Kinetics

Question 1

What factors affect rates of reaction?

Answer 1

Rates of reaction vary lots
and depend on different factors.
For example:
• Cookies bake faster at higher temperatures. (temperature).
• The body cant adequately digest lactose without catalytic assistance of lactase enzyme (catalyst).
• Theres risk of explosion in factories when handing fine combustible powders such as custard powder (SA).
• By adjusting concs of two chemicals inside, manufacturers produce glow sticks that either glow bright for a short amount of time
or dimly for longer length of time (conc of reactants).

Temperature, catalysts, surface area, pressure and concentration all affect the rate of these different chemical reactions.

Question 2

Whats collision theory (concentration, pressure, temperature)

Answer 2

Most collisions are unsuccessful.
> Only a small proportion are successful and cause a reaction.
> Collision theory explains main factors wch affect rate of a chemical reaction.

1 Concentration:
• If conc of a reactant is increased
• More particles of reactant are present
• Leads to more successful collisions between reactant particles in a given period of time
• Causes the rate of reaction to increase.

2 Pressure:
• If pressure on gaseous reaction system increases
• The particles are forced closer together
• Leads to more successful collisions between reactant particles in given period of time
• Causes rate of reaction to increase.

3 Temperature:
• If temperature increases, particles gain energy and move faster
• Leads to more frequent and successful collisions (collisions with energy greater than activation energy)
• between reactant particles in given period of time
• Causes rate of reaction to increase

Question 3

What is rate of reaction and activation energy

Answer 3

The rate of reaction is the change in concentration of a reactant/product
in a given period of time.

The activation energy is the minimum amount of energy
which the reacting particles require for a successful collision.

Question 4

Whats collision theory (SA, catalyst presence)

Answer 4

4 Surface area of solid reactants:
• Increasing surface area of solid reactants (by grinding them up)
• increases exposed surface of reactants.
• This increases number of successful collisions in given period of time
• wch increases rate of the reaction.

5 Presence of a catalyst:
• A catalyst increases rate of reaction without being used up.
• A catalyst works by providing an alternative reaction pathway of lower activation energy.
• By lowering activation energy, more collisions are successful in given period of time
• and so rate of reaction increases.

Catalysts are important in many industrial processes
by speeding up process and so reducing costs.

Question 5

What are Enthalpy profile diagrams

Answer 5

Enthalpy or reaction profile diagrams were examined
- for an exothermic and an endothermic reaction.
- These diagrams also help to explain the definition of a catalyst.

The catalyst simply provides an alternative reaction pathway of lower activation energy.
You may have to interpret differences between the four levels on this diagram.

** Diagram 1**

Question 6

What is the Distribution of molecular energies

Answer 6

• A Maxwell-Boltzmann distribution is a plot of number of gaseous molecules
• against energy they have at a fixed temperature.
  > is also called a molecular energy distribution graph
  > or a distribution of molecular energies.
  >
  A single plot on the graph shows
  distribution of molecular energies at a constant temperature.

It should appear as a roughly normal distribution
- which is asymptotic to horizontal axis (gets closer but never touches the axis).
- The main points from a Maxwell-Boltzmann distribution at constant temperature are shown…
** diagram 2**

Question 7

Hows Activation energy shown on a Maxwell-Boltzmann distribution

Answer 7

As the Maxwell-Boltzmann Distribution represents energy of reactant molecules
- there will be an energy value on x-axis wch is the activation energy

• A catalyst increases rate of reaction by providing an alternative reaction route with a lower activation energy.
• Adding catalyst lowers value of activation energy, as can be seen from position of activation energy on both distributions.
• The area above activation energy on the second distribution is greater.
  > This indicates there are more molecules with sufficient energy to undergos successful collision.
  >This increases the rate of the reaction.
  ** diagram 3**

A catalyst doesnt affect shape of distribution as long as
temperature and total number of molecules are not changed.

Question 8

Whats the Maxwell-Boltzmann distribution at different temperatures

Answer 8

When the temperature of reactant molecules is increased,
this will increase energy of gaseous reactant molecules.
&raquo_space; will change the shape of the Maxwell-Boltzmann distribution.

You will be asked to sketch a distribution for same sample of gas
at a lower or higher temperature.
• Lower temp distributions are moved to left and peak is higher.
• Higher temperature distributions are moved to right and peak is lower.
> The graphs should only cross once.
•
• The curves should always start at the origin
and shod end up being asymptotic to energy axis at higher energy values.

diagram 4

..
- If comparing shaded areas above activation energies, at the higher temp,
- there are more reactant molecules with enough energy for a successful collision.
> explains why theres a higher rate of reaction at higher temperature.
>
> the shaded area is significantly larger at 320 K than at 300 K.
> A small increase in temp causes a large increase in rate of reaction
> as theres a significant increase in number of molecules
> w enough energy for successful collision.

Question 9

Maxwell boltzmann distribution at different concentrations

Answer 9

If the conc of reactant molecules is increased,
> the shape of a Maxwell-Boltzmann distribution will again change.

The curve retains the same basic shape.
- This means the most likely energy of the molecules remains the same
- so peak should be higher but at same energy value on horizontal axis.
-
- As are more total reactant molecules at same temp, overall area under curve increases.
> increases number of reactant molecules
> wch have enough energy for a successful reaction.
> leads to a higher rate of reaction.

diagram 5

..
If comparing shaded areas above the activation energy,
- can be seen at higher concs there are only a few more reactant molecules
- with enough energy to undergo a successful collision.
-
- It does explain why rate of reaction increases at higher concs
- but effect is much less significant than when increasing temp.
- Small increases in temp increase rate of reaction more significantly than conc.