Chemical Reactions: Energy, Enthalpy, and Equilibrium
The Power of Chemical Reactions: Energy and Chemical Balance
6.1 Energy in Chemical Reactions
Chemical reactions are always accompanied by an exchange of energy. This can be either a release or absorption of energy because the energy that the reactants have is different from the energy that the products possess. A chemical reaction involves the breaking of bonds in reactants and the formation of new bonds in products. Breaking bonds requires energy, while the formation of bonds normally entails a release of energy.
The question is how to calculate that difference in energy. If the products have less energy than the reactants (the initial process), the reaction can be:
- Exothermic (releases heat)
- Endothermic (absorbs heat)
The enthalpy of a reaction is defined as the sum of its internal energy plus the product of its volume and external pressure. It is represented by the letter H. Its formula is:
H = U + PV
Where:
- The heat absorbed or evolved in a chemical reaction at a constant volume (in a closed container) equals the change in internal energy that has occurred.
- The heat absorbed or evolved in a chemical reaction at a constant pressure (in an open container) equals the enthalpy change that has occurred.
6.2 Formation Enthalpy and Reaction Enthalpy
The formation enthalpy of a substance is defined as the amount of heat absorbed or evolved in the formation of one mole of that substance from its simple elements at 1 atm pressure and 25°C. The reaction enthalpy is the heat lost or absorbed in a chemical reaction at 1 atm and 25°C.
Hess’s Law: The heat of a reaction is the same whether the reaction occurs in one stage or indirectly in several stages. That is, the heat of a reaction depends only on the initial state (reactants) and the final state (products), not on the path taken by the reaction.
6.3 Spontaneity of Chemical Reactions
Normally, exothermic reactions are spontaneous, but there are also endothermic reactions that are spontaneous. This means that energy is not the only factor that influences the determination of the direction of spontaneous changes. Another factor is disorder: an increase in disorder in a chemical reaction favors its spontaneous transformation. Solid bodies have highly ordered molecules, while in liquids and gases, the molecules are more disordered.
Entropy (S) is a quantity that measures the degree of disorder in a chemical reaction.
6.4 Chemical Equilibrium
A system is in a state of equilibrium when its composition does not vary with time. If the substances in the system are the result of a chemical reaction, in the state of equilibrium, there is a certain amount of reactants and products.
Features of Chemical Equilibrium:
- Chemical equilibrium is reversible; the same state of equilibrium can be reached starting from either the reactants or the products.
- Chemical equilibrium is dynamic. When chemical equilibrium is reached, the reaction does not stop. What happens is that the conversion of reactants to products occurs at the same rate as the conversion of products to reactants.
Equilibrium Constants Kc and Kp
Consider a reaction:
aA (g) + bB (g) ⇌ cC (g) + dD (g)
Where (g) signifies that the substance is gaseous, and [A] represents the concentration of substance A in moles/liter. Experimental checks show that when equilibrium is reached in a chemical reaction, the value of the equilibrium constant (Kc) is as follows:
Kc = ([C]^c * [D]^d) / ([A]^a * [B]^b)
When all substances are gases, the equilibrium constant can also be expressed in terms of partial pressures (Kp).