Chemical Reactions, Stoichiometry, and Chemical kinetics
2. CHEMICAL KINETICS (Rates of Reactions)
The branch of chemistry that deals with the study of reaction rates is known as Chemical Kinetics or Reaction Kinetics.
These include:
· Rate of reaction
· Mechanism/sequence of steps by which a reaction occurs
· Factors influencing the rate of reaction
Rates of reaction (ROR) are a measure of how fast a reaction will occur.
The rate of a chemical reaction is the number of moles of reactants converted or products formed per unit time.
Usually, the rate of reaction is determined experimentally by measuring the change in concentration of one
of the components in the reaction with time.
Thus,
Rate of reaction = Change in concentration of reactant or product (mol/dm3)
Time taken for the change (seconds)
Consider the reaction: A → B
Rate R = -d[A]/dt = d[B]/dt or -∆[A]/∆t = ∆[B]/∆t
where ∆ = change, t= time, A = reactant, B = product
The unit of the rate of reaction is mol/dm-3S-1 or gdm-3S-1.
The rate of reaction can also be expressed as:
Rate of reaction = Change in number of moles or mass of reactant or product
Time taken for the change
Then the unit of rate is molS-1 or gS-1
EXAMPLES:
1. When 0.5g of calcium trioxocarbonate (IV) was added to excess dilute hydrochloric acid, carbon (IV) oxide was evolved. The complete reaction took 5 minutes. What was the rate of reaction?
SOLUTION:
Rate of reaction = Change in number of moles or mass of reactant or product
Time taken for the change
Rate of reaction = (0.5-0) g = 1.67 x 10-3 gS-1
(5 x 60) S
2. Consider the following reaction:
H2O2 (aq) + 3I– (aq) + 2H+ (aq) → I3– (aq) + 2H2O (l)
In the first 10.0 seconds of the reaction, the concentration of I– dropped from 1.000 M to 0.868 M.
Calculate the average rate of this reaction in this time interval.
Solution:
Average Rate = −Δ[Reactant]
Δt
Given Data:
- Initial concentration of I⁻: 1.000 M
- Final concentration of I⁻: 0.868 M
- Time interval: 10.0 seconds
Eqn: H2O2 (aq) + 3I– (aq) + 2H+ (aq) → I3– (aq) + 2H2O (l)
Rate I− = −Δ[I−] = - (-0.132M) = 0.0132 M/s
Δt (10s)
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COLLISION THEORY
Reactions happen when molecules, elements, or atoms collide with the proper orientation and sufficient energy, but not all collisions result in a reaction.
The collision theory states that for a chemical reaction to occur, the reactant particles must collide and they must collide with a certain minimum amount of energy known as activation energy.
Collisions, which result in chemical reactions, are called EFFECTIVE COLLISIONS. The minimum amount of energy required by reacting particles for a chemical reaction to occur is called ACTIVATION ENERGY. Activation energy is the ENERGY BARRIER that the reactants must overcome for the reaction to occur. It is the minimum energy required for bond breaking for a chemical reaction to occur.
Chemical reactions occur only when the energy of the colliding reactant particles is equal to or greater than the activation energy. Activation energy must be equal to the energy barriers, also for a chemical reaction to occur.
Note: Every reaction has its energy of activation. Reactions with low activation energy have a high rate of reaction and occur spontaneously. Reactions with high activation energy have a low rate of reaction and are not spontaneous.
FACTORS AFFECTING THE RATE OF REACTION
From the collision theory, it can be seen that the rates of reaction depend on the following features.
1. The energy of the particle.
2. The frequency of collision of the reaction.
3. The activation energy of the reaction.
These features of a chemical reaction are, in turn, affected by some factors, which can cause them to change and consequently affect the rate of reaction. These are factors that affect the rate of reactions.
Some important ones are:
1. Nature of reactants.
2. Concentration/pressure (for gases) of reactants.
3. Surface area of reactants
4. Temperature of the reaction mixture
5. Presence of light
6. Presence of catalysts
To study the effect of any one of these factors on the rate of reaction, all other factors must be kept constant.
EFFECT OF NATURE OF REACTANTS
If all other factors are kept constant, different substances will have different rates of reaction with dilute HCl, for example. When dilute HCl reacts with zinc, iron, and gold under the same conditions, hydrogen gas is evolved fast with zinc, slow with iron, and no gas is evolved with gold.
The difference in rate of reaction is due to the chemical nature of the elements as they naturally possesses a different amount of energy content.
EFFECT OF CONCENTRATION OF REACTANTS
The frequency of collision among particles is high when the particles are crowded in a small space, i.e high concentration. This leads to highly effective collision and thus a high rate of reaction. An increase or decrease in the concentration of the reactants will result in a corresponding increase or decrease in effective collisions of the reactants and hence the reaction rate.
EFFECT OF SURFACE AREA OF REACTANTS
This is a very important factor to be considered when a solid is involved in a chemical reaction. Lumped solids offer a small surface area of contact for reaction, while powdered solids offer a large surface area for reaction. The rate of reaction is slow with lumped solids but high with powdered solids.
EFFECT OF TEMPERATURE
Increasing the temperature of a system can lead to an increase in reaction rate in two ways. When heat is raised, energy in the form of heat is supplied to the reactant particles, so that
1. The number of particles with energy equal to or greater than the activation energy increases.
2. The velocity of all the reactant particles increases due to the greater kinetic energy, leading to
a higher frequency of collision.
As a result, the number of effective collisions increases and the reaction proceeds at a faster rate. Decreases in temperature lead to a decrease rate of reactions.
EFFECT OF LIGHT
Some reactions are influenced by light. The rate of reaction is high when the light intensity is high, low when the intensity is low and does not proceed at all in the absence of light. Such reactions are known as photochemical reactions. Examples of photochemical reactions include.
1. Reaction between hydrogen and chlorine and
2. Decomposition of hydrogen peroxide
3. Reactions between methane and chlorine
4. Photosynthesis in plants
5. Conversion of silver halides to grey metallic silver
EFFECT OF CATALYST
A Catalyst is a substance which alters the rate of a reaction, but itself does not undergo any change at the end of the reaction.
A positive catalyst increases the rate of reaction by lowering the activation energy of the reaction whereas, the one which increases the activation energy is known as a negative catalyst or an inhibitor.
RATE LAW
The rate law (also called the rate equation) expresses the rate of a reaction as a function of the concentration of reactants, each raised to a power called the order of the reaction. Rate law tries to explain the dependence of reaction rate on concentration.
For a general reaction:
aA + bB → Products
The rate law is:
Rate = k[A]m[B]n
Rate = Reaction rate (usually in mol·L⁻¹·s⁻¹)
- k = Rate constant (depends on temperature and nature of reaction)
- [A], [B] = Concentrations of reactants A and B
- m, n = Reaction orders with respect to A and B (determined experimentally)
REACTION ORDER
The order of a reaction with respect to a reactant tells us how the rate is affected by changes in that reactant’s concentration.
· If m = 1, the rate is directly proportional to [A] → First order
· If m = 2, the rate is proportional to [A]² → Second order
· If m = 0, the rate is independent of [A] → Zero order
· Overall order = m + n + …
Overall Reaction Order
The overall reaction order is the sum of the orders of the reactant species
For a reaction: A + 2B → AB2
Rate = k[A]1[B]2
Overall Reaction Order = m + n = 1 + 1 = 2
Reaction is of the Second Order Overall
Examples
EXPERIMENTAL DETERMINATION OF RATE LAW
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