Molecules of a system pass energy in various forms such as sensible and latent energy (associated with change of state), chemical energy (associated with the molecular structure), and nuclear energy (associated with the atomic energy).

During a chemical reaction, some chemical bonds that bind the atoms into molecules are broken, and new ones are formed.

The chemical energy associated with these bonds, in general, is different for the reactants and the products.

During a chemical reaction:

DE_system = DE_state + DE_chem

When the products formed during a chemical reaction exit the reaction chamber at the inlet state of the reactants, we have,

DE_state = 0

Energy change of system is only due to change in chemical composition.

To have a common reference state for all substances, a reference state 25^oC and 1 atm. Is taken as a standard reference state. Property value at the standard reference state is indicated by a superscript “^o” (such as h^o and u^o).

Enthalpy of reaction

It is the difference between the enthalpy of the products at the specified state and the enthalpy of the reactants at the same state for a complete reaction.

DH_R = -393,520 kJ/kmol C

1 kmol C

1kmolO₂ 1kmol CO₂

25^oC,1atm

Enthalpy of combustion h_c, which represents the amount of heat released during a steady-flow combustion process when 1 kmol (or 1kg) of fuel is burned completely at a specified temperature and pressure.

A more fundamental property to represent the chemical energy of an element or compound at a reference state is enthalpy of formation.

It is defined as enthalpy of a substance at a specified state due to its chemical composition.

The enthalpy of formation of all stable elements (such as O₂, N₂, H₂ and C) is assigned a value of zero at the standard state of 25^oC and 1atm.

For all stable elements DH^o_f298 is zero. The stable form of an element is simply the chemically stable form of that element at 25^oC and 1 atm. The diatomic form (N₂) for example is the stable form at 25^oC and 1 atm and not monoatomic nitrogen N.

DH_f = Q = -393,520 kJ/kmol CO₂

Text Box: Combustion chamber 1 kmol C

1kmolO₂ 1kmol CO₂

25^oC,1atm 25^oC,1atm

The standard heats of formation for common compounds are available in tabulated form.

Example:

Calculate the standard heat DH^o_f298 for the following reaction

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (g)

Solution:

The standard heats of formation DH^o_f298 of the chemical species involved in the reaction are.

CO₂(g) : - 393.51 kJ; H₂O(g): -241.82 kJ

C₅H₁₂(g): -146.76 kJ; O₂(g): 0 kJ

The following combination of the formation reactions gives the desired reaction.

C₅H₁₂(g) à 5C(s) + 6H₂(g) DH^o₂₉₈ = 146.76 kJ

5{C(s) + O₂(g) àCO₂(g)} DH^o₂₉₈ = 5(-393.51)kJ

6{H₂(g) + ½ O₂(g) àH₂O(g)} DH^o₂₉₈ = 6(-241.82)kJ

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (g)

DH^o₂₉₈ = -3271.71 kJ

Example:

Estimate the standard heat DH^o_f298 of the following reaction:

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (l)

Assume that the latent heat of vaporization of water at 298.15 K is 2442.6 kJ/kg

Solution:

The standard heat DH^o₂₉₈ of the reaction

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (g)

DH^o₂₉₈ = -3271.71 kJ

6{H₂O(g) à H₂O}(l) DH^o₂₉₈ = 6(-43.97)kJ

The latent heat of vaporization of water =

(2442.6 x 18)/1000 = 43.97 kJ/mol

Hence the required

DH^o₂₉₈ = -3271.71 + 6(-43.97)

= -3535.53 kJ

Standard heat of combustion

A combustion reaction is defined as a reaction between the element or compound and oxygen to form specified combustion products.

The heat of reaction for a combustion reaction when the reactants and products are in their respective standard states is called the standard heat of combustion.

Another term commonly used is “heating values” of the fuel. If the water in the products is in the liquid form, it is called higher heating value and if it is in vapor form, it is called lower heating value.

Example:

Estimate the gross heating value and net heating value of pentane if the reactants and products are at 25^oC.

Solution:

The combustion reaction is given by

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (g)

The standard heat of the above reaction is estimated as

DH^o₂₉₈ = -3271.71 kJ per mol of pentane.

Therefore,

Net heating value = lower heating value

= -DH^o_C = 3271.71 kJ

If the water in the products is in the liquid state, the combustion reaction is given by

C₅H₁₂ (g) + 8O₂(g) à 5CO₂ (g) + 6H₂O (l)

The standard heat of this reaction is

DH^o₂₉₈ = -3535.53 kJ per mol of pentane

Gross heating value or higher heating value,

- DH^o_C = 3535.53 kJ

Effect of temperature on the standard heat of reaction

The standard heat of reaction at temperature T can be estimated in the following steps:

Reactants at Desired change Products at

temperature, T à temperature, T

DH^o_T Step 3

DH^o_R DH^o_P Heating Step1

cooling

Step 2

Reactants at à Products at

T = 298.15 K DH^o₂₉₈ T = 298.15 K

1. The reactants in their standard states are cooled at constant pressure (0.1 MPa) from T to 298.15 K. The change in the enthalpy associated with this process is

2. The reaction is allowed to proceed at 298.15 K. The change in enthalpy associated with this process is given by DH^o_298.

3. The products in their standard states are heated from 298.15 K to T. The change in enthalpy associated with this process is given by

Adiabatic flame temperature

The chemical energy released during a combustion process is either lost as heat to the surroundings or is used internally to raise the temperature of combustion products.

If no loss to the surrounding occurs, the temperature of the products will reach a maximum, which is called the adiabatic flame temperature.

The adiabatic flame temperature depends on:

1. state of the reactants

2. the degree of composition of reaction

3. the amount of air used

The adiabatic flame temperature is maximum when the complete combustion occurs with theoretical amount of air.

In combustion chambers, the highest temperature to which a material can be exposed is limited by metallurgical considerations. Hence, the value of adiabatic flame temperature is an important consideration for combustion chambers. Actual temperatures are usually lower than the adiabatic flame temperature.

Example:

Estimate the adiabatic flame temperature that can be reached by the combustion of n-Pentane with 25 percent excess air. Both the fuel and air enters the burner at 25^oC. Assume complete combustion.

Solution:

The first law of thermodynamics for a steady flow process, ignoring the changes in the kinetic energy and potential energy is given by:

H_e – H_i = Q = 0 or DH = 0

The burner is adiabatic: Q = 0, W_sh = 0

C₅H₁₂(g) + 10O₂(g) à 5CO₂(g) + 6H₂O(g) +2O₂(g) + 37.6N₂(g)

+ 37.6N₂

298 K DH = 0 DH_P

5CO₂(g) +6H₂O(g) + 2O₂(g) + 37.6N₂(g) (T)

The process that occurs in the reactor can be treated as consisting of two steps. First the reactants are converted into products at 298 K. The energy change associated with this step is equal to DH^o₂₉₈ . In the second step, the products are raised to temperature, T. The energy change associated with this step is equal to DH_P and is given by

The energy transferred as heat to the surroundings is Q which is equal to the overall change in the enthalpy (DH^o₂₉₈ +DH^o_P).

Q =DH^o₂₉₈ +DH^o_P = 0

DH^o₂₉₈ = -3271.71 kJ

The average molar heat capacities of CO₂, H₂O, O₂ and N₂ are 62.75 J/mol K, 52.96 J/mol K, 38.67 J/mol K and 37.13 j/mol K, respectively.

Therefore,

DH^o_P = (5 x 62.75 + 6 x 52.96 + 2 x 38.67 + 37.6 x 37.13) x 10^-3 (T-298)

Hence,

-3271.71 + (5 x 62.75 + 6 x 52.96 + 2 x 38.67 + 37.6 x 37.13) x 10^-3 (T-298) = 0

or,

T = 1852.3 K

Example:

An internal combustion engine uses octane as fuel. The air and fuel vapour mixture enter the engine at 25°C and 0.1 MPa and the engine uses 120 percent theoretical air. Supposing 75% of the fuel’s carbon is converted into CO₂ and the rest is converted to CO, and the combustion products leave the engine at 800K, calculate the amount of energy transferred as heat to the engine (per kg of fuel).

If the molar specific heat capacities in the ideal gas state for the reactants and products are expressed as a function of temperature by equations of the form,