Energy, Energy Changes and Biotechnology – SS2 Chemistry Lesson Note (Second Term)

Energy is the ability to do work. It can change from one form to another but cannot be created or destroyed. In chemical reactions, the energy change is measured as the enthalpy change (∆H): negative for exothermic reactions, which give out heat, and positive for endothermic reactions, which absorb heat.

Behavioural objectives

By the end of the lesson, students should be able to:

• define energy and state the law of conservation of energy;
• define enthalpy change and write its expression;
• distinguish between exothermic and endothermic reactions and give examples of each;
• explain how the breaking and forming of chemical bonds produce the heat of a reaction;
• define the heats of formation, neutralisation, combustion and solution.

What is energy?

Energy is defined as the ability to do work. It exists in different forms such as heat, light, sound, electrical, chemical, potential (stored) and kinetic energy.

What is the law of conservation of energy?

Energy can be changed from one form to another. The total amount of energy before and after the change remains the same. This observation is the law of conservation of energy. It states that energy can neither be created nor destroyed but can be changed from one form to another.

What is heat content (enthalpy) of a substance?

Heat content or enthalpy of a substance is the characteristic internal energy possessed by the substance, which is due to the structure and physical state of the substance. The potential energy is due to the structure while the kinetic energy is due to the physical state.

The enthalpy of one substance is different from another. Total enthalpy cannot be measured but only enthalpy changes. Generally, an enthalpy change (∆H) is the heat that would be exchanged with the surroundings, that is, it is the amount of energy involved in a reaction.

Thus,

Enthalpy change = Heat of products – Heat of reactants

That is, ∆H = H_products – H_reactants

The enthalpy change of a given reaction is always written side by side with the given equation and it may be either a positive or negative value.

Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)    ∆H = -57.3 kJ

The unit of enthalpy change (∆H) is joules (J) or kilojoules (kJ).

What are exothermic and endothermic reactions?

Exothermic reaction

A chemical reaction in which heat is given off to the surroundings is known as an exothermic reaction. When an exothermic reaction occurs, heat is liberated and transferred from the chemicals to the surroundings and the temperature of the reaction mixture rises. The reaction vessel will feel hot.

Examples of exothermic reactions include:

• Reaction between calcium oxide and water
• Reaction between an acid and a base
• Combustion of fuel
• Corrosion of metals
• Respiration

In an exothermic reaction, the enthalpy change is negative since the heat content of the products is less than the heat content of the reactants.

Endothermic reaction

An endothermic reaction is a type of reaction in which heat is absorbed from the surroundings. When an endothermic reaction occurs, heat energy is absorbed and transferred from the surroundings to the reactants and the temperature of the reaction mixture falls. The reaction vessel will feel cold.

Examples of endothermic reactions are:

• Thermal decomposition of calcium trioxocarbonate (IV)
• Thermal dissociation of ammonium chloride
• Action of light on silver bromide in photographic film
• Photosynthesis in plants

In an endothermic reaction, the heat content of the products is more than the heat content of the reactants; hence the enthalpy change is positive.

What are energy level diagrams?

Energy changes can be presented by diagrams, which show at once whether reactions are exothermic or endothermic.

How are heat of reaction and chemical bonds related?

During chemical reactions, chemical bonds are broken, atoms are regrouped and new bonds are formed. Bond breaking requires energy and bond forming evolves energy. The minimum amount of energy required for bond breaking is called activation energy. While bond breaking is endothermic, bond forming is exothermic.

Thus, the heat of the reaction comes from the breaking and forming of chemical bonds. The heat of reaction is negative (exothermic) when bond-breaking energy is less than bond-forming energy. The heat of reaction is positive (endothermic) when bond-breaking energy is more than bond-forming energy.

What are the types of heat changes in chemical reactions?

Heat of formation

The amount of heat evolved or absorbed when one mole of a substance is formed from its elements is known as the heat of formation (or enthalpy of formation). The standard heat of formation of a substance (∆H_f^θ) is the heat evolved or absorbed when one mole of that substance is formed from its elements under standard conditions.

For the formation of 1 mole of liquid water, the equation is:

H₂(g) + ½O₂(g) → H₂O(l)    ∆H_f^θ = -285 kJ mol^-1

Thus, ∆H_f^θ of water = -285 kJ mol^-1

Heat of neutralisation

Neutralisation is an exothermic reaction. The amount of heat evolved during a neutralisation reaction in which one mole of water is formed is known as the heat of neutralisation (or enthalpy of neutralisation). The standard heat of neutralisation (∆H_n^θ) is the amount of heat evolved when 1 mole of hydrogen ions, H⁺, from an acid reacts with 1 mole of hydroxide ions, OH^–, from an alkali to form 1 mole of water under standard conditions.

The heat of neutralisation is also known as the heat of formation of one mole of water from its ionic components.

H⁺(aq) + OH^–(aq) → H₂O(l)    ∆H_n^θ = -57.4 kJ mol^-1

Heat of combustion

Combustion reaction is always exothermic. The amount of heat that evolves when one mole of a substance is burned completely in oxygen is known as the heat of combustion or enthalpy of combustion. The standard heat of combustion of a substance (∆H_c^θ) is the heat evolved when one mole of the substance is burned completely in oxygen under standard conditions.

A bomb calorimeter is usually used for accurate determination of heat of combustion.

The heat of combustion can be determined from the relation below:

Heat of combustion = (Heat energy produced ÷ Mass burnt) × Molar mass

When the heat evolved by the burning substance is used to raise the temperature of a known mass of water, the expression for the heat of combustion becomes:

Heat of combustion = (mC∆θ ÷ Mass burnt) × Molar mass

Where:

• m = mass of water
• C = specific heat capacity of water
• ∆θ = change in temperature, that is, θ₂ – θ₁

Heat of solution

The heat of solution can be exothermic or endothermic. It is the heat evolved or absorbed when one mole of a substance is dissolved in so much water that further dilution results in no detectable heat change.

The standard heat of solution (∆H_s^θ) is the amount of heat evolved or absorbed when 1 mole of a substance is dissolved in so much water that further dilution results in no detectable heat change at standard conditions.

Evaluation

1. State the law of conservation of energy.
2. Define enthalpy change and write its expression.
3. Give two examples each of exothermic and endothermic reactions.
4. When is the heat of a reaction negative, in terms of bond breaking and bond forming?
5. Define the heat of neutralisation and write the equation for it.

Assignment

1. Define the heat of combustion and write the two expressions used to determine it.
2. Explain what is meant by the heat of solution.
3. Compare exothermic and endothermic reactions in terms of temperature change and the sign of ∆H.