The heat of reaction is defined as the amount of heat absorbed or released during a chemical reaction. It is also called the enthalpy change of a reaction. It is represented by H.

The enthalpy change is used to calculate the heat of the reaction when work is done at constant pressure because in this case change in internal energy becomes difficult to determine.

So we can write

(dQ)p=(dH)p

This equation states that the amount of heat transferred in and out of the system is equal to enthalpy change at constant pressure.

The symbol ‘d’ represents a very very small change.

The symbol ‘H’ represents the term enthalpy. It is defined as the total heat content of the system. It can also be defined as the amount of heat added to a system at constant pressure.

H=U+PV

It is the sum of internal energy and product of pressure-volume work. Since internal energy, pressure and volume are state functions so enthalpy is also a state function.

# Numerical way to find the heat of reaction

The heat of reaction can be determined numerically by the sum of heat of formation of products(multiply with the number of moles) minus the sum of heat of formation of reactants(multiply with the number of moles).

Hreaction=n-m

Where,

Hreaction=heat of reaction

= heat of formation of products

= heat of formation of reactants

n and m are the numbers of moles of products and reactants from a balanced chemical equation.

The heat of the reaction is measured in kJ.

Outline

## Sign Convention

If Hreaction is positive it indicates heat of formation of products is greater than the heat of formation of reactants i.e, Endothermic reaction.

If Hreaction is negative it indicates heat of formation of reactants is greater than the heat of formation of products i.e, Exothermic reaction.

### Heat of formation Table

Heat of formation (kJ/mol) at 25oC and 1 atm

AgBr(s) -99.5 CaO(s) -635.5
AgCl(s) -127 CO(g) -110.5
AgI(s) -62.4 CO2(g) -393.5
Ag2O(s) -30.6 HCl(g) -92.3
Ag2S(s) -31.8 HBr(g) -36.2
Al2O3(s) -1669.8 HI(g) 25.9
BaCl2(s) -860.1 HF(g) -268.6
BaCO3(s) -1218.8 HNO3(l) -173.2
BaO(s) -558.1 H2O(g) -241.8
BaSO4(s) -1465.2 H2O(l) -285.8
CaCl2(s) -795.0 SO2(g) -296.1
CaCO3(s) -1207.0 SO3(g) -395.2

This data of heat of formation is very useful in determining the heat of reaction which is difficult to measure directly.

Let’s discuss some examples.

Example-1

4Al(s)+3O2(g)→2Al2O3(s)

Heat of formation of reactants is 0 because the heat of formation of elements in a pure state is zero.

Heat of formation of product=-1669.8kJ/mol

Hreaction=n-m

Hreaction=2(-1669.8)

Hreaction=-3339.6kJ/mol

The negative heat of the reaction indicates that the formation of Al2O3 is an exothermic reaction.

Example-2

H2(g)+I2(g)→2HI(g)

The heat of formation of reactants is zero and the heat of formation of hydrogen iodide is +25.9kJ/mol.

So the heat of the reaction is 2(25.9)=51.8kJ/mol

Example-3

C3H8(g)+5O2(g)→3CO2(g)+4H2O(g)

Calculate heat of reaction.

Heat of formation of reactants=(1mol of C3H8)(-103.8kJ/mol)+5(0)

Heat of formation of reactants=-103.8kJ

Heat of formation of products=(3mol of CO2)(-393.5kJ/mol)+(4mol of H2O)(-241.8kJ/mol)

Heat of formation of products=-1180.5kJ-967.2kJ

Heat of formation of products=-2147.7kJ

Hreaction=n-m

Hreaction=-2147.7kJ-(-103.8kJl)

Hreaction=-2043.9kJ

This is the combustion reaction of propane. Negative heat of reaction means that combustion reaction releases -2043.9kJ of energy. So it is an exothermic reaction.

Example-4

Mg+2HCl→MgCl2+H2

Calculate the heat of the reaction.

The formula for the heat of reaction is

Hreaction=n-m

Heat of formation of reactants= (1mol of Mg)(0)+(2mol of HCl)(-167.2kJ/mol)

Heat of formation of reactants=-334.4kJ

Since the heat of formation of Mg in the standard state is zero.

Heat of formation of products=(1mol of MgCl2)(-796.9kJ/mol)+(1mol of H2)(0)

Heat of formation of reactants=-796.9kJ

Since the heat of formation of H2 in the standard state is zero.

Hreaction=n-m

Hreaction=(-796.9kJ)-(-334.4kJ)

Hreaction=-462.5kJ

It is an exothermic reaction.

## Concepts Berg

1. What is the formula for this compound?

The formula of the compound gives information about the proportions of atoms that combine to form a compound. The formula of a compound is written by using

1. Symbols of atoms
2. Numbers of atoms
3. Commas
4. Brackets

For example, the formula of water is H2O and sugar is C12H22O11.

2. How do You calculate the Heat of Reaction or Reaction Enthalpy?

The heat of reaction can be determined numerically by the sum of heat of formation of products(multiply with the number of moles) minus the sum of heat of formation of reactants(multiply with the number of moles).

Hreaction=n-m

Where,

Hreaction=heat of reaction

= heat of formation of products

= heat of formation of reactants

n and m are the numbers of moles of products and reactants from a balanced chemical equation.

3. Calculating heat of reaction from bomb calorimetry data?

For the bomb calorimeter, we used equation

Q=mcT

Where,

q=heat transfer in KJ

m=mass of substance

T=change in temperature

At constant pressure

(dq)p=(dH)p

So we can write

dH=mcT

dH is the heat of reaction or change in enthalpy.

4. How to calculate the amount of heat released?

The amount of heat released can be calculated by using the following equation

q=mcT

Where,

q=amount of heat released or absorbed measured in joules

m= mass of liquid in kg

c= specific heat capacity in j/kg.K

T= change in temperature in kelvin

5. How do you calculate delta H for a reaction?

There is a list of some methods to find delta H.

(i). From the formula of enthalpy

ΔH= ΔU+P ΔV

(ii). From the formula of the heat of formation

Hreaction=nHf(products)-mHf(reactants)

(iii). From Hess’s law

(iv). From bond energies

(v). From the formula of heat capacity

H=mCpT

6. How to calculate the heat of reaction in kJ/mol?

The amount of heat evolved or absorbed can be calculated by using equation

q=mcT

At constant pressure

(dq)p=(dH)p

So we can write enthalpy in the place of heat.

H=mCpT

The heat of reaction or reaction enthalpy is measured in kilojoules. When we divide H by a number of moles then it can be measured in kJ/moles.

7. How to calculate the heat of reaction from a graph?

8. How do you calculate the total heat capacity of a reaction?

Heat capacity is defined as the amount of heat required to raise the temperature of a mass of substance through 1 kelvin.

q=mcT

c=q/mT

Heat capacity can be calculated if the values of the amount of heat transfer, the mass of substance, and the change in temperature are known.

9. What is the heat of neutralization?

The heat of neutralization is the amount of heat evolved when acid and base react with each other to form salt and water. The reaction is always exothermic so H is negative.

HCl+NaOH→NaCl+H2O

The heat of neutralization is -50.16kjmol-1.

10. What is the difference between enthalpy and heat of reaction?

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

H=U+PV

At constant pressure

(dH)p=(dQ)p

The change in enthalpy is equal to the amount of heat transferred at constant pressure. It is an intrinsic property of the thermodynamic system. It is a measure of heat absorbed or released during a chemical reaction.

The heat of reaction is also enthalpy change but in condition when reactants and products are in standard state. It can be measured by using the heat of formation.

When we are dealing with sensible heat effects we use the term enthalpy and for all other processes, we use the term heat of reaction.

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

The enthalpy change for the isomerization reaction of ethanol to dimethyl ether is 74.3kjmol-1. It means this is an endothermic reaction.

12. How do I calculate a theoretical value for the total change in temperature or energy in exothermic chemical reactions?

13. What happens when H3PO3 is heated?

On heating, it is converted into phosphoric acid and phosphine.

4H3PO3→3H3PO4+PH3

It is a disproportionation reaction i.e the reaction in which the same substance undergoes oxidation as well as reduction e.g phosphorus.

14. How much heat will be released from the reaction of calcium oxide and water?

Calcium oxide reacts with water to form calcium hydroxide.

CaO(s)+H2O(l)→Ca(OH)2

The amount of heat released during this reaction is -64kJ/mol.

15. How do heat losses to the surroundings affect the calculated value of specific heat?

The calculated value of specific heat does not depend on heat losses if the calculation is done properly.

16. What is the enthalpy change for a reversible reaction?

For a reversible reaction, the enthalpy change for the reverse reaction is equal to the enthalpy change of forwarding reaction but with the opposite sign.

For example, if the forward reaction is endothermic then the reverse reaction will be exothermic.

17. What is the difference between the enthalpy of formation and reaction?

Enthalpy of formation is defined as enthalpy change when one mole of a compound is formed from its constituent elements.

Enthalpy of reaction is the amount of heat released or absorbed during any type of chemical reaction such as combustion, hydrogenation, and neutralization, etc.

18. What is the effect of pressure and temperature on enthalpy?

Enthalpy is a state function so it depends on T, P, and internal energy. Enthalpy is the sum of internal energy and product of pressure-volume work.

H=U+PV

When the temperature is increased, the kinetic and potential energy of atoms increases which results in an increase in internal energy so enthalpy is increased.

If the pressure of the system increases then the pressure of the surroundings decreases so enthalpy of the system increases due to a decrease in external pressure.

19. In chemistry, how do you use the equation Q=mc∆t?

By using this equation we can calculate heat energy by putting the values of the mass of substance, specific heat, and change in temperature.

If the resulting value is positive then the reaction will be endothermic and vice versa.

20. What is the change in enthalpy in a constant volume process?

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

H=U+PV

Change in enthalpy is equal to

H=U+PV

At constant volume V=0, so

H=U

The change in enthalpy is equal to the change in internal energy.

21. What is the difference between heat absorption and heat released?

The reaction in which heat is absorbed from the surroundings to the system is called the endothermic process.

The reaction in which heat is released from the system to surroundings is called an exothermic process.

For example, freezing of water is endothermic while evaporation of water is exothermic.

22. What is the reaction between sodium hydroxide and water?

When sodium hydroxide is dissolved in water it dissociates into Na+ and OH ions. The Na+ interacts with OH of water and OH ions interact with the hydrogen of water. It is completely dissolved in water to form a solution.

The process is highly exothermic as it liberates a large amount of energy.

23. Is the reaction between NH3 + HCl →NH4++Cl a neutralization reaction?

It is not a neutralization reaction according to Arrhenius’s concepts of acid and base. According to this concept, an acid is a substance that gives H+ ion in solution and a base is a substance that gives OH ion in solution.

So we can say HCl is Arrhenius acid but NH3 is not Arrhenius base.

It is a neutralization reaction according to the Bronsted-Lowery concept. According to this concept, acid is a proton donor while the base is a proton acceptor. HCl is a strong acid so its conjugate base Cl is weak. NH3 is a weak base so its conjugate acid NH4+ will be strong.

24. What is the enthalpy of neutralization of acetic acid and sodium bicarbonate?

The enthalpy of neutralization of acetic acid and sodium bicarbonate is -55.4kJ. It is an exothermic reaction.

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