9 Heat release and enthalpy

Question 1

How to calculate enthalpy change in combustion reaction

Answer 1

1. Apply first law of thermo to show change in internal energy (enthalpy) of the system
2. hence derive enthalpy of combustion

Question 2

What is the 1st Law of Thermodynamics?

Answer 2

(The increase in internal energy (H) of a thermo system) is equal to (the amount of heat energy added to the system) minus (any work done by the system on the surroundings)

Question 3

What is Enthalpy?

Answer 3

H (aka Heat content)
= sum (internal energy of a thermodynamic system) + (energy associated with work done by the system on the atmosphere)

Question 4

What is the SFEE?

Answer 4

Steady Flow Energy Equation

Relates Ek, Ep, Heat Transfer and Work done on the system

Question 5

SFEE

Answer 5

Q-W = (h2-h1) + 0.5(v2^2-v1^2) + g(z2-z1)

Question 6

Assumptions that need to be made to isolate enthalpy from SFEE

Answer 6

• No work is transferred across system boundary (W=0)
• No change in Ep or Ek (v and z = constant)
• Gas mixture is ideal (so ΔU independent of V and T)

Question 7

Reduced SFEE when assumptions applied

Answer 7

Q = ΔH0

= Enthalpy of Combustion equation
(0 refers to datum)

Question 8

How do pressures and temps change within combustion processes?

Answer 8

The combustion acts as the change in enthalpy needed by the reactants to transition to different temps and pressures of the products

Question 9

How to calculate energy change from reactants to products in combustion reaction

Answer 9

(Change in energy between products and reactants) = (change in internal energy of reactants) + (change in internal energy of reactants) + (internal energy of combustion)

Δ(Up-Ur)1 = Δ(Up)0,2 + Δ(Ur)0,1 + ΔU0