Reaction Rates and Equilibrium

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Reaction Rates and
Equilibrium
Chemistry
Chapter 18
Rates of Reaction

The rate of a chemical reaction is usually expressed as
the change in the amount of a reactant per unit of
time.
 Collision theory – particles collide & react if enough
energy
 Activation

energy – minimum energy required to react
What factors affect reaction rates?

The rate of a chemical reaction depends upon temp,
concentration, particle size, and the use of a catalyst.
 A catalyst lowers the activation energy without taking part in
the reaction.
 An inhibitor is a substance that interferes with a catalyst
Reversible Reactions

A reversible reaction is one in which the conversion
of reactants to products and products to reactants
occur simultaneously.
When the forward & reverse reaction are equal the reaction
has reached a state of balance; chemical equilibrium.
 At a chemical equilibrium, no net change occurs in the
amounts of components of the system.

Factors Effecting Equilibrium

Equilibrium position shifts as a result of changing
conditions.
 LeChaterler’s Principle – if stress is applied to a
system in dynamic equilibrium, the system changes in a
way that releases the stress. These stresses include:
 Changes in the concentration of reactants or
products
 Changes in the temp
 Changes in the pressure
LeChaterler’s Principle

Suppose you have an equilibrium
established between four substances A, B, C
and D.
Using Le Chatelier's Principle with a
change of concentration

What would happen if you changed the conditions
by increasing the concentration of A?
Using Le Chatelier's Principle with a
change of pressure


This only applies to reactions involving
gases:
What would happen if you changed the
conditions by increasing the pressure?
Using Le Chatelier's Principle with a
change of temperature

For this, you need to know whether heat
is given out or absorbed during the
reaction.

Assume that our forward reaction is exothermic
(heat is released):
Using Le Chatelier's Principle with a
change of temperature (cont)

What would happen if you changed the
conditions by increasing the temperature?
What would happen if you changed the conditions by increasing the temperature?
Equilibrium Constant

In a general reaction, a mol of reactant A and b mol of
reactant B react to give c mol of product C and d mol of
product D at equilibrium.

The equilibrium constant (Keq) is the ratio of product
concentrations to reactant concentrations at equilibrium, with each
concentration raised to a power equal to the number of moles of
that substance in the balanced chemical equation.
A value of Keq greater than 1 means that
products are favored over reactants; a value of
Keq less than 1 means that reactants are
favored over products.
Solubility Product Constant

The solubility product constant (Ksp), equals the
product of the concentrations of the ions each raised
to a power equal to the coefficient of the ion in the
dissociation equation.

The lower the numerical value, the lower the solubility
of the compound.
The Common Ion Effect

If the product of the concentrations of two ions in
the mixture is greater than the Ksp of the compound
formed from the ions, a precipitate will form.
A precipitate of barium sulfate forms
as barium nitrate (Ba(NO3)2 and
sodium sulfate (Na2SO4) solutions are
mixed.
Applying Concepts - What is the product of
the concentrations of barium ion and sulfate
ion after precipitation is complete?
Free Energy & Spontaneous
Reactions



Excess energy released in a chemical reaction is
referred as free energy as is thus available to do
work.
A spontaneous reaction is one that occurs naturally
and favors the formation of substantial amounts of
products at the specified conditions; also releases
free energy.
A nonspontaneous reaction is one that does not
favor the formation of products at the specified
conditions.
Entropy
 Entropy is a measure of disorder in a system.
 The law of disorder states that the natural tendency of
a system is to move in the direction of maximum
disorder or randomness.
An increase in entropy favors the spontaneous
chemical reaction; decrease favors the nonspontaneous reaction.
Applying Concepts - Which has the greater
entropy, an assembled jigsaw puzzle, or the
pieces in the box?
Enthalpy, Entropy, & Free Energy

The size & direction of enthalpy changes & entropy
changes together determine whether a reaction is
spontaneous; that is, whether it favors products and
releases free energy.
Gibbs Free-Energy

All spontaneous reactions release some energy (“Gibbs” Free
Energy) that becomes avalible to do work.
 Gibbs
Free Energy change is the max. amount of
energy that can be used to do work.. A change in Gibbs
free energy is related to the change in entropy (ΔS) and the
change in enthalpy (ΔH) of the system by the free-energy
equation.
ΔG = ΔH − TΔS (The temperature (T) is in kelvins.)
The numerical value of ΔG is negative in spontaneous
processes because the system loses free energy.
Rate Laws
A→B

The rate at which A forms B can be expressed as the change
in A (ΔA) with time, where concentration A1 is the initial
concentration of A at time t1 and concentration A2 is the
concentration of A at a later time, t2.
Rate Laws (cont)

A rate law is an expression for the rate of a reaction in
terms of the concentration of reactants.

The specific rate constant (k) for a reaction is a
proportionality constant relating the concentration of
reactants to the rate of the reaction.

The value of the specific rate constant, k, is large if the
products from quickly; the value is small if the products
form slowly.
Reaction Mechanisms

Graphs (a reaction progress curve) of the
progress of a reaction show peaks that
correspond to the energies of activated
complexes and valleys that correspond to the
energies of intermediates & products.
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