Energy, Energy Changes and Biotechnology SS2 Chemistry Lesson Note

ENERGY

Energy is defined as the ability to do work. It exists in different forms like heat, light, sound, electrical, potentials (stored), kinetic etc.

LAWS 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 stated in the law of conservation of energy which states that energy can neither be created nor destroyed but can be changed from one form to another.

There are types of energy such as chemical energy, heat energy, and light energy.

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 a 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 = Products – Hreactants

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 = -57.3kJ

Unit of enthalpy change(∆H) is Joules(J) or kilojoules(kJ)

EXOTHERMIC AND ENDOTHERMIC REACTIONS

1. 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:

i. Reaction between calcium oxide and water 

ii. Reaction between an acid and a base.

iii. Combustion of fuel

iv. Corrosion of metals

v. 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.

2. 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 product is more than the heat content of the reactant; hence the enthalpy change is positive.

ENERGY LEVEL DIAGRAMS

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

HEAT OF REACTION AND CHEMICAL BONDS 

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. Heat reaction is negative [exothermic] when bond-breaking energy is less than bond-forming energy. The heat of the reaction is positive [endothermic] when bond-breaking energy is more than bond-forming energy. 

TYPES OF HEAT CHANGES IN CHEMICAL REACTIONS

1. 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(∆Hfθ) 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)  + 1/2O2(g)( H2O(1)          ∆Hfθ  = – 285kJmol-1 

Thus, ∆Hfθ of water = – 285 Jmol-1 

2. Heat Of Neutralization

Neutralization is an exothermic reaction. The amount of heat evolved during a neutralization reaction in which one mole of water is formed is known as the heat of neutralization (or enthalpy of neutralization). The standard heat of neutralization ∆Hnθ 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 neutralization is also known as the heat of the formation of one mole of water from its ionic components. 

     H+(aq)  +  OH-(aq)   →  H2O(l)            ∆Hnθ  = – 57.4kJmol-1

3. 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, ∆HCθ; 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:

The heat of combustion = Heat energy produced  x molar mass

Mass burnt       1

When the heat evolved by the burning substance is used to raise the temperature of a known mass of water, then the expression for the heat of combustion can be given as:

The heat of combustion =  

mC∆θ        x  molar mass 

Mass burnt 1

Where m = mass of water

            C = Specific heat capacity of water

           ∆θ = change in temperature, that is, θ2 – θ1

4. Heat Of Solution

The heat of the solution can be exothermic or endothermic. The heat of solution 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, ∆Hsθ, 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.