The Equilibrium Constant Expression Here comes K!!

GG HH SS-T  S Comments +-+- Both  H &  S are favourable so the reaction is favourable at all temperatures -+-+ Neither  H &  S are favourable so the reaction is unfavourable at all temperatures +--+  H is favourable &  S is not so the reaction is only favourable at lower temperatures +++-  S is favourable &  H is not so the reaction is only favourable at higher temperatures  G =  H – T  S

Describing equilibria The state of equilibrium can be described in two ways; percentage reaction equilibrium constant, K eq The progress or status of the equilibrium can be assessed through the; reaction quotient, Q.

Percent reaction at equilibrium Percentage Reaction The product measured at equilibrium expressed as a percentage of the theoretical yield.

Percent reaction at equilibrium Percentage reaction can be used to classify chemical reactions at equilibrium. Description of equilibrium Percentage reaction of equilibrium No reaction (NR)<1% Reactants are favoured <50% (  ) Products are favoured >50% (  ) Quantitative reaction >99% (  )

Equilibrium Constant At equilibrium the concentration of the reactants and products are constant and the equilibrium constant, K c or K or K eq, is a mathematical expression of these concentrations. Discovered in 1864 by two Norwegian chemists, Cato Maximilian Guldberg & Peter Waage it is also called the Equilibrium Law if a chemical system is in equilibrium, there will be a constant ratio between the concentrations of the products and the concentrations of the reactants if a chemical system is in equilibrium, there will be a constant ratio between the concentrations of the products and the concentrations of the reactants

For the general chemical reaction: aA + bB  cC + dD Equilibrium Constant (K eq or K c or K)

Where; aA + bB  cC + dD A, B, C, and D are the concentrations of the reactants and products in gas or aqueous phases, after the reaction has reached equilibrium and the concentrations are no longer changing, and a, b, c, and d are the coefficients in the balanced chemical equation.

Equilibrium Constant characteristics 1.The product of the molar concentrations of the products is always in the numerator, while the product of the molar concentration of the reactants in in the denominator. 2.The exponents are the same as the coefficients in the balanced reaction. 3.The constant is dimensionless. 4.Liquids and solids never appear in the expression as their concentrations are constant. 5.The equilibrium constant value is specific to the temperature of the system.

The meaning of Equilibrium Constant values 1.K eq > 1 The products are favoured and the equilibrium lies far to the right. If the K eq value is greater than 10 10, it is considered to go to completion (a quantitative reaction) 2.K eq = 1 The concentrations of the reactants and products are approximately equal. 3.K eq < 1 The reactants are favoured and the equilibrium lies far to the left. If the K eq value is greater than , it is considered to be a non- reacting reaction.

K eq & Percent reaction comparison Description of system K eq valuePercentage reaction Graphic of system No reaction (NR) < <1% Reactants are favoured <50% (  ) Products are favoured >50% (  ) Quantitative reaction > >99% (  ) Reactants Products [ ] Time Reactants Products [ ] Time Reactants Products [ ] Time Reactants Products [ ] Time

Uses for the Equilibrium Constant 1.Testing to see if an equilibrium has been established. 2.Predicting the effects of changing the concentrations, temperature and pressure conditions of the system. 3.Calculating the equilibrium concentrations.

Analyzing the Equilibrium Law ICE TABLE In the quantitative analysis of an equilibrium system is may be advantageous to chronicle the progression of the system over time. The ICE table is a method of recording the Initial concentration, the Change in concentration during the equilibrium process, and the final Equilibrium concentrations.

2 CO 2(g)  2 CO (g) + O 2(g) Time (s) Concentration (mol/L) [O 2(g) ] [CO 2(g) ] [CO (g) ] Reduction in [CO 2(g) ] by “2x” Increase in [CO (g) ] by “2x” Increase in [O 2(g) ] by “x” [O 2(g) ] [CO 2(g) ] [CO (g) ] Initial concentrations of reactants & products Changes occur in the concentrations as the reactants become used up (decrease) & the products are created (increased) in proportions predicted in the balanced chemical reaction. Once the equilibrium process has stabilized, Equilibrium concentrations will be established ICE Table

Time (s) Initial concentrations Change concentrations Equilibrium concentrations 2 CO 2(g)  2 CO (g) + O 2(g) In the following chemical system, a mol/L sample of carbon dioxide (CO 2 (g) ) forms an equilibrium as suggested in the following chemical equation. C [Change]-2x+2x+x E [ Equilibrium ] x0+2x0+x I [Initial] ICE Table

Initial, Change in & Equilibrium concentrations are accounted for in the ICE table. The ICE Table is an effective tool in the analysis & processing of quantitative characteristics in an equilibrium system. ICE Table

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