Chemical Reactions and Heat of Reaction

Study Notes

Reactants are transformed into products through a series of steps, e.g., ( N_2 + 3 H_2 \rightarrow 2 NH_3 ).
Energy is absorbed to break existing bonds at the beginning of the reaction.
Energy is released when new bonds are formed at the end of the reaction.
An activated complex is a transient, unstable state between reactants and products.
Activation energy (( E_A )) represents the minimum energy required to initiate a reaction.
A positive catalyst increases reaction rates by lowering activation energy without undergoing chemical change.

Enthalpy (( H )) measures a chemical system's internal energy, with changes in enthalpy (( \Delta H )) being the only measurable values.
The heat of reaction (( \Delta H )) quantifies energy absorbed or released per mole through the formula ( \Delta H = H_{products} - H_{reactants} ).
Practically, ( \Delta H ) is calculated as the energy absorbed (to break bonds) minus the energy released (as new bonds form).
Units for energy change are expressed as ( kJ \cdot mol^{-1} ).

Exothermic reactions release energy; the energy absorbed to break bonds is less than the energy released during bond formation (( \Delta H < 0 )).
Endothermic reactions absorb energy; the energy absorbed to break bonds is greater than the energy released during bond formation (( \Delta H > 0 )).
Potential Energy Diagrams illustrate the changes in energy during reactions, depicting energy levels of reactants and products.

Specific bond energies provided useful for calculating ( \Delta H ) in chemical reactions.
For example, bond energies for nitrogen (( N \equiv N : 942 , kJ \cdot mol^{-1} )) and hydrogen (( H - H : 435 , kJ \cdot mol^{-1} )) are key in determining the heat of reactions.

Tasks include completing specific exercises, calculating energy changes, and drawing energy diagrams.
Refer to various exercises and past papers to reinforce concepts learned.