Chemical Equilibrium Concepts

Chemical Equilibrium Study Notes

What is Chemical Equilibrium?

• Definition: Chemical equilibrium refers to a state in a reversible reaction where the forward and reverse reactions occur at the same rate, leading to no net change in the concentrations of reactants and products.

• Representation: The state of equilibrium is typically indicated in chemical equations by a double arrow (↔).

Overview of Reversible Reactions

• Example Reaction:

  • Forward Reaction: $2SO<em>2(g) + O</em>2(g) <br>ightleftharpoons 2SO_3(g)$

  • Reverse Reaction: $2SO<em>3(g) ightleftharpoons 2SO</em>2(g) + O_2(g)$

• When both the forward and reverse reactions are occurring at the same rate, the system reaches equilibrium.

Chemical Equilibrium Explained

• Dynamic Process:

  • Chemical equilibrium is a dynamic process, meaning that reactions continue to occur in both directions (i.e., the conversion of reactants to products and vice versa).*

  • No visible change in the amount of reactants and products; they remain constant over time.

• Equilibrium Example:

  • For the reaction $A + B <br>ightleftharpoons AB$:

  • The forward reaction: $A + B <br>ightarrow AB$

  • The reverse reaction: $AB <br>ightarrow A + B$

  • Both reactions are happening at the same rate, leading to equilibrium.

Le Chatelier's Principle

• Principle Statement:

  • Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract the effect of that change and restore a new equilibrium.

• Forward Reaction:

  • $A + B <br>ightarrow AB$

  • If this reaction is favored, the concentrations of reactants $[A]$ and $[B]$ will decrease, while the concentration of product $[AB]$ will increase over time.

• Reverse Reaction:

  • $AB <br>ightarrow A + B$

  • If this direction is favored, the concentrations of reactants $[A]$ and $[B]$ will increase, while the concentration of product $[AB]$ will decrease over time.

Types of Changes Affecting Equilibrium

• Changes or Stresses to a System:

  • Concentration changes

  • Temperature changes

  • Pressure changes

Specific Examples of Le Chatelier's Principle

• Stress: Addition of CO2

  • Shift: The reaction shifts towards the reactants. This shift leads to an increase in the formation of $H<em>2CO</em>3$ (carbonic acid) in the bloodstream, which increases blood acidity.

• Stress: Increased Breathing

  • Shift: The reaction shifts towards the products. The body responds by breathing rapidly to remove CO2 from the bloodstream.

Factors That Affect Equilibrium

• Concentration Changes:

  • Adding or removing reactants/products affects the system's equilibrium.

  • Scenario:

    • Add reactant (either $A$ or $B$) → Forward reaction is favored.

    • Add product ($AB$) → Reverse reaction is favored.

    • Remove reactant (either $A$ or $B$) → Reverse reaction is favored.

    • Remove product ($AB$) → Forward reaction is favored.

Temperature Changes

• Exothermic Reactions:

  • Defined as reactions that release heat.

  • Example:

    • $N<em>2(g) + 3H</em>2(g) <br>ightleftharpoons 2NH_3(g) + 91 ext{ kJ}$

  • Adding heat favors the reverse reaction.

• Endothermic Reactions:

  • Defined as reactions that absorb heat.

  • Example:

    • $N<em>2O</em>4(g) + ext{heat} <br>ightleftharpoons 2NO_2(g)$

  • Adding heat favors the forward reaction.

Pressure Changes in Gaseous Reactions

• Effect of Pressure Changes:

  • Increasing pressure favors the direction that produces fewer gas molecules.

  • Decreasing pressure favors the direction that produces more gas molecules.

Practical Examples of Pressure Effects

• Scenario 1:

  • Reaction: $CaCO<em>3(s) ightleftharpoons CaO(s) + CO</em>2(g)$

  • Not affected by pressure changes as there are no gas molecules on both sides.

• Scenario 2:

  • Reaction: $N<em>2(g) + 3H</em>2(g) <br>ightleftharpoons 2NH_3(g)$

  • An increase in pressure favors the forward reaction, while a decrease in pressure favors the reverse reaction.

• Scenario 3:

  • Reaction: $H<em>2(g) + Cl</em>2(g) <br>ightleftharpoons 2HCl(g)$

  • This reaction is unaffected by pressure changes as there are equal moles of gas on each side.

• Example Reaction:

  • Forward Reaction: $2SO<em>2(g) + O</em>2(g) \rightleftharpoons 2SO_3(g)$

  • Reverse Reaction: $2SO<em>3(g) \rightleftharpoons 2SO</em>2(g) + O_2(g)$

• Equilibrium Example:

  • For the reaction $A + B \rightleftharpoons AB$:

  • The forward reaction: $A + B \rightarrow AB$

  • The reverse reaction: $AB \rightarrow A + B$

  • Both reactions are happening at the same rate, leading to equilibrium.

• Stress: Addition of CO2

  • Shift: The reaction shifts towards the reactants. This shift leads to an increase in the formation of $H<em>2CO</em>3$ (carbonic acid) in the bloodstream, which increases blood acidity.

• Stress: Increased Breathing

  • Shift: The reaction shifts towards the products. The body responds by breathing rapidly to remove CO2 from the bloodstream.