Unit 9 Reaction Rates

  1. Reaction Rates. Some chemical reactions go fast, some slowly, some reactions go to completion, some seem to get stuck halfway, and still others do not seem to occur at all. This unit is about the factors that control the speed and outcome of a reaction.
  2. What is the reaction rate?To determine the speed or rate of a reaction, you would measure the in concentration over time, or the in concentration over time:

rate =decrease in reactant concentration

time interval

or

rate = increase in product concentration

time interval

  1. Anything that would the number of collisions would be likely to increase the rate of the reaction.This is called, theory of reaction rates. From collision theory, you predict that increasing concentration increases reaction rate. (more collisions)
  2. increasing the of reactant molecules in the mixture (increasing concentration)
  3. increasing the of a solid reacting with a liquid or gas
  4. to increase contact between a solid and a liquid
  5. The overall speed of the reaction is determined by the slowest step, called the . For the reaction of NO2 and CO, themechanism that has been suggested is

(1) NO2 + NO2 → NO3 + NO slow

(2) NO3 + CO → NO2 + CO fast

D.The is involved in the reaction mechanism but is not changed in the overall reaction.

Reaction can proceed at a lower temperature than would be required for the uncatalyzed reaction mechanism.

  1. Reversibility of Reactions.
  2. In a reaction the reactants react to form products and the products react to form reactants until a state of equilibrium is reached.

1.Most chemical reactions are .

  1. Equilibrium is indicated by a .
  2. When a reaction begins there is no reverse reaction since there are no products NaOH + HCl
  3. The reverse reaction begins and the forward reaction decreases. When the forward reaction and the reverse are constant equilibrium is reached. The rate of the forward reaction is equal to the reverse reaction.

NaOH + HClNaCl + H2O

NaOH + HCl NaCl + H2O

  1. There are reactions

Examples: Cooking an egg. This denatures the protein in the egg and cannot be undone by cooling.

  1. When has not been set the arrows can be different lengths.

Forward arrow can be longer than the reverse arrow.

  1. The size of the K tells the and the position of the equilibrium when compared to similar reactions. The larger the K value the more are formed.

K< 1 more

K = 1 products = reactants

K > 1 more

  1. Equilibrium constant for a reaction is K.

aA + bBcC + dD

K= [C]c [D]d

[A]a [B]b

K= [products]

[reactants]

Examples

CO(g) + Cl2(g) COCl2

K= [COCl2]

[CO] [Cl2]

1. are left out of the equilibrium equation

CaCO3(s)CaO(s) + CO2(g)

K = [CO2]

N2(g) + 3H2(g) 2NH3(g)

K = [NH3]2

[N2] [H2]3

If you are given the concentrations [NH3] = 1.0M, [N2] = 0.05M, and [H2] = 0.05M, what is the K?

K=

Does it favor reactants or products?

  1. Le Châtelier's Principle
  2. Le Châtelier's Principle guides us in predicting the effect various changes in conditions have on the position of equilibrium.
  3. a reactant will cause the reaction to compensate for the addition by forming more products. Shift to the .
  4. a reactant willcause the reaction tocompensate for the reduction by forming fewer products. Shift to the .
  5. a product will cause the reaction to form more products to compensate for the loss.Shift to the .
  6. a product will cause the reaction to form fewer products to compensate for the loss.Shift to the .
  7. Equilibrium shifts away from side with added chemicals or toward side with removed chemicals.
  8. Remember, adding more of a solid or liquid does not change its concentration – and therefore has no effect on the equilibrium
  9. the size of the container increases the concentration of all the gases in the container, when the volume , the equilibrium shifts to the side with fewer gas molecules.

4 Fe (g) + 3 O2 (g) → 2 Fe2O3 (g)

7 moles of gas2 moles of gas

Decreasing the volume will shift to the right.

  1. the size of the container decreases the concentration of all the gases in the container, when the volume , the equilibrium shifts to the side with more gas molecules
  1. Practice

The reaction 2 SO2(g) + O2(g) 2 SO3(g) with H° = -198 kJ is at equilibrium. How will each of the following changes affect the equilibrium concentrations of each gas once equilibrium is re-established?

adding more O2 to the container

condensing and removing SO3

compressing the gases

cooling the container

doubling the volume of the container

warming the mixture

adding the inert gas helium to the container

adding a catalyst to the mixture