Kinetics 1

Question 1

What is the rate and how is it calculated?

Answer 1

The rate is the amount of reactant used up or the amount of product formed in a certain time.

(initial concentration - final concentration)/ time

Question 2

How do you calculate instantenous rate?

Answer 2

You draw the tangent at a time where you want to find the rate. You find the gradient of the tangent.

Question 3

How are differential rate equations written for reactants and products?

Answer 3

-d[reactant]/dt reactant is used up so it is negative

d[product]/dt product is formed so positive

Question 4

Give me 3 factors which affect rate of reaction and give a reason why.

Answer 4

• concentration- as we increase concentration, we increase number of molecules in a given volume, more successful collisions will take place and reaction will be faster.
• temparature- it increases kinetic energy of particles so more particles will have energy greater than activation energy. This means that more collisions will take placce in a given time and reaction will be faster.
• pressure- similar reasoning to concentration but applies to gases

Question 5

What is a complex reaction?

Answer 5

A complex reaction is an overall reaction that consists of many elementary reactions (where two molecules or atoms are involved). They add up to give this overall reaction.

Question 6

What is an elementary reaction and what are its characteristics?

Answer 6

Elementary reaction is reaction where only 2 molecules or atoms are involved and it shows what happens to reactants at a molecular level. Furthermore, stoichiometry equals to the reaction order in rate equation for each reactant.

Question 7

For an elementary reaction:

A+B⇒C+D

Write a rate equation. What does it show?

Answer 7

rate=k[A][B]

stoichiometry equals to the reaction order for each reactant. Overall order is 2.

k is a constant at a given temperature and it relates to how quickly the reaction proceeds.

rate is dependent on concentration of 2 reactants.

Question 8

How are units of k calculated?

Answer 8

They are calculated from the rate equation.

Question 9

How to work out the order of a complex reaction?

Answer 9

With elementary reaction, it is easy but for a complex reaction it is much more advanced.

• You isolate one reactant by keeping it in large excess. This will mean that it doesn’t change during the reaction so you only analyse how rate depends on one reactant.
• You vary initial concentration of your reactant and then plot a concentration versus time graph for each concentration.
• You find initial concentration by working out the gradient.
• from equation: log(rate)=logk + xlog[A] you plot your rates against corresponding concentrations
• the gradient of that straight line is your order with respect to [A].

Question 10

Give intergral rate equation and half life equation for:

0th order reaction

1st order reaction

2nd order reaction

Answer 10

Question 11

What is a different way to work out the order, other than initial rates method?

Answer 11

By plotting our intergral rate equations on a straight line graph. If it gives the straight line, it obeys the equation and predicts order of the reaction for us.

Question 12

What is half life?

Answer 12

half life is the time taken for concentration of the reactant to decrease by a half.

Question 13

How does the half life vary for:

0th order reaction

1st order reaction

2nd order reaction

Answer 13

0th order reaction as initial concentration gets smaller, half life becomes shorter

1st order reaction half life stays constant, regardless of concentration.

2nd order reaction half life gets larger as we decrease initial concentration.

Question 14

What happens to k as we increase temparature?

Answer 14

k increases exponentially with increasing temperature. This means that rate becomes higher too and reaction gets faster.

Question 15

What is arrhenius equation?

Answer 15

Question 16

How do we find values for Ea and A from arrhenius equation?

Answer 16

We calculate k for different temparatures and we plot a graph of lnk against 1/T.

The gradient of this graph is -Ea/R.

and y intercept equals to lnA

Question 17

What is the equation that we can use to find k at a different temparature if we know the k at one temparature and activation energy for a reaction?

Answer 17

lnk’=lnk- Ea/R(1/T’ - 1/T )

Question 18

What are characteristics of Boltzmann distribution?

Answer 18

• asymptotic, values for energy never reach 0
• none molecules foud at 0K
• most molecules have energy= most probable energy
• mean energy slightly to the right
• only small proportion of molecules have either small or high energy
• molecules to the right of Ea are able to react
• at high temperatures, graph will shift to the right and it will be lower than at lower temperature.

Question 19

What is activation energy?

Answer 19

Activation energy is the minimum energy that a molecule must possess to break pre-existing bonds and create new bonds.

Question 20

What does collision theoy state?

Answer 20

Only molecules with kinetic energy greater than activation energy will be able to react. They must have the right orientation to react as well.

Question 21

What is an exothermic reaction?

Answer 21

In exothermic reaction, energy is released to surroundings. Stronger bonds are formed than broken so more energy is released than absorbed. Overall, energy of the system is lowered and stabilised.

Question 22

What is an endothermic reaction?

Answer 22

In endothermic reaction, energy is absorbed from the surroundings, lowering their temperature. Weaker bonds are formed than are broken which is associated with greater energy absorbed than released. Products end up higher in energy than reactants so they are more unstable.

Question 23

What does each term in Arrhenius equation relate to?

Answer 23

A- pre exponential factor

A= steric factor x k collision

k collision is the frequency of all collisions. Steric factor takes into account only proportion of these collisions which have the right orientation.

e term

this takes into account only a fraction of these collisions which have energy greater than activation energy.

So arrhenius equation is a way of calculating k and rate of reaction, by taking into account only collisions which are succesful.