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| Publisher | VTU eLearning |
| Number of Pages | 253 |
| Available | Available in all digital devices |
Every chemical species has a certain amount of "heat content," or enthalpy, H, which cannot be measured. However, differences in enthalpy can be measured. The net energy change for a reaction performed at constant pressure is the enthalpy change for the reaction. This enthalpy change, H, has units kJ mol and is defined
C H fuel O2 N2 Air - Combustion Process - CO2 H2O N2 Heat where C Carbon, H Hydrogen, O Oxygen, N Nitrogen 1
H H products H reactants
If energy is given off during a reaction, such as in the burning of a fuel, the products have less heat content than the reactants and H will have a negative value the reaction is said to be exothermic. If energy is consumed during a reaction, H will have a positive value the reaction is said to be endothermic. The enthalpy change for a chemical change is independent of the method or path by which the
change is carried out as long as the initial and final substances are brought to the same temperature. This observation, known as HESS'S LAW, has important practical utility. Thermochemical equations may be treated as algebraic equations they may be written in the reverse direction with a change in the sign of H even though the reverse reaction may not actually occur they may be added and subtracted algebraically the equation and associated H value may be multiplied or divided by factors. Hess's Law allows the calculation of enthalpy changes that would be difficult or impossible to determine directly, i.e. by experiment. The enthalpy change for the reaction 2
2C s O2 g
2CO g
cannot be determined directly because carbon dioxide will also form. However, measured for 3
C s O2 g
CO2 g
4
2CO g O2 g
2CO2 g
H can be
H 393.5 kJ H 566.0 kJ
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