# What is Thermochemistry?

What is thermochemistry in chemistry? This is the branch of chemistry that deals with the heat energy released or absorbed during chemical reactions.

The chemical reactions can occur either in an open, closed, or isolated system. It is very pertinent I mention that some chemical reactions can absorb heat and are called endothermic reactions while some chemical reactions release energy and are thus called exothermic reactions.

Actually, thermochemistry is a branch of thermodynamics and it also deals with calculations involved in heat energy calculations.

Types of systems associated with Thermochemistry

1. Open system: This is the type of system in which there is an exchange of heat energy and materials between the system and the surrounding.

2. Closed system: This is the type of system in which there is an exchange of heat energy but no exchange of material between the system and the environment.

## What is the difference between a closed system and an open system?

In a closed system, there is an exchange of heat but no material exchange but in an open system, there is an exchange of beat and materials between the system and the surroundings.

3. An isolated system; An isolated system is a system in which there is no exchange between the environment and the system.

## How to Solve Thermochemistry Problems

Types of Heat energies and Calculations

There are various types of thermochemistry calculations but they are very easy to solve if one can master the concepts and the law guiding them.

1. Heat of combustion/ Enthalpy of Combustion: Heat of combustion is the heat evolved when one of a substance burns completely in oxygen.

I will just solve one of the common calculations in heat of combustion so we can see the trend and pattern used in solving this type of question.

Example 1

Calculate the heat evolved when 25g of ethanol burns completely in oxygen to raise the temperature of 25g of water from 200C to 300C.[ C= 4.2J/gK-1mol-1]

Solution

Calculate the molar mass of Ethanol C2H5OH

Molar mass = [12×2+1 x5+16+1]

Molar mass = 46g

Number of moles =25/46

Number of moles = 0.543 moles

Heat evolved = MCT

= 25 X 4.2 X 10

= -1050J

If I 0.543 mole of Ethanol — 1050J

Therefore I mole of Ethanol —-x

X = 1050/0.543

X = 1933.7J

Example 2

Calculate the heat evolved when 16g of ethane burns completely in oxygen to raise the temperature of 20g of water from 220C to 400C.[ C= 4.2J/gK-1mol-1]

Solution

Calculate the molar mass of Ethane C2H6

Molar mass = [12×2+1×16]

Molar mass = 40g

Number of moles = 16/40

Number of moles = 0.4 moles

Heat evolved = MCT

= 20 X 4.2 X 10

= -840J

If I 0.4 mole of Ethanol —840J

Therefore I mole of Ethanol —-x

X = 840/0.4

X = 2100.7J

2. Heat of Neutralization

Heat of neutralization is the heat evolved when one mole of hydrogen ion from an acid combines with one mole of hydroxide ion to form one mole of water.

Heat of neutralization is highest for a strong acid versus strong base and lowest for a weak acid versus weak base.

3. Heat of Formation:

This is the heat evolved or absorbed when one mole of a compound is formed from its constituent elements.

4. Heat of solution

This is the heat evolved or absorbed when a substance dissolves completely in oxygen that further dilution causes no noticeable effect on the heat involved.

Actually, there are so many types of heat energies and its calculations we can go and on and talk about like heat of vaporization, heat of fusion and fission, bond energy calculations

## Why is Thermochemistry Important?

Thermochemistry is very important to us because we virtually apply the laws and calculations in almost everything we do, industries, factories, construction etc