How Does Energy Shift Affect Chemical Equilibrium?
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 Chemistry
1. What is meant by chemical equilibrium in a reaction?
Chemical equilibrium is a state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction. This means that while reactants are still turning into products and vice versa, the overall concentrations of both remain constant. It's a dynamic state, not a static one where reactions have stopped.
2. What are the main types of chemical equilibrium?
Chemical equilibrium is mainly classified into two types based on the phases of the substances involved:
- Homogeneous Equilibrium: This occurs when all reactants and products are in the same phase, such as all being gases or all being dissolved in a liquid solution.
- Heterogeneous Equilibrium: This occurs when the reactants and products are in two or more different phases. For example, a reaction involving a solid reactant and a gaseous product.
3. What is the relationship between Gibbs free energy and the equilibrium constant?
The relationship between the standard Gibbs free energy change (ΔG°) and the equilibrium constant (K) is described by the equation: ΔG° = -RT ln K. In this formula, 'R' is the universal gas constant, and 'T' is the temperature in Kelvin. This equation shows how the spontaneity of a reaction under standard conditions is directly linked to its equilibrium constant.
4. Why is the change in Gibbs free energy (ΔG) exactly zero at equilibrium?
The change in Gibbs free energy (ΔG) represents the driving force of a reaction. At equilibrium, the system is in its most stable state because the forward and reverse reactions are perfectly balanced. Since there is no net change occurring and the system cannot perform any more work, the driving force for the reaction is zero. Therefore, the Gibbs free energy change (ΔG) is also zero.
5. How does changing the temperature affect a reaction at equilibrium?
According to Le Chatelier's principle, the effect of temperature depends on whether the reaction is exothermic or endothermic.
- For an exothermic reaction (releases heat), increasing the temperature will shift the equilibrium to the left, favouring the reactants.
- For an endothermic reaction (absorbs heat), increasing the temperature will shift the equilibrium to the right, favouring the products.
6. What happens to the equilibrium if you increase the pressure on a gaseous reaction?
Increasing the pressure on a system at equilibrium will cause it to shift in the direction that has fewer moles of gas. This is how the system counteracts the pressure increase. If the number of gas moles is the same on both the reactant and product sides, a change in pressure will have no effect on the equilibrium position.
7. Can a catalyst change the equilibrium position of a reaction? Why or why not?
No, a catalyst cannot change the equilibrium position or the value of the equilibrium constant (K). A catalyst works by speeding up both the forward and the reverse reactions equally. This means it helps the reaction reach equilibrium much faster, but it does not alter the final concentrations of reactants and products once equilibrium is achieved.
8. How can you predict if a reaction will move forward or backward using the reaction quotient (Q)?
You can predict the direction of a reaction by comparing the reaction quotient (Q), which is the ratio of products to reactants at any given time, to the equilibrium constant (K).
- If Q < K, the reaction will proceed in the forward direction to make more products.
- If Q > K, the reaction will proceed in the reverse direction to make more reactants.
- If Q = K, the reaction is already at equilibrium and no net change will occur.
9. What is the difference between dynamic and static equilibrium?
In dynamic equilibrium, the forward and reverse reactions are still occurring at the same rate, so there's continuous movement at the molecular level, but no overall change. In contrast, static equilibrium is a state where all movement has ceased, and the reaction has completely stopped. Chemical equilibria are always dynamic.
10. If the standard Gibbs free energy change (ΔG°) is positive, does it mean the reaction can never be spontaneous?
Not necessarily. A positive ΔG° only means the reaction is non-spontaneous under standard conditions (1M concentration, 1 atm pressure). The actual spontaneity of a reaction is given by ΔG, which depends on the current concentrations (or the reaction quotient, Q). By changing the reaction conditions, like removing a product as it forms, it is possible to make ΔG negative, allowing the reaction to proceed.
