rate constant
m = order with respect to A
n = order with respect to B

Question 6

How can concentration-time graphs be plotted?

Answer 6

Continuous monitoring (by mass loss or gas collection)
Calorimeter (wavelength of light passing through a coloured solution is controlled using a filter)

Question 7

Orders from concentration-time graph shapes

Answer 7

Zero order = straight line with negative gradient = k

First order = exponential curve with negative gradient

Question 8

Half-life

Answer 8

Time taken for half of a reactant to halve.

Question 9

How to calculate rate constant from half-life

Answer 9

k = ln2/t1/2

Question 10

Orders from rate-concentration graph shapes

Answer 10

Zero order = horizontal straight line with 0 gradient.
First order = straight line graph through the origin as rate is directly proportional to the concentration.
Second order = upward curve with increasing positive gradient

Question 11

How can you find the rate constant k from a second order rate-cone graph?

Answer 11

Plot a second graph of conc^2 against rate, which will result in a straight line that goes through the origin. The gradient of this line = k.

Question 12

Initial rates

Answer 12

Draw a tangent at t=0 and calculate the gradient.

Question 13

What can a clock reaction be used for?

eg iodine clock

Answer 13

Time t from the start of an experiment is measured for a visual change to be observe (eg colour or precipitate)

Question 14

Rate-determining step

Answer 14

Slowest step in the reaction mechanism of a multi

Question 15

Intermediate

Answer 15

Species formed in one step of the mechanism and used up in another step. It can not be seen with the reactants or products in the overall equation.

Question 16

Effect of temperature on the rate constant (how?)

Answer 16

As the temperature increases, the rate constant increases. (Boltzmann’s curve is shifted to the right)
For each 10 degrees C rise in temperature, the rate constant k doubles, therefore the rate of reaction doubles.

Question 17

Arrhenius equation

Answer 17

k = Ae^(-Ea/RT)
where
A = pre-exponential factor (frequency factor - frequency of collisions with the correct orientation)
e^(-Ea/RT) = exponential factor (proportion of molecules that exceed Ea and have enough energy for a reaction to occur)

Question 18

Logarithmic form of Arrhenius equation

Answer 18

ln k = (-Ea/R)(1/T) + ln A

y = mx + c