Chapter 17 Equilibrium
Reversible Reactions
Collision Model
• Chemical reactions occur because of
collisions between reactants.
• In that collision old bonds are broken and
new bonds are formed.
• Not all collisions are effective in causing a
reaction.
Factors That Effect Collisions
• Collision Frequency
– Increase in frequency increases the rate of
the reaction. Increased pressure or increased
temperature.
• Collision Energy
– A minimum kinetic energy is required but too
much energy is also detrimental.
• Collision Geometry
– Molecules must be oriented in the proper
geometry for a reaction to occur.
Factors Effecting Reaction Rate
• Reaction Concentration
– Increasing concentration increases the
chance of collisions and thus speeds up the
reaction
• Reaction Temperature
– Increasing temperature increases particle
kinetic energy which speeds up most
reactions
• Addition of a Catalyst
– A catalyst lowers activation energy and thus
speeds up the reaction.
Role of a Catalyst
Enzymes
• Every raw food contains exactly the right quantities and
types of enzymes necessary to digest that particular
food. For example, fruits high in carbohydrates , such as
apples, contain high amounts of amylase.
• Like Catalysts, they increase the rate of chemical
reactions without themselves being consumed.
• Secondly like Catalysts, they increase reaction rates
without altering the chemical equilibrium between
reactants and products.
Chemical Equilibrium
• Most chemical reactions do not continue until all
of the reactant is used up.
• At first the forward reaction proceeds quickly.
• As the amount of product builds, a reverse
reaction starts to occur as products collide and
form reactants.
• When the forward and reverse reactions occur at
the same rate, equilibrium is established.
• The concentration of products and reactants
remain the same.
Concentration Graphs
Equilibrium Constant
• There is a mathematical value which indicates
when equilibrium is established.
• The Law of Chemical Equilibrium states that
the molar concentrations of the products
divided by the molar concentration of the
reactants equals a constant value, Kc.
• If Kc >> 1 the reaction goes to near
completion.
• If Kc << 1 equilibrium is quickly established in
the system.
Equilibrium Constant
Equilibrium Constant
• For the values plugged into the equation
for Kc:
– Gases are entered as their pressure
– Aqueous solutions are entered as their molar
concentrations
– Liquids and solids are not entered in the
equation as they do not affect equilibria in
heterogeneous systems.