Download Chemical Equilibrium

Chemical Equilibrium
A. Concentration Units: The concentration of a solution defines the amount of substance in a given
volume or mass of the mixture. Two of the most common concentration units are:
1. Molarity (M): molarity is defined as the number of moles of solute dissolved in 1 liter of
solution.
Molarity (M) = moles of solute/1 liter of solution
(solution = solute + solvent)
2. Molality (m): molality is defined as the number of moles of solute dissolved in 1 kilogram of
solvent.
Molality (m) = moles of solute/1 kilogram of solvent
B. Reversible Reactions: Most chemical reactions are reversible, meaning that they can run in both
directions: from reactants to products and from products to reactants. This is represented by a
double arrow between reactants and products in a chemical equation.
aA + bB  cC+ dD
1. When a reaction begins, the forward rate of reaction is greater than the reverse rate.
2. Over time, as the reactants are used up and products accumulate, the rate of the reverse
reaction increases and becomes equal to the rate of the forward reaction (i.e. reactants and
products are being formed at the same rate). At this point the reaction is said to be in
equilibrium.
C. Equilibrium – a condition in which two opposing physical or chemical changes occur at equal rates in
the same closed system. This does not mean that the concentrations of products and reactants are
equal.
D. Equilibrium Constant: A constant used to determine the relative concentrations (moles/liter) of
reactants and products for a reaction at equilibrium. The equilibrium constant (Keq) for the given
reaction is:
aA + bB  cC+ dD
Keq = products = CcDd
reactants AaBb
Steps to Determine the Equilibrium Constant (Keq) of a Reaction:
1. In the equilibrium constant Keq, the products are in the numerator and the reactants are in the
denominator.
2. The coefficient of each substance becomes the exponent.
3. Solids and pure liquids are not part of the equilibrium constant. They should be thought of as
having a value of one and left out of the equation. All solids and pure liquids that are reactants
or products in reactions are indicated by (s) and (l), respectively.
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4. Aqueous dissolved particles are part of the equilibrium constant. Aqueous dissolved particles
are indicated by (aq).
5. If all the reactants and products in a reaction are gases, partial pressures may be used instead of
molar concentrations to calculate Keq (Kp).
Interpretation of Equilibrium Constants:
aA + bB  cC+ dD
Keq = CcDd
AaBb
1. Keq is always positive since concentrations of reactants and products can never be negative.
2. If Keq 1, this indicates that equilibrium favors the formation of products in a forward
reaction: i.e. the concentration of products is higher than the concentration of reactants (C and
D are formed in greater concentrations).
3. If Keq 1, this indicates that equilibrium favors the reverse reaction: i.e. the concentration of
reactants is higher than the concentration of products. (A and B are formed in greater
concentrations)
Le Chatelier’s Principle: If an external stress (change of condition) is applied to a system at equilibrium,
the position of the equilibrium will shift in a direction that reduces the stress (change).
2CO(g) + O2(g)  2CO2(g)
External stress includes changes in concentration, pressure, and temperature.
Stress 1: Changes in Concentration
If the concentration of a reactant is increased (more added) or if the concentration of a product is
decreased (some is removed), the reaction shifts to the right (forward reaction is favored) and a new
equilibrium is established having more products.
If the concentration of a reactant is decreased (some is removed) or if the concentration of a product is
increased (more is added), the reaction shifts to the left (reverse reaction is favored) and a new
equilibrium is established having more reactants.
Changes in concentration do not change the equilibrium constant.
Stress 2: Changes in Pressure
A change in pressure on a system affects the system only if the products and reactants are in the gaseous
phase.
An increase in pressure favors the direction that produces a smaller number of moles of gas (smaller
volume).
A decrease in pressure favors the direction that produces a larger number of moles of gas (larger
volume).
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A change in pressure has no effect on the equilibrium if the number of moles (or volumes) of gaseous
products equals the number of moles (or volumes) of gaseous reactants.
A change in pressure does not change the value of the equilibrium constant.
Stress 3: Change in Temperature
H2(g) + Br2(g)  2HBr2 + heat
An increase in temperature favors the endothermic reaction (forward reaction) and a decrease in
temperature favors the exothermic reaction (reverse reaction).
Consider heat as being a product (exothermic) or a reactant (endothermic): when the temperature is
increased in an exothermic reaction, the concentration of heat is increased on the product side and the
equilibrium shifts to the left (reverse reaction) favoring the reactants.
When the temperature is increased in an endothermic reaction, the concentration of heat is increased on
the reactant side and the equilibrium shifts to the right (forward reaction) favoring the products.
A change in temperature does change the value of the equilibrium constant.
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