Characteristics of Chemical Reactions
The rate of a reaction is the change in concentration of a reactant or a product per unit time. Reactions take place when molecules or ions collide. The rate of a reaction increases with an increasing number of effective collisions or collisions that lead to a reaction. The energy necessary for a reaction to take place is the activation energy. Effective collisions are those that have (a) sufficient energy in overcoming the activation energy barrier and (b) the proper spatial orientations of reactants. The lower the activation energy, the faster the reaction. An energy diagram shows the progress of a reaction. The intermediate state when reactants are being transformed into products is referred to as the transition state, which is at the top of the curve tracking the energy changes during a reaction in the energy diagram.
Molecular Collision Theory
Chemical Equilibrium
Many reactions are reversible and eventually reach equilibrium - some slowly, some quickly. At equilibrium, the forward and reverse reactions take place at equal rates, and concentrations do not change. Every equilibrium reaction has an equilibrium constant, Keq, which is defined by its equilibrium expression with concentration terms of reactants and products. Keq does not change when reactant or product concentrations change at a given temperature. But, it does change when the reaction temperature changes. The equilibrium constants are large (Keq>>>1) for reactions yielding large proportions of products and are small (Keq<<<1) for reactions yielding small proportions of products. There is no relationship between K and the rate of a reaction.
Le Chatelier's principle tells us what happens when we put stress on a system in equilibrium. Addition of a component causes the equilibrium to shift to the opposite side. Removal of a component causes the equilibrium to shift to the side the component is taken from. Increasing the temperature drives an exothermic equilibrium to the side of the reactants. Addition of a catalyst has no effect on the position of equlibrium.
THE RATE OF ANY CHEMICAL REACTION DEPENDS ON TWO THINGS: CONCENTRATION TERM(S) AND AN ENERGY TERM.
rate = k[ ][ ][ ]
The energy term is represented by the symbol k;
concentration terms are represented by brackets, [ ], and the number of them will depend on the number of reactants involved in the reaction.
The energy term changes with temperature;
the concentration terms change with concentration of reactant molecules and are commonly expressed in the units of molarity (M) or pressure (P)
rate = k[ ][ ][ ]
The energy term, k, is called the rate constant.
It contains information about the activation energy or the amount of energy a reaction must overcome before a reaction may occur.
The rate constant, k, has the following form: k = Ae-Ea/RT
or ln k = ln A - (Ea/RT)
where Ea = activation energy
A and R are constants; T is absolute temperature, K.
Equilibrium: Reactions that go in both directions
Equilibrium is the situation in which the rate of a chemical reaction in the forward direction equals that in the reverse direction.
A + B <====> C + D
The ratio of product concentrations to reactant concentrations is determined by the equilibrium constant (Keq) which depends on the reaction temperature.
At equilibrium, rateR = rateL
kR [A][B] = kL [C][D]
kR [C][D]
--- = --------
kL [A][B]
kR/kL = Keq = [C][D]/[A][B]
Equilibrium: Reactions that go in both directions
In general, for any chemical reaction such as
aA + bB ------>cC + dD
the corresponding equilibrium constant is
Keq=[C]c[D]d/[A]a[B]b
LeChatelier’s Principle
When a chemical system in equilibrium is subjected to an external stress, the system reacts in a way to relieve this stress.
For example, consider the reaction
A + B-------> C + D
If the concentration of C were increased, which direction would the equilibrium shift?
It would shift to the left because the extra C would react with D and form more A and B.
If some A were removed from the above reaction mixture at equilibrium which direction would the equilibrium shift?
It would shift to the left because C and D would have to react to make A to replace the A removed.
Entropy
The second law of thermodynamics states that a system and its surroudings spontaneously tend toward increasing disorder (randomness). A measure of the randomness of a chemical system is referred to as entropy. A random, or disordered system is characterized by high entropy; a well-ordered system is said to have low entropy.
Often, reactions that are exothermic and whose products are more disordered (higher in entropy) occur spontaneously, while endothermic reactions that produce products of lower entropy are not spontaneous. If they occur at all, they will require external energy input (i.e. heating a mixture of reactants).
Disorder or randomness increases as we proceed from a solid to liquid to the vapor state. Solids often have an ordered crystalline structure, and liquids have a somewhat disordered structure, while gas particles are virtually random in their distribution. The entropy value for solids with a crystalline structure will be smaller than that of amorphous solids.
Free Energy
In chemical reactions, the maximum amount of energy that can be converted to a useful form is called Gibb's free energy (DG). Free energy incorporates the two factors above, the energy factor and the entropy factor, into a single expression for predicting the spontaneity of chemical change:
DG = DH - TDS
In the expression DG is the difference in free energy between products and reactants, DH is the difference in heat energy (enthalpy) between products and reactants, and DS is the difference in entropy between products and reactants. The temperature of the reaction is symbolized by T.
A spontaneous reaction will have a negative DG whereas a non-spontaneous reaction will have a positive DG. A reaction for which DG is positive will only occur if the energy is added to the system.
The Chemical Reaction
If the energy made available by the collision of reacting molecules exceeds the bond energies, the bonds will break, and the resulting atoms will recombine in a lower energy configuration. A collision meeting the above conditions and producing one or more product molecules is referred to as an effective collision. Only effective collisions lead to chemical reaction.
Activation Energy and the Activated Complex
The minimum amount of energy required to produce a chemical reaction is the activation energy for the reaction. As implied above, a large component of the activation energy is the bond energy of the reacting molecules.
The chemical reaction may be represented in terms of the changes in potention energy that occur as a function of the time of the reaction. Several important characteristics of this relationship follow:
1. The reaciton proceeds from reactants to products trhough an extremely unstable intermediate state that we term the activated complex.
2. Formation of the activated complex requires energy. The difference between the energy of the reactants and activated complex is the activation energy.
3. For an exothermic reation, the overall energy change much be a net release of energy. The net release of energy is the difference in energy between the products and the reactants.
Experimental Factors Affecting Reaction Rate
Five major experimental conditions influence the rate of chemical reaction.
1. Structure of the reacting species
2. The concentration of reactants
3. The temperature of reactants
4. The physical state of reactants
5. The presence of a catalyst
A catalyst is a substance that increases the rate of a reaction. If added to a reaction mixture, the catalytic substance undergoes no net change, nor does it alter the outcome of the reaction.
Rate and Reversibility of Reactions.
Many reactions may proceed in either direction, left to right or right to left. The concentration of the various species is fixed at equilibrium because product is being consumed and formed at the same rate. In other words, the reaction continues indefinitely (dynamic) but the concentrations of products and reactants is fixed (equilibrium). This is a dynamic equilibrium.
Chemical Equilibrium
Chemical change, as well as physical change, can attain equilibrium. For example:
H2(g) + I2(g) <=> 2HI(g)
At equilibrium, the rate of disappearance of H2 and I2 is equal to the rate of formation of H2 and I2. The rates of the forward and reverse reactions are equal and the concentrations of H2, I2 and HI are fixed. The process is dynamic, with a continuous interconversion of products and reactants. We represent this process as:
Keq = [H2] [I2]
[HI2]
Effect of Concentration
Addition of extra product or reactant to a fixed reaction volume is just another way of saying that we increased the concentration of product and reactant. Removal of material from a fixed volume decreases the concentration. Therefore changing the concentration of one or more components of a reaction mixture is a way to alter the equilibrium composition of an equilibrium mixture.
Effect of Heat
The change in equilibrium composition caused by the addition or removal of heat from an equilibrium mixture can be explained by treating heat as a product or reactant. Adding heat to an exothermic reation is similar to increasing the amount of product, shifting the equalibrium to the left. Removing heat from a exothermic reaction shifts the equilibrium to the right.
Heat is a reactant in an endothermic reaction; its removal shifts the equilibrium to the left. Adding heat favors product formation.
Effect of Pressure
Only gases are affected significantly by changes in pressure because gases are free to expand and compress in accordance with Boyle's law. However, liquids and solids are not compressible, so their volumes are unaffected by pressure.
Therefore pressure changes will alter equilibrium composition only when they involve gass or variety of gases as products and/or reactants. If the reaction shifts to conserve volume:
If the number of moles of gaseous product and reactant is identical, pressure will have no effect on the equilibrium because there is no volume advantage.
Effect of a Catalyst
A catalyst has no effect on the equilibrium composition. A catalyst increases the rates of both forward and reverse reactions to the same extent. The equilibrium composition and equilibrium concentration do not change when a catalyst is used.