Energy Level Diagrams

Objectives

Understandings

• A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.
• The equilibrium law describes how the equilibrium constant (Kc) can be determined for a particular chemical reaction.
• The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.
• The reaction quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time. Q is the equilibrium expression with non-equilibrium concentrations. The position of the equilibrium changes with changes in concentration, pressure, and temperature.
• A catalyst has no effect on the position of equilibrium or the equilibrium constant.

Applications and skills:

• The characteristics of chemical and physical systems in a state of equilibrium.
• Deduction of the equilibrium constant expression (Kc) from an equation for a homogeneous reaction.
• Determination of the relationship between different equilibrium constants (Kc) for the same reaction at the same temperature.
• Application of Le Châtelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant.

Guidance:

• Physical and chemical systems should be covered.
• Relationship between Kc values for reactions that are multiples or inverses of one another should be covered.
• Specific details of any industrial process are not required.

Activation Energy

In terms of iGCSE Chemistry - the Activation Energy is defined as 'The Minimum Energy particles require for a successful collision'

When particles collide - If particles do not have the required activation energy to react then this will result in an unsuccessful collision - however if they do have this energy then they will successfully collide and make a new substance called a product.

This threshold is represented in two different graphs:

Energy Level Diagrams

The above graphs are called Energy Level Diagrams and they show how energy is stored and transferred during a Chemical Reaction.

In the Left hand Graph we can see a typical Exothermic Energy Level Diagram.

Exothermic can be broken down into 'Exo' meaning 'Outside' and 'Thermic (or Thermal)' meaning 'Heat Energy'.

Thus Exothermic is a chemical reaction where heat energy is transferred to the surroundings

In the diagram we can observe that reactants have Energy stored within their bonds

When we break these bonds (Breaking bonds is an Endothermic Process) this Requires Energy

As the reaction proceeds and we form the Products we can observe that the Energy stored in the Products bonds is Lower than in the reactants. When the Bonds are formed (Making Bonds is an Exothermic Process) there is a difference in Energy between the reactants and Products. This remaining Energy is released to the surroundings.

Exothermic Reactions are characterised by:

• Products are more Stable than reactants
• Thus they are a Lower Energy Level when compared to the Reactants
• The Temperature of the Reaction Increases and you can feel this as exothermic reactions HEAT UP The surroundings

Energy Level Diagrams also show us how the energy changes as we proceed through the reaction. The Activation Energy is therefore included as part of this diagram and is the difference between the Reactants and the highest peak of the graph.

As Exothermic Reactions start with Reactants having a higher energy - this is usually relatively small for these reactions and easier to achieve than in their counterpart Endothermic Reactions.

Whereas Exothermic Reactions are described as Heat releasing Reactions Endothermic Reactions are the opposite.

Endothermic Reactions are derived from 'Endo' meaning 'Inside' and 'Thermic' meaning 'Heat Energy'

Therefore an Endothermic Reaction is when Heat Energy is ABSORBED From the surroundings.

In the diagram we can observe that reactants have Energy stored within their bonds

When we break these bonds (Breaking bonds is an Endothermic Process) this Requires Energy

As the reaction proceeds and we form the Products we can observe that the Energy stored in the Products bonds is HIGHER than in the reactants. When the Bonds are formed (Making Bonds is an Exothermic Process) there is a difference in Energy between the reactants and Products. This remaining Energy is ABSORBED from the surroundings.

Endothermic Reactions are characterised by:

• Products are LESS Stable than reactants
• Thus they are a HIGHER Energy Level when compared to the Reactants
• The Temperature of the Reaction DECREASES and you can feel this as endothermic reactions COOL DOWN The surroundings

Energy Level Diagrams also show us how the energy changes as we proceed through the reaction. The Activation Energy is therefore included as part of this diagram and is the difference between the Reactants and the highest peak of the graph.

As Endothermic Reactions start with Reactants having a LOWER energy - this is usually relatively HIGH for these reactions and HARDER to achieve than in their counterpart Exothermic Reactions.

Catalysts and Energy Level Diagrams

Both the above graphs demonstrate the effect of adding a catalyst to a chemical reaction. It can be observed that in both types of reaction adding a catalyst causes the highest peak to fall as the activation energy required for a successful collision is decreased