3.1.5 Kinetics

What is Collision Theory?

According to this theory, for a reaction to occur, particles (atoms, molecules, or ions) must collide with each other. However, not all collisions result in a reaction.

What are two main conditions must be met for a collision to be successful and lead to a reaction:

1.) Sufficient energy: The colliding particles must have enough kinetic energy to overcome the energy barrier known as the activation energy (Ea).

2.) Correct orientation: The particles must be oriented properly during the collision for the bonds to break and new bonds to form.

What is the activation energy (Ea)

The activation energy (Ea) is the minimum energy that reacting particles must have for a successful reaction to take place. It represents the energy barrier that must be overcome for a chemical reaction to occur.

What are the conditions needed to reach activation energy

-Reactions only occur when the particles have kinetic energy greater than or equal to the activation energy.

-If particles collide with less energy than the activation energy, they will simply bounce off each other, and no reaction will occur.

Why do most Collisions Do Not Lead to a Reaction

In any reaction mixture, most particles do not have enough energy to meet the activation energy threshold. As a result:

-Only a small proportion of particle collisions have the required energy to result in a reaction.

-The majority of collisions are unsuccessful because the energy of the colliding particles is too low, or they do not collide in the correct orientation.

What is the Rate of Reaction?

The rate of reaction measures how quickly a reactant is consumed or a product is formed during a chemical reaction. It is defined as the change in concentration of a reactant or product over a given period.

What is The formula for rate?

Rate of reaction = Change in concentration/ Time

Factors Affecting the Rate of Reaction

1. Concentration

2. Pressure (for Gaseous Reactions)

3. Temperature

4. Surface Area of Solid Reactants

5. Presence of a Catalyst

1. Concentration

Effect: Increasing the concentration of a reactant increases the number of particles in a given volume, leading to more frequent collisions.

Outcome: As the number of successful collisions per unit time increases, the rate of reaction increases.

Concentration↑ ⇒ Rate↑

2. Pressure (for Gaseous Reactions)

-Effect: Increasing the pressure of a gas forces the particles closer together, increasing the frequency of collisions.

-Outcome: More successful collisions per unit time mean the rate of reaction increases.

Pressure↑ ⇒ Rate↑

3. Temperature

Effect: Increasing the temperature provides particles with more kinetic energy, causing them to move faster.

Outcome: More particles collide with energy greater than or equal to the activation energy, increasing the number of successful collisions and speeding up the reaction.

Temperature↑ ⇒ Rate↑

4. Surface Area of Solid Reactants

Effect: Increasing the surface area of a solid reactant exposes more particles to collisions.

Outcome: This leads to more successful collisions per unit time, increasing the rate of reaction.

Surface Area↑ ⇒ Rate↑

5. Presence of a Catalyst

Effect: A catalyst provides an alternative reaction pathway with a lower activation energy.

Outcome: With a lower activation energy, a greater proportion of particles will have sufficient energy to react, leading to more successful collisions and an increased rate of reaction.

Catalyst ⇒ Rate↑

Why are catalyst used?

Catalysts are widely used in industrial processes to increase efficiency and reduce costs by speeding up reactions without being consumed in the process.

What is the Maxwell-Boltzmann Distribution?

The Maxwell-Boltzmann distribution describes the distribution of molecular energies in a gas at a given temperature. It provides a graphical representation of how many gas molecules have various amounts of kinetic energy.

Key Features of the Maxwell-Boltzmann Distribution

-The graph is asymptotic, meaning the curve approaches the horizontal axis but never touches it.

-The curve shows a peak where most molecules have a moderate amount of energy. This is the most probable energy.

-There are fewer molecules with very low or very high energies, as shown by the tails of the curve.

-The area under the curve represents the total number of molecules.

Maxwell-Boltzmann Distribution at high temperature

-The curve shifts to the right and flattens. This is because the average kinetic energy of the molecules increases.

-More molecules have energies above the activation energy, which leads to a higher rate of successful collisions and thus a faster reaction rate.

-The peak of the curve is lower and occurs at a higher energy.

Maxwell-Boltzmann Distribution at low temperature

-The curve shifts to the left and becomes taller. This means more molecules have lower kinetic energy.

-Fewer molecules have enough energy to surpass the activation energy, so the reaction rate is slower.

-The peak of the curve is higher and occurs at a lower energy.

Interpreting the Maxwell-Boltzmann Distribution

-Most probable energy: The peak of the curve represents the energy that the largest number of molecules possess.

-Average energy: Located to the right of the peak, the average energy is higher than the most probable energy because the distribution is skewed.

-Activation energy (Ea): The energy threshold that molecules must exceed for a reaction to occur. Only the molecules with energy greater than (Ea) will successfully react.

-As the temperature increases, more molecules will have energies exceeding the activation energy, increasing the rate of reaction.

The Effect of Catalysts on the Maxwell-Boltzmann Distribution

A catalyst does not change the shape of the Maxwell-Boltzmann distribution curve but it lowers the activation energy (Ea).

-This means that a greater proportion of molecules now have enough energy to surpass the activation energy, even without increasing the temperature.

-As a result, the rate of reaction increases because more molecules can successfully collide and react.

How Temperature Affects Molecular Energies

Increasing the temperature causes a shift in the Maxwell-Boltzmann distribution of molecular energies:

-At a higher temperature, the distribution curve shifts to the right and the peak becomes lower. This indicates that more molecules have higher kinetic energy.

-At a lower temperature, the curve shifts to the left and the peak becomes higher. This shows that most molecules have lower kinetic energy.

Why Does Temperature Increase Reaction Rate?

-A small increase in temperature can lead to a large increase in the rate of reaction. This occurs because:

-Increased kinetic energy: As temperature rises, particles move faster and collide more frequently.

-More successful collisions: A greater proportion of particles have energy equal to or greater than the activation energy, resulting in more successful collisions per unit of time.

How Does Concentration Affects Collision Frequency

1.) Increased concentration means there are more particles in the same volume.

2.) With more particles present, the frequency of collisions between reactant particles increases.

3.) More collisions mean more opportunities for particles to collide with sufficient energy to overcome the activation energy, leading to an increase in the rate of successful reactions.

What is the effect of Maxwell-Boltzmann Distribution at Higher Concentration:

-The shape of the curve remains the same, meaning the most probable energy does not change.

-The peak of the curve stays at the same energy level but is higher because there are more molecules in total.

-The area under the curve increases, representing the larger number of reactant molecules.

Though increasing concentration raises the rate of reaction, its effect is less significant than the effect of increasing temperature. Explain why

This is because only a slightly higher number of molecules gain enough energy to undergo successful collisions.

How Does Pressure Affects Collision Frequency

1.) Higher pressure compresses gas particles into a smaller volume, effectively increasing the number of particles per unit volume.

2.) This leads to more frequent collisions between gas particles.

3.) Like concentration, an increase in pressure increases the likelihood of successful collisions, thereby increasing the rate of reaction.

What is the difference of pressure and concentration

While both concentration (in solutions) and pressure (in gases) influence the rate of reaction by increasing the collision frequency, the underlying mechanism is the same:

-more particles in the same space leads to more collisions and, therefore, more chances for reactions to occur.

How Do catalysts Work

-Catalysts work by providing an alternative reaction pathway with a lower activation energy (Ea) than the uncatalyzed reaction.

-The activation energy is reduced, meaning more molecules have sufficient energy to react. This increases the rate of successful collisions and thus speeds up the reaction.

Catalysts Using the Maxwell-Boltzmann Distribution

-The shape of the distribution curve remains unchanged because neither the temperature nor the number of molecules is affected.

-However, the activation energy (Ea) is lowered.

-The area under the curve beyond the new, lower activation energy is greater, meaning a larger proportion of molecules have enough energy to successfully react.