- Energy absorption or release can be placed directly in equation
- For example (the formation of water): 2H2 (g) + O2 (g) --> 2H2O (l) + 572 kJ
- Exothermic reactions have the energy term on the right hand side and a negative change in enthalpy (H). Therefore the formation of water is an exothermic reaction
- Endothermic reactions have the energy term on the left hand side and a positive change in enthalpy (H).
Energy Calculations:
- Change in enthalpy is the energy change of a reaction and is expressed in kJ/mole of 1 of the chemicals
- We take the water formation sample again: 2H2 (g) + O2 (g) --> 2H2O (l) + 572 kJ
- Change in enthalpy for the above reaction is expressed using coefficients of the balanced equation:
- -572 kJ / 1 mol O2 or
- -572 kJ / 2 mol H2 = -286 kJ / 1 mol H2 or
- -572 kJ / 2 mol H2O = -286 kJ / 1 mol H2O
- Therefore, the value of change in enthalpy depends on which chemical in the chemical equation is referred to
Again, an example for the exothermic reaction:
Note:
- The value of change in enthalpy with different reactions
- change in enthalpy depends on the chemical reaction
- change in enthalpy is NOT a constant like Avogadro's number
Now, we officially welcome the return of moles and significant figures!!!
Sample question you can be asked:
e.g. For the reaction 2H2 (g) + O2 (g) --> 2H2O (l) + 572 kJ, if 2.3 moles of oxygen gas is burned in the presence of hydrogen gas, how much heat energy would be released?
2.3 mol O2 * (-572 kJ/1 mol O2) = 1315.6 kJ
Don't forget sig. figs: = 1300 kJ
Relationship between Moles and energy:
Moles * (kJ/1mol) = Energy
Energy * (1 mol/kJ) = Moles
Links:
(Enthalpies of reactions)
(Enthalpy changes)
(Practice problems with answers)
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