Sunday, February 3, 2008

Revision: Heat of reaction, fusion and vapourization

Materials can contain heat energy.

They can absorb heat without changing temperature while they change phase.
These energy changes can be accounted for with constants specific to the material.

Heat of Fusion, Hf is the energy required to change a solid to a liquid, or the amount of heat per gram given off when a liquid freezes.

Heat of vaporization, Hv, is the amount of energy required to change a liquid to a gas at its boiling point. It is also the amount of heat released when a liquid condenses.

Water has a heat of fusion of 334 J/g
Water has a heat of vaporization of 2,270 J/g

The specific heat of a substance depends on its state.

Solid water has a heat capacity of 2.108 J/g °C
Liquid water has a heat capacity of 4.186 J/g °C
Water vapor has a heat capacity of 1.996 J/g °C

So how much energy heat is required to heat 24.0 g of water from ice at -10°C to vapor at 120°C?

(A)First heat to melting point, 24.0 x 2.108 x (0- -10) = 506
(B)Then melt the ice, 24.0 x 334 = 8016
(C)Then heat to boiling, 24.0 x 4.186 x (100-0) =10046
(D)Then boil the water, 24.0 x 2,270 =54480
(E)Then heat the vapor to 120° 24.0 x 1.996 x (120-100) = 958

Total A+B+C+D+E = 74,006 J = 74 KJ

Heat of reaction

There is also energy contained in the bonds of a chemical compound. To break the bonds requires energy, energy is released when the bonds are formed. Every compound has a characteristic bonding energy called enthalpy, ∆H, which can be used to determine the heat released or required during chemical reactions. Let’s see how it’s used.

Substance ∆H (kJ/mol)
C2H4 52.30
O2 0
CO2 -393.51
H2O -285.85

With these values we can determine the heat released in the reaction:

C2H4 + 3O2 → 2CO2 + 2H2O

Reactants Products

1 x 52.30 +3 x 0 → 2 x (-393.51) + 2 x (-285.85)

+52.30 + 0 → -787.02 -571.70

+52.30 → -1358.72

-1411.02 kJ


Multiply each reactant ∆H by its coefficient in the balanced equation.

Add both sides separately.

Subtract reactant side by transferring it over to the product side.

∆Heat of Reaction = Sum of ∆H products – Sum ∆H reactants

Negative ∆Heat of Reaction means the heat is released, the chemicals have lost the heat to the surroundings.

This is an exothermic reaction, the heat is produced.

The reaction can be completed:

C2H4 + 3O2 → 2CO2 + 2H2O + 1411.02 kJ

Notice the change of the sign of the net ∆H in the reaction. The + sign on the product shows that energy is also released along with products.

But ∆H is negative. Enthalpy change is negative. energy is lost to surroundings.

A positive ∆H means that the products bond energy is higher, they soaked up energy from their surroundings. A reaction with +∆H is endothermic it feels cold.

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