How Energy Change Due To Equilibrium Relates To Enthalpy Gibbs Free Energy And Equilibrium Constant
Definition of Equilibrium
When there is no change in the concentration of the reactants as well as the products with time, then that point is called the chemical equilibrium. When we say equilibrium, it means the reactants are going to products where the rate of forward reaction (reactant to products) is equal to the rate of backward reaction (products to reactions). We can also say the rate at which the reactants turn into products is equal to the rate at which products turn into reactants. Equilibrium is represented with a symbol ⇌.
Reactants ⇌ Products
There are many examples of chemical equilibrium around us. One example is a bottle of a fizzy drink. The bottle has the liquid of dissolved carbon dioxide into it. There is also CO2 gas present in the space between the liquid and bottle cap. There is a constant movement of CO2 from the liquid to the gas phase, and from gas to the liquid phase. However, if you look at the bottle, there does not appear to be any change. This is the point at which the system has reached chemical equilibrium (where the rate of the forward reaction is equal to the rate of backward reaction).
Equilibrium- Key Points
The State of Equilibrium is the one in which there is no net change in the concentration of reactants and products, but this doesn’t mean the reactions have stopped. At equilibrium, the forwards and backward reactions continue but at the identical rate.
The concentration of reactants represented as [Reactant]
The concentration of products represented as [Products]
At Equilibrium, the ratio of [Reactants] and [Products] are constant and this constant is known as Equilibrium Constant and represented with the symbol Keq.
For Example: consider this below chemical reaction:
A ⇌ B
Where A is the Reactant and B is a product
Forward reaction: A → B, where A goes to form B and it has some rate constant kf (the rate at which the reaction is taking place). The forward rate equation will look like this,
Ratef = kf [A], here rate of reaction is dependent on the concentration of reactant to form the product B.
Backward reaction: B → A, where B goes to form A and it has some rate constant kr (the rate at which the reaction is taking place). The backward reaction will look like this,
Rater = kr [B],
So, based on the definition of equilibrium:
The rate at which the product is formed = rate at which the reactant is formed
kf [A] = kr [B] (rate of the forward reaction is equal to the rate of backward reaction)
= Constant
Equilibrium condition can be achieved from either direction, either going from reactants to the product or going back from products to reactants.
Graphical Representation
The concentration of Products & Reactants at Equilibrium
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In the above graph, prior to equilibrium, the concentration of the products is increasing, and the concentration of the reactants is decreasing. Rates of forward reactions and reverse reactions can be seen as equal in equilibrium. Therefore, there is no change in the concentration of reactants and products.
Rate of Forward & Backward reactions over time and at Equilibrium
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Over time, rates of the forward reactions and the reverse reactions become equal and due to this, the reaction system can be seen at equilibrium.
Br₂(l) ↔ Br₂(g)
Energy Changes Due to Equilibrium:
1. Change in Concentration:
For example, any reaction mixture is at equilibrium and you add some more reactant. Then according to Le Chatelier’s principle, the reaction will change itself in order to counteract the change. Therefore, if reactants are added, then the reaction will shift towards the products to form more products and if products are added, then the reaction will shift towards the reactant.
2. Change in Pressure:
This can be achieved by changing the volume of the container. For example, if we mechanically decrease the volume of a container (of gases), then the pressure inside the container will increase. Since the change we made was to increase the pressure inside the container, the reaction will drift in a way to decrease the pressure. This could be attained with few gas molecules, by moving to the edge of the reaction.
3. Addition of an Inert Gas:
Adding Inert gases like (Argon, Neon Krypton) to the reaction mixture at constant volume, it has no effect. At the constant volume with the addition of inert gas to the system, total pressure will be changed but the partial pressure of compounds will remain the same.
4. Change in Temperature:
A temperature change occurs when the temperature is increased or decreased by the flow of heat. This shifts the chemical equilibria towards the product or the reactant, which can be determined with the study of the reaction and also deciding whether the reaction is endothermic or is exothermic.
For Exothermic reactions, the Equilibrium constant decreases when the temperature increases.
For Endothermic reactions, the Equilibrium constant increases when the temperature increases.
5. Effect of Catalyst on Equilibrium:
By adding a catalyst to a reaction, the energy of activation of both forward and backward reactions are lowered. Thereby, both forward and backward reactions increase in the same amount and thus, the equilibrium remains unaffected. Catalysts are basically compounds which accelerate the rate of the reaction without being consumed.
FAQs on Energy Change Due To Equilibrium in Chemical Reactions
1. What is energy change due to equilibrium in chemistry?
Energy change due to equilibrium refers to the enthalpy change (ΔH) associated with a reversible reaction as it reaches dynamic equilibrium. In a chemical equilibrium system:
- The forward and reverse reactions occur at equal rates.
- The overall energy change is determined by the reaction’s ΔH value.
- If ΔH is negative, the reaction is exothermic; if positive, it is endothermic.
2. How does temperature affect chemical equilibrium and energy change?
Temperature changes shift equilibrium according to Le Châtelier’s principle, depending on whether the reaction is exothermic or endothermic.
- For an exothermic reaction (ΔH < 0), increasing temperature shifts equilibrium to the left (toward reactants).
- For an endothermic reaction (ΔH > 0), increasing temperature shifts equilibrium to the right (toward products).
- Heat can be treated as a reactant (endothermic) or product (exothermic).
3. What is the relationship between enthalpy change (ΔH) and equilibrium constant (K)?
The relationship between enthalpy change and the equilibrium constant is given by the van ’t Hoff equation, which shows how K changes with temperature.
- If ΔH > 0 (endothermic), increasing temperature increases K.
- If ΔH < 0 (exothermic), increasing temperature decreases K.
- The simplified form is: ln(K2/K1) = −ΔH/R (1/T2 − 1/T1).
4. Is there any energy change at equilibrium?
At dynamic equilibrium, there is no net energy change because the forward and reverse reactions occur at equal rates.
- Individual molecules still react and exchange energy.
- The system’s overall concentrations and total energy remain constant.
- The forward reaction’s enthalpy change is equal and opposite to that of the reverse reaction.
5. How do you determine if an equilibrium reaction is exothermic or endothermic?
You determine whether an equilibrium reaction is exothermic or endothermic by checking the sign of ΔH or observing the effect of temperature on equilibrium.
- If ΔH is negative, the reaction is exothermic.
- If ΔH is positive, the reaction is endothermic.
- If increasing temperature shifts equilibrium to products, the forward reaction is endothermic.
6. What happens to equilibrium when heat is added to an exothermic reaction?
When heat is added to an exothermic reaction, the equilibrium shifts toward the reactants to absorb the excess heat.
- Heat acts as a product in an exothermic reaction.
- Adding heat increases the reverse reaction rate.
- The equilibrium constant K decreases with increasing temperature.
7. How is Gibbs free energy related to chemical equilibrium?
Chemical equilibrium is reached when the Gibbs free energy change (ΔG) equals zero under constant temperature and pressure.
- If ΔG < 0, the reaction is spontaneous in the forward direction.
- If ΔG > 0, the reverse reaction is favored.
- At equilibrium, ΔG = 0 and ΔG° = −RT ln K.
8. Why does increasing temperature increase the equilibrium constant for endothermic reactions?
Increasing temperature increases the equilibrium constant for endothermic reactions because heat acts as a reactant.
- Endothermic reactions absorb heat (ΔH > 0).
- Adding heat shifts equilibrium toward products.
- As product concentration increases, the equilibrium constant K increases.
9. What is the energy profile diagram for a reaction at equilibrium?
An energy profile diagram for a reaction at equilibrium shows equal forward and reverse reaction rates with different activation energies.
- The vertical axis represents potential energy.
- The peak represents the activation energy (Ea).
- The difference between reactants and products represents ΔH.
10. Can you give an example of energy change affecting equilibrium in industry?
A classic example is the Haber process, where energy change affects ammonia production at equilibrium.
- Reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)
- ΔH = −92 kJ/mol (exothermic).
- Lower temperatures favor ammonia formation but slow the reaction rate.
- Industrially, a moderate temperature (~450°C) balances yield and rate.
