Chemical Kinetics II

Question 1

Complex Reactions
aA + bB > products

Answer 1

Mechanism involved a series of steps (elementary reactions)

Overall Rate Equation:Cannot be deduced from stoichiometric equation

k = overall rate constant

m & n = orders of the reaction with respect to A and B (can be 0,1,2 . . . Can even be fractions). The overall order of the reaction is (m + n)

Question 2

Complex reactions - exceptions:

Answer 2

The stoichiometric coefficients do not appear in the rate equation

Sometimes m or n (or both) might be the same as the values in the chemical equation, but this is coincidence only

A substance not in the chemical equation can be in the rate equation e.g. catalyst, product of reaction

Question 3

Does the concentration of reactant affect the overall rate of the reaction?

Answer 3

No, unaffected

Question 4

m = 0, [A]0 = 1 and so _______ is not in the rate equation

Answer 4

concentration

Question 5

The _____ _____ of the reactant in a zero order reaction is proportional to the initial concentration and inversely proportional to the rate constant. It is not constant.

Answer 5

half life

Question 6

What does each letter mean in The Arrhenius Equation?

Answer 6

Constant A = Arrhenius Factor (or pre-exponential factor). It has the same units as k.

The value of k increases exponentially as temperature increases

Experimental observations tell us that the reaction rate approximately doubles every 10 ⁰C

A plot of ln k vs 1/T is a straight line
y = mx + c
ln A = y intercept
-Ea / R = m (the slope)

The rate constant k , and therefore the rate of reaction, increase with increasing temperature and decrease with increasing Ea

Measuring k at different temperatures allows for the determination of Ea for the reaction

Question 7

What is the importance of Ea (activation energy)

Answer 7

Value of Ea determines how sensitive the reaction is to changes in T

Higher Ea = more sensitive to T

If Ea = 0, rate constant (k) is independent of T

If k1 is known at a particular T1 and Ea is known, then k2 at any other T2 can be calculated

Ea can also be determined by measuring k at two different T

Question 8

first order reaction units of k

Answer 8

s^-1

Question 9

2nd order reactions units of k

Answer 9

dm^3 mol^1 sec^1

Question 10

third order reaction units of k

Answer 10

dm^6 mol^-2 sec^-1

Question 11

zero order reaction units of k

Answer 11

mol dm^-3 sec^-1

Question 12

Reaction Theory meaning

Answer 12

Based on a model of how a reaction occurs at a molecular level - Collision Theory and Transition State Theory

Question 13

What is Collision Theory?

Answer 13

Reactant molecules behave like hard spheres. Collisions between the spheres may result in a reaction

Question 14

Transition State Theory

Answer 14

Assumes the reactants form a transition state at the maximum on the energy profile – more sophisticated theory

Question 15

how does collision theory work?

Answer 15

1) two molecules must collide in the correct orientation with particular min amount to break existing bonds

2) energy profiles used in collision theory represent the potential energy of twi molecules as they approach then collide and react to from products

Question 16

Determinations of collision theory?

Answer 16

Only molecules with kinetic energy greater than the Ea will react
There are more of these molecules at T2 than T1
As T increases, Ea/RT decreases and approaches zero, so e-Ea/RT approaches 1

Question 17

Transition state theory

Answer 17

Transition State: Highest point on energy profile

Ea: Activation Energy, difference between reactants and transition state
∆rU: Internal energy change

Ea (reverse) > Ea (forward) because the reverse reaction is endothermic

Exothermic Reaction: products are at a lower potential energy than the reactants

Question 18

what is transition state theory?

Answer 18

A pre-equilibrium is established between reactants and the transition state, ABCŦ, which then goes on to form the products

Question 19

Gibbs energy of activation

Answer 19

Change in Gibbs energy for the formation of the transition state

Question 20

Transition state theory shows that the rate constant (k) depends on the Gibbs energy of activation ∆GŦ.
Transition state theory shows that the rate constant (k) depends on;

Answer 20

Gibbs energy of activation ∆GŦ.

Question 21

Explain what a catalyst is;

Answer 21

substance that changes the rate of a reaction without being consumed in the reaction

Question 22

Catalysis process&raquo_space;>

Answer 22

Does not appear in the overall chemical equation for the reaction

Can appear in rate equations

Does not affect the amount of product formed

Not changed chemically (can be physically altered) so can be recovered at the end of the reaction

Negative catalysts / inhibitors slow down reaction rates

Question 23

What is the advantage of catalysis?

Answer 23

The catalyst provides an alternative pathway for the reaction with a lower Gibbs activation energy for the rate-determining step

Question 24

The process of catalysis lowers the activation energy. TRUE or FLASE?

Answer 24

FALSE
It does not lower the activation energy

Question 25

At a given ___, the rate constant is greater for the catalysed reaction, so the reaction is faster

Answer 25

T = temperature

Question 26

____ ______ reaction has a single transition state, the catalysed reaction involves the formation of an intermediate

Answer 26

Non-catalysed

Question 27

What is Heterogeneous Catalysis?

Answer 27

Different physical state than the reactants
Products and any unreacted starting materials are easily separated from the catalyst

Question 28

What is Homogeneous Catalysis?

Answer 28

Same physical state as the reactants
More difficult to recover catalyst potentially
Enzyme catalyses reactions in cells take place in aqueous solution and involve homogeneous catalysis

Question 29

Enzymes: what are they?

Answer 29

Protein molecules that catalyse a specific reaction and remain unchanged by the reactions they catalyse

They catalyse a wide range of biochemical reactions (37 ⁰C – Human Body). Processes would happen much to slowly to sustain life without enzymes.

Lock and key model

Question 30

Enzyme-Catalysed Reactions

Answer 30

Just like with other catalysts, enzymes provide an alternative pathway with a lower activation energy for the rate-determining step