Thermochemical Equation

When a balanced chemical equation not only indicates the quantities of the different reactants and products but also indicates the amount of heat evolved or absorbed, it is called thermochemical equation.

Fractional coefficients may be used in writing a thermochemical equation.

H2 ( g ) + ½ O2 ( g ) ———> H2O ( l ) +285.8 KJ mol-1

H2 ( g ) + ½ O2 ( g ) ———> H2O ( l )  ΔH = – 285.8 KJ mol-1

285.8 KJ mol-1 of heat is produced when 1 mole of hydrogen reacts with 0.5 mole of Oxygen. If the quantities of reactants are doubled, the heat produced will also be doubled.

2H2 ( g ) + O2 ( g ) ———> 2 H2O ( l ) + 571.6 KJ mol-1

2H2 ( g ) + O2 ( g ) ———> 2H2O ( l ) ,ΔH = – 571.6 KJ mol-1

Conventions about thermochemical equation

1)For exothermic reaction, ΔH is negative whereas for endothermic reaction, ΔH  is positive.

2)ΔH values are for the standard state of the substance (  298 K and 1 bar pressure )

3) The coefficients of different substances represents the number of moles reacted and formed for the heat change represented in the equation.

4) The physical state of the different substances must be mentioned as the heat evolved or absorbed depends upon the physical state.

5) If the coefficient of the substances are multiplied or divided by some number , the value of ΔH is multiplied or divide by the same number.

6) If the reaction is reversed ,the sign of ΔH changes but the magnitude remains the same.

Heat of reaction or Enthalpy of reaction or Enthalpy change of reaction

The amount of heat evolved or absorbed in a chemical reaction when the number of moles of reactants as represented by the chemical equation have completely reacted, is called the heat of reaction or enthalpy of reaction or enthalpy change of reaction.

It is represented by Δr H.

For example

CH4 ( g ) + O2 ( g ) ————> CO2 ( g ) + 2 H2O ( l ) ΔH = – 890.4 KJ mol-1

C ( s ) + H2O ( g ) ———> CO ( g ) + H2 ( g ) ΔH = 131 Kj mol-1

1 mole of methane combines completely with 2 moles of Oxygen gas, 890.4 KJ of heat is produced.

1 mole of solid carbon react completely with one mole of steam ,131.4 KJ of heat is absorbed.

Enthalpy of reaction, ΔH = Sum of enthalpies of products – Sum of enthalpies of reactants

ΔH = ∑ai H (products) – ∑bi H (reactants)

where symbol ∑ represent  summation, ai and bi represents the coefficient of the products and reactants.

If the reaction is reversed , the sign of ΔH changes

H2 ( g ) + ½ O2 ( g ) ———> H2O ( l )  ΔH = – 285.8 KJ mol-1

H2O ( l ) ——–>H2 ( g ) + ½ O2 ( g )  ΔH = 285.8 KJ mol-1

Factors on which the heat of reaction depends

1) Quantities of the reactants involved

If the quantities of reactants are doubled, the heat of reaction is also doubled.

2)Physical state of the reactants and products

The physical state of the reactants and products affects the heat of reaction.

When hydrogen and oxygen gas combine to form liquid water, the heat of reaction is different than when they combine to form water in the gaseous state.

H2 ( g ) + ½ O2 ( g ) ———> H2O ( l )  ΔH = – 285.8 KJ mol-1

H2 ( g ) + ½ O2 ( g ) ———> H2O ( g )  ΔH = – 241.8 KJ mol-1

It is essential to write the symbols s, l, g  or aq to indicate whether a particular substance is a solid ,liquid ,gaseous or an aqueous solution.

3)Allotropic modification

For elements like Sulphur and carbon which exist in different allotropic modification, the heat of reaction is different in different allotropic form is involved in the reaction.

C ( diamond ) + O2 ( g ) ——–> CO2 ( g) , ΔH = -395.4 KJ mol-1

C ( graphite ) + O2 ( g ) ——–> CO2 ( g) , ΔH = -395.4 KJ mol-1

4)Concentration of solution

Heat changes occurs when a solute is dissolved in a solvent to form a solution or when solution is diluted.

Therefore, if the solutions are involved in a reaction, their concentration affect the heat of reaction.

5)Temperature

The heat of reaction depends upon the temperature at which the reactants and products are taken. The values are usually reported at 298 K.

6)Conditions of constant pressure or constant volume

Whether the reaction take place at constant pressure or at constant volume.

Standard enthalpy of reaction

The enthalpy change of a reaction depends upon the condition under which the reaction is carried out. Hence it is essential to specify some standard conditions.

The standard enthalpy of a reaction is the enthalpy change accompanying the reaction when all the reactants and products are taken in their standard state.

A substance is said to be in standard state when it is in the purest and most stable form at 1 bar pressure and the specified temperature.

This temperature is usually taken as to 298 K.

Standard state of pure ethanol at 298 k is pure liquid ethanol at 298 K and 1 bar pressure.

in terms of S.I. unit, 1 atm=101.325 KPa whereas 1 bar=105  Pa .

The enthalpy change of a reaction in the standard state is represented by the symbol ΔH°

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