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Equilibrium : A State of Dynamic Balance

Equilibrium : A State of Dynamic Balance. Reversible Reactions. When a reaction results in complete conversion of reactants to products chemists say it goes to completion Not all reactions go to completion. They appear to stop because they are reversible

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Equilibrium : A State of Dynamic Balance

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  1. Equilibrium : A State of Dynamic Balance

  2. Reversible Reactions • When a reaction results in complete conversion of reactants to products chemists say it goes to completion • Not all reactions go to completion. They appear to stop because they are reversible • Reversible reactions can occur in both the forward and reverse directions

  3. Reversible Reactions cont. . . • Forward N2 + 3H2 2NH3 • The reactants are N2 and 3H2; the product is NH3 • Reverse N2 + 3H2  2NH3 • The reactant is 2NH3; the products are N2 and 3H2 N2 + 3H2⇆ 2NH3 The forward and reverse reactions are happening at the same time.

  4. Chemical Equilibrium • Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates. • Rateforward reaction = Ratereverse reaction • This does notmean the concentrations of the products and reactants are the same • Equilibrium is a state of action, not inaction. This process is dynamic; dynamic equilibrium.

  5. http://www.youtube.com/watch?v=JsoawKguU6A

  6. Equilibrium Expression and Constant (Keq) • Law of chemical equilibrium states that at a given temperature, a chemical system may reach a state in which a particular ratio of reactant and product concentrations has a constant value known as Keq or equilibrium constant.

  7. Equilibrium Constant Keq • Keq is the ratio of product concentrations to reactant concentrations. • Keq only includes compounds that are in the gaseous or aqueous state, not solid or liquid • A large Keq, Keq > 1 means the products are favored over the reactants • A small Keq, Keq < 1 means the reactants are favored over the products

  8. Homogenous vs. Heterogeneous Equilibrium • Homogenous equilibrium means all reactants and products are in the same physical state • Ex: H2(g) + O2(g)  H20(g) • Heterogeneous equilibrium is when the reactants and products are in more than one physical state. • Ex: H2O(g) + C(s)  H2(g) + CO(g)

  9. Chemical Equilibrium—Keq aA + bB  cC +dD products [C]c [D]d reactants [A]a [B]b Keq = = **only use Keq for gases and aqueous compounds, cross out all liquids and solids

  10. Homogenous Equilibria • Given N2(g) + 3H2(g) 2NH3(g) write the equilibrium constant expression for the reaction.

  11. Given 2SO2(g) + O2(g) 2SO3(g) write the equilibrium constant expression for the reaction.

  12. Heterogeneous Equilibria • Given 2NaHCO3(s) Na2CO3(s) + CO2(g) +H2O(g) write the equilibrium constant expression for the reaction. *Products are in the numerator **Remember not to use solids or liquids in the Keq expression.

  13. Calculating the Value of Equilibrium Constants At equilibrium and 100°C a flask contains: [PCl5]=0.0325M [H2O]=0.025M [HCl]=0.375M [POCl3]=0.250M Calculate the Keq for the reaction PCl5(g) + H20(g) 2HCl(g) + POCl3(g) **We don’t use units for Keq, or it would get messy!

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