According to the first law of thermodynamics, energy can neither be created nor destroyed but can be converted from one form into another. When a chemical reaction takes place, an exchange of energy occurs between the system and its surroundings. This energy or heat change is known as **Enthalpy** represented by **H**.

Give some detail about enthalpy.

Outline

## Measurement of enthalpy

Enthalpy cannot be measured directly. We can’t determine the absolute value of entropy rather we measure the absolute value of entropy.

## The first method to find the enthalpy of a reaction

The formula for enthalpy change is;

H=msT

where,

- H= enthalpy change
- m= mass of reactants
- s= specific heat of products
- T= change of temperature during the reaction

## Steps to Calculate the enthalpy change

## Identify reactants and products

In a chemical reaction, the substances that combine are called reactants and the substances that are produced as a result are called products. For example

2H_{2}+O_{2}→2H_{2}O

On the left-hand side, hydrogen and oxygen are reactants while on the right side the water is a product.

## The total mass of reactants

Calculate the total mass of reactants by adding the molar masses of all reactants. For example

2H_{2}+O_{2}= 2(2)+2(16)=4+32=36g

## Calculate the specific heat of the product

Every element or molecule has its own specific heat. These values are constant. For example in the above reaction

The specific heat capacity of water is 4.18J/g^{0}C.

## Calculate temperature difference

Now calculate the temperature change before and after the reaction. Suppose we have an initial temperature is 170^{0}C and the final temperature is 90^{0}C.

T=T_{2}-T_{1}

T=90-170=-80^{0}C

## Put values in the formula

As;

H=msT

By putting the values in the above equation we get the value of enthalpy change and it comes to be negative. It means the reaction is exothermic.

### Significance of sign

H=-ve Exothermic reaction

H=+ve Endothermic reaction

## The second method to find the enthalpy of a reaction

**By using Bond Energies**

## Concepts Berg

**How to calculate the enthalpy of a reaction?**

Enthalpy of reaction can be calculated by the given formula

H=msT

where,

- H= enthalpy change
- m= mass of reactants
- s= specific heat of products
- T= change of temperature during the reaction

**Need a little refresher on Thermo**

**How to find enthalpy/entropy from vaporization?**

The entropy of vaporization is the ratio of heat of vaporization to the boiling temperature.

S_{vap}=H_{vap}/T

Enthalpy of vaporization is the amount of energy needed by a liquid substance to change into a gas. It is also known as latent heat of vaporization.

**How do I find volume at given pressure and temperature?**

If the values of temperature and pressure are known, then volume can be calculated by using an ideal gas equation

PV=nRT

V=nRT/P

where,

- P=pressure
- V=volume
- n= no. of moles
- R=general gas constant 0.821atmdm
^{3}mol^{-1}K^{-1} - T=temperature

**How to calculate enthalpy change per mole?**

The enthalpy change can be calculated by the following the formula

H=msT

When enthalpy change is divided by a number of moles we get enthalpy change per mole.

**How to calculate the enthalpy change of formation?**

Enthalpy of formation can be calculated by the sum of enthalpy change of products minus the sum of enthalpy change of reactants.

H_{formation}= Productus-Reactants

**What is a change in enthalpy formula thermodynamics?**

In thermodynamics, Enthalpy is the sum of internal energy and product of pressure-volume work.

H=U+PV

**How to calculate the enthalpy change of combustion?**

Enthalpy change of combustion is the amount of heat evolved when 1 mole of a substance is completely burnt in the air under standard conditions. It is represented by Combustion.

It can be calculated by the enthalpy of formation.

Combustion= H_{formation}(products)-H_{formation}(reactants)

**How to calculate enthalpy change using Hess’s Law?**

Hess’s law states that;

*‘*’The amount of heat evolved or absorbed during chemical reaction remains the same, whether that change takes place in one or several steps.’’

If the reaction takes place in three steps and heat change during these reactions are H_{1}, H_{2,} and H_{3} respectively. The total enthalpy change

H_{total}= H_{1}+H_{2} +H_{3}

**How to calculate the enthalpy of neutralization?**

Enthalpy of neutralization is the amount of heat evolved when one mole of acid reacts with a base to form salt and water.

Following steps should be followed to determine the enthalpy of neutralization.

- Calculate the mass of acid
- The heat capacity of the aqueous solution should be known
- Observe the change in temperature during the reaction
- Put these values in the formula

Q=mcT

where Q is the heat change of neutralization.

Then find the number of moles of base and divide the Q by n to get enthalpy change.

H=Q/n

**What are some different ways to calculate the enthalpy of a chemical reaction?**

Enthalpy of a chemical reaction can be calculated by

- Using a formula H=mcT
- By using bond energy
- By using Hess’s law

- What is the enthalpy change for the isomerization reaction of ethanol to dimethyl ether?

The reaction of isomerization is

C_{2}H_{5}OH→CH_{3}OCH_{3}

H_{reaction}=H_{product}-Reactant

=-203.3kJ/mol-(-277.6kJ/mol)

=74.3kJ/mol

The reaction is endothermic.

**How do we find the enthalpy change of a reversible reaction? Does it equal the sum of the enthalpies of the front and reverse reactions?**

Enthalpy change of forward reaction is equal to the enthalpy change of reverse reaction but with opposite sign.

**What is the effect of pressure and temperature on enthalpy?**

When the temperature increases, kinetic energy and potential energy of the molecule increase so the internal energy of the system increases. This results in an increase in enthalpy.

When the temperature of the system decreases, the temperature of the surrounding area increases. So the enthalpy decreases due to an increase in external pressure.

**How to determine the lattice energy of CaF _{2}?**

**Under what condition is the heat of the reaction equal to the enthalpy change?**

When heat is added to the system at constant pressure then the heat of reaction becomes equal to enthalpy change.

**How do you calculate the heat of the formation of naphthalene?**

The heat of formation is the amount of enthalpy change when a compound is formed from its constituent’s elements.

Naphthalene is formed from carbon and oxygen so by using heat of formation of carbon and oxygen, heat of formation of naphthalene can be calculated.

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