Chemistry- Kinetics & Rate Equations

Question 1

What is a zero order concentration time graph?

Answer 1

IF the order is 0 with resepect to a reactant, then the rate is unaffected by changing the concentration of it.
eg. Rate [A]^0

Question 2

What is a first order concentration time graph?

Answer 2

If the order is 1 with respect to the reactant, then when the reactant concentration doubles so dows the rate.
eg Rate [B]^1

Question 3

WHat is a second order concentration time graph?

Answer 3

IF the oder is 2 with respect to a reactant then when the reactant concentration increases by x2, the rate increases by 2^2 = x4.
Eg. Rate [C]^2

Question 4

What is the general form of rate law for a general cehmical reaction?

Answer 4

Rate = k[A]^m [B]^n
where k= constant
m & n = reaction order.

If any reactant is zero order then it does not appear in the rate equation.

Question 5

What is the overall oder of a rate equation?

Answer 5

Sum of individual orders
eg. rate = k[B]^1 [C]^2
then overall oder = 1+2
= 3

Question 6

How do you find units for the rate equation?

Answer 6

Rate =k[A] [B]^2
k= rate / [A] [B]^2
so units = moldm^-3 s^-1 / (moldm-3)(moldm^-3)^2
=mol^-2 dm^6 s^-1

Question 7

What are the different orders for a reaction?

Answer 7

Order = Effect of each reactant
zero order - Rate unaffected y changing [A]
first order-DIrectly proportional
second order- Rate increases by [A]^2

Question 8

What are the units for a first order, second order and third order.

Answer 8

1st order - s^-1
2nd order - mol^-1 dm^-3 s^-1
3rd order - mol^-2 dm^6 s^-1

Question 9

Define Half life.

Answer 9

The time taken for a variable to fall to half the time taken. t(1/2)
Half life decreases with time for 0 order reactions.
Half life is constant for 1st order reactions.

Question 10

How can you find rate constant using half life?

Answer 10

k= ln(2) / t(1/2)
eg. t(1/2): 3 mins —> 180 seconds
k= ln2 /180 =3.85x10^-3 s^-1

Question 11

Define the rate determining step.

Answer 11

The slowest step in a multi step reaction that determines the overall reaction.

Question 12

Define activation energy.

Answer 12

THe minimum energy required by particles for a collisio between them to result in a reaction.

Question 13

Define kinetics.

Answer 13

Chemical kinetics/reaction kinetics is the study of rates of reactions & factors affect rates.

Question 14

Define first order Reactions.

Answer 14

A reaction is first order with respect to a particular species if the rate of reaction is directly proportional to the concentration of that species.

Question 15

Define colourimetry.

Answer 15

A technique that measures the concentration of a coloured solution by measurig the amount of a specific wavelength of light absorbed by the solution.

Question 16

Define a catalyst.

Answer 16

A substance that speeds up a chemical reaction without being used up in the reaction. It does this by providing an alternative pathway for a reaction to take place with a lower activation energy.

Question 17

Define zero order reactions.

Answer 17

A reaction is zero order to a particular species if the rate of reaction is independent of the concentration of that species.

Question 18

Define second order reactions.

Answer 18

A reation is second order with respect to a particular species if the rate of reaction is proportional to the concentration of that species squared.

Question 19

Define overall oder of reactions

Answer 19

The sum of the powers to which the concentrations have been raised in the equation.

Question 20

Define rae equation.

Answer 20

An equation that relates the concentration of the substrates involved in the reaction to the rate of reaction.

Question 21

Define rate of reaction.

Answer 21

The rate at which reactants are consumed or products are formed in a reaction.

Question 22

What is the rate constant?

Answer 22

The constant of proportinality in the rate equation given the symbol k
Larger k value = faster rate of reaction.

Question 23

What is the effect of temperature of the rate constant?

Answer 23

Increasing the temperature gives more energy to molecules so the rate increases.
Rate = k[A][B][C]
if the rate increases then so does k

Question 24

What is the arrhenius equation?

Answer 24

k=Ae^-Ea/RT
k= Rate constant
Ea= Activation energy
R= Gas constant (8.31)
T= Temperature in kelvin
A= Pre-exponential factor
e= Mathematical constant.

Question 25

Rearrange the arrhenius equation for Ea, A, T

Answer 25

Ea = RT(lnA-lnK)

A= Ke^Ea/RT

T= Ea/ R(lnA-lnK)

Question 26

What is the initial rate of reaction?

Answer 26

Rate at the start when t=0

Question 27

How can a clock reaction give initial rate of reaction and what experiment can be used for this?

Answer 27

Some reactions produce visible changes. TIme take for changes to appear is measured.
Rate = 1/t
- Place a piece of paper with a (X) under reacting vessle. Start reaction and timer until (X) is obscured at this point stop the timer.
Initial rate = 1/t

Question 28

How can you predict a reaction mechanism for RDS (rate determining step) ?

Answer 28

Order tells us number of molecules of each species in RDS.

Only molecules in rate equation are involved in RDS.