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

Energy is the ability to do work. In chemical reactions, the energy change is measured as the enthalpy change (∆H): negative for exothermic reactions and positive for endothermic reactions.

Behavioural objectives

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

1. Define energy and state the law of conservation of energy.
2. Define enthalpy change and write its expression.
3. Distinguish between exothermic and endothermic reactions and give examples of each.
4. Explain how the breaking and forming of chemical bonds produce the heat of a reaction.
5. Define the heats of formation, neutralisation, combustion and solution.

What is energy?

Energy is the ability to do work.

• Heat energy: energy that flows from a hot body to a cold body.
• Light energy: energy carried by light, as from the sun.
• Sound energy: energy produced by vibrating objects.
• Electrical energy: energy of moving charges, as in a battery circuit.
• Chemical energy: energy stored in the bonds of substances, as in food, fuel and cells.
• Potential energy: stored energy due to position or structure.
• Kinetic energy: energy due to motion.

What is the law of conservation of energy?

The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another.

• The total energy before a change equals the total energy after it.
• Example: chemical energy in firewood changes to heat and light when it burns.
• Example: electrical energy in a torch changes to light and a little heat.

What is heat content (enthalpy) of a substance?

Enthalpy, or heat content, is the internal energy a substance has because of its structure and physical state.

• Potential energy part: comes from the structure of the substance.
• Kinetic energy part: comes from the physical state of the substance.
• Each substance has a different enthalpy.
• Total enthalpy cannot be measured. Only enthalpy changes can be measured.

What is enthalpy change (∆H)?

Enthalpy change is the heat exchanged with the surroundings during a reaction, that is, the energy involved in the reaction.

• Expression: ∆H = H_products – H_reactants
• In words: enthalpy change = heat of products – heat of reactants.
• Unit: joules (J) or kilojoules (kJ), often written as kJ mol^-1.
• Sign: may be positive or negative.
• Writing it: ∆H is written beside the balanced equation.

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

What are exothermic and endothermic reactions?

An exothermic reaction gives out heat to the surroundings, while an endothermic reaction takes in heat from the surroundings.

What is an exothermic reaction?

An exothermic reaction is a reaction in which heat is given off to the surroundings.

• Heat passes from the chemicals to the surroundings.
• The temperature of the reaction mixture rises.
• The reaction vessel feels hot.
• The heat content of the products is less than that of the reactants, so ∆H is negative.

Examples of exothermic reactions:

• Calcium oxide and water: the mixture becomes very hot.
• Acid and base: neutralisation gives out heat.
• Combustion of fuel: burning kerosene, wood or gas.
• Corrosion of metals: for example, rusting of iron.
• Respiration: food is oxidised to release energy.

What is an endothermic reaction?

An endothermic reaction is a reaction in which heat is absorbed from the surroundings.

• Heat passes from the surroundings to the reactants.
• The temperature of the reaction mixture falls.
• The reaction vessel feels cold.
• The heat content of the products is more than that of the reactants, so ∆H is positive.

Examples of endothermic reactions:

• Thermal decomposition of calcium trioxocarbonate (IV): heating limestone gives calcium oxide and carbon dioxide.
• Thermal dissociation of ammonium chloride: heat splits it into ammonia and hydrogen chloride.
• Action of light on silver bromide: used in photographic film.
• Photosynthesis: plants take in light energy to make food.

What is the difference between exothermic and endothermic reactions?

What are energy level diagrams?

An energy level diagram shows the energy of reactants and products, so that one can see at once whether a reaction is exothermic or endothermic.

• Exothermic: products sit lower than reactants, with the arrow pointing down.
• Endothermic: products sit higher than reactants, with the arrow pointing up.

How are heat of reaction and chemical bonds related?

The heat of a reaction comes from the breaking of old bonds and the forming of new bonds.

1. Bonds in the reactants are broken.
2. The atoms are regrouped.
3. New bonds form in the products.

• Bond breaking: needs energy, so it is endothermic.
• Bond forming: gives out energy, so it is exothermic.
• Activation energy: the minimum energy that reacting particles must have before a reaction can start.

What are the types of heat changes in chemical reactions?

There are four types of heat change studied at this level: heat of formation, neutralisation, combustion and solution.

What is heat of formation?

Heat of formation is the heat evolved or absorbed when one mole of a substance is formed from its elements.

• Standard heat of formation (∆H_f^θ): the same change measured under standard conditions.
• Other name: enthalpy of formation.

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

What is heat of neutralisation?

Heat of neutralisation is the heat evolved when an acid and a base react to form one mole of water.

• Type: always exothermic.
• Standard heat of neutralisation (∆H_n^θ): heat evolved when 1 mole of H⁺ from an acid reacts with 1 mole of OH^– from an alkali to form 1 mole of water.
• Link: it is also the heat of formation of one mole of water from its ions.

H⁺(aq) + OH^–(aq) → H₂O(l) ∆H_n^θ = -57.3 kJ mol^-1

What is heat of combustion?

Heat of combustion is the heat evolved when one mole of a substance is burned completely in oxygen.

• Type: always exothermic.
• Standard heat of combustion (∆H_c^θ): the same change under standard conditions.
• Apparatus: a bomb calorimeter gives accurate values.

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

When the heat raises the temperature of a known mass of water:

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

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

Steps to find the heat of combustion of a fuel such as ethanol:

1. Measure a known mass of water into a metal can.
2. Record the starting temperature of the water, θ₁.
3. Weigh the spirit burner with the fuel.
4. Light the burner and heat the water, stirring gently.
5. Put out the flame and record the highest temperature, θ₂.
6. Weigh the burner again to find the mass of fuel burnt.
7. Calculate mC∆θ, then use the formula above.

What is heat of solution?

Heat of solution is the heat evolved or absorbed when one mole of a substance dissolves in so much water that further dilution gives no detectable heat change.

• Type: can be exothermic or endothermic.
• Standard heat of solution (∆H_s^θ): the same change at standard conditions.

How do the four heat changes compare?

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

Define the heat of combustion, write the expression for finding it with a water calorimeter, and state what each symbol in the expression means.