KINETICS

How Fast Does A Reaction Occur?

Energy Diagrams

Reactants always start a reaction so they are on the left side of the diagram.

Reactants

Products are on the right.

Products

Exothermic Reactions

The exothermic reaction gives off heat because the products are at a lower energy level than the reactants.

Graph of anExothermic Reaction

Graph of anExothermic Reaction

In an exothermic graph, the reactants have greater energy than the products.

Reactants

The change in energy is a negative value.

Products

Exothermic And Endothermic Reactions

The endothermic reaction absorbs heat because the products are at a higher energy level than the reactants.

Graph of anEndothermic Reaction

Graph of anEndothermic Reaction

In an endothermic graph, the products have greater energy than the reactants.

Reactants

The change in energy is a positive value. Products

Exothermic And Endothermic Reactions

Scientists have observed that the energy released in the formation of a compound from its elements is always identical to the energy required to decompose that compound into its elements.

Energy Diagrams

Activation energy is the minimum amount of energy that reacting particles must have to form the activated complex.

Activation energy

activated complex

Energy Diagrams

The activated complex is a short-lived, unstable arrangement of atoms that may break apart and re-form the reactants or may form products.

activated complex

Energy Diagrams

To calculate the activation energy, subtract the energy of the reactants from the energy at the top of the peak.

Activation energy

Energy Diagrams

The enthalpy or heat of reaction (ΔH) is the amount of heat released or absorbed in the reaction.

Enthalpy/heat of reaction (ΔH)

Energy Diagrams

To determine ΔH, take the energy of the products and subtract the energy of the reactants.

Enthalpy/heat of reaction (ΔH)

Problem

The heat content of the reactants of the forward reaction is about ______ kilojoules.

(80 kJ)

Problem

The heat content of the products of the forward reaction is about ______ kilojoules.

(160 kJ)

Problem

The heat content of the activated complex of the forward reaction is about _____ kilojoules.

(240 kJ)

Problem

The activation energy of the forward reaction is about _____ kilojoules.

(160 kJ)

Problem

The heat of reaction (ΔH) of the forward reaction is about _____ kilojoules.

(80 kJ)

Problem

The forward reaction is (endothermic or exothermic).

(endothermic - products are higher in energy)

Problem

The heat content of the reactants of the reverse reaction is about ______ kilojoules.

(160 kJ)

Problem

The heat content of the products of the reverse reaction is about ______ kilojoules.

(80 kJ)

Problem

The heat content of the activated complex of the reverse reaction is about _____ kilojoules.

(240 kJ)

Problem

The activation energy of the reverse reaction is about _____ kilojoules.

(80 kJ)

Problem

The heat of reaction (ΔH) of the reverse reaction is about _____ kilojoules.

(- 80 kJ)

Problem

The reverse reaction is (endothermic or exothermic).

(exothermic - products are lower in energy)

Energy DiagramsThe activation energy can be lowered by

adding a catalyst.

effect of the catalyst

Energy DiagramsThe catalyst lowers the activation

energy by providing an alternate pathway for the reaction to occur.

effect of the catalyst

Expressing Reaction Rates

As you know, some chemical reactions are fast and others are slow; however, fast and slow are inexact, relative terms.

Chemists often need to be more specific.

Expressing Reaction Rates

We generally define the average rate of an action or process to be the change in a given quantity during a specific period of time.

Expressing Reaction Rates

Reaction rates cannot be calculated from balanced equations as stoichiometric amounts can.

Reaction rates are determined experimentally by measuring the concentrations of reactants and/or products in an actual chemical reaction.

Collision Theory

According to the collision theory, atoms, ions, and molecules must collide with each other in order to react.

Collision Theory

The following three statements summarize the collision theory. 1. Particles must collide in order to

react. 2. The particles must collide with the

correct orientation.

Collision Theory

3. The particles must collide with enough energy to form an unstable activated complex, also called a transition state, which is an intermediate particle made up of the joined reactants.

Collision Theory

activated complex

Collision Theory

The minimum amount of energy that colliding particles must have in order to form an activated complex is called the activation energy of the reaction.

Particles that collide with less energy than the activation energy cannot form an activated complex.

Collision Theory

In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier, form an activated complex, then release energy and form products at a lower energy level.

Ene

rgy

Reaction coordinate

Reactants

Products

Ene

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Reaction coordinate

Reactants

Products

Activated Complex or Transition State

Ene

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Reaction coordinate

Reactants

Products

Activation Energy

Ene

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Reaction coordinate

Reactants

Products

Overall energy change

Collision Theory

In the reverse endothermic reaction, the reactant molecules lying at a low energy level must absorb energy to overcome the activation energy barrier and form high-energy products.

Factors Affecting Reaction Rates

The reaction rate for almost any chemical reaction can be modified by varying the conditions of the reaction.

Factors Affecting Reaction Rates

1) An important factor that affects the rate of a chemical reaction is the reactive nature of the reactants. As you know, some substances react more readily than others. The more reactive a substance is, the faster the reaction rate.

Factors Affecting Reaction Rates

2) Another important factor that affects the rate of a chemical reaction is the concentration of the reactants. Reactions speed up when the concentrations of reacting particles are increased. Increasing the number of reactants increases probability of collisions.

Factors Affecting Reaction Rates

2) continued. The rate of gaseous reactions can be increased by pumping more gas into the reaction container.

Factors Affecting Reaction Rates

3) Surface area of the reactants affects the rate of a chemical reaction. Increasing the surface area of reactants provides more opportunity for collisions with other reactants, thereby increasing the reaction rate.

Factors Affecting Reaction Rates

4) Temperature affects the rate of a chemical reaction. Generally, increasing the temperature at which a reaction occurs increases the reaction rate. Raising the temperature raises both the collision frequency and the collision energy.

Factors Affecting Reaction Rates

5) Adding a catalyst affects the rate of a chemical reaction. A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed in the reaction. In fact, catalysts are not included in the chemical equation.

Factors Affecting Reaction Rates

A catalyst lowers the activation energy of a reaction by providing an alternate pathway for the reaction to occur (the blue line).

Ene

rgy

Reaction coordinate

Reactants

Products

Factors Affecting Reaction Rates

6) Compressing gases affects the rate of a chemical reaction. When two gases react, compressing the gases generally increases the rate of the reaction.