# 7.4 Predicting the Direction of a Reaction

Predicting the Direction of a Reaction Equilibrium Constant (Kc) and the Reaction Quotient (Qc) Review of Le Chteliers Principle SCH4U Grade 12 Chemistry, University Preparation DVORSKY We learned in previous lessons that

Law of Chemical Equilibrium At equilibrium there is a constant ratio between the concentration of reactants and products in any change. Using ICE charts and the equilibrium equation we can solve for the: Concentrations at: I = initial, C = change, E = equilibrium Equilibrium constant Initial or Equilibrium concentrations of chemical species in a reaction May need to use the quadratic equation to solve problems. There may be more than one solution, but only one of the solutions will make sense.

The equilibrium constant (K) describes the extent of a reaction. K > 1 means equilibrium favours the products (more products made than reactants) K = 1 means at equilibrium there are equal concentrations of products & reactants. K < 1 means equilibrium favours the reactants (less products made than reactants) Reaction Quotient, Qc How do you know a reaction is at equilibrium? How can you predict the direction in which the reaction must proceed to reach equilibrium? The reaction quotient (Qc) is an expression that

is identical to the equilibrium constant expression, but its valueaA is calculated concentrations + bB using cC + dD that are not necessarily those at equilibrium. c d [C] [D] Qc = a b

[A] [B] Reaction Quotient (Qc) Q < K denominator is large system attains equilibrium by moving to the right, move towards making products Q = K system at equilibrium Q > K numerator is large system attains equilibrium by moving to the left, move towards making reactants

What happens if we change the concentration of a chemical entity when a reaction is at equilibrium? Le Chteliers Principle A dynamic equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect the equilibrium. Predicts the way that an equilibrium system responds to change

Involves: Initial equilibrium system Shifting non-equilibrium state New equilibrium state Henry-Louis Le Chtelier, 1850-1936, a French chemist and engineer Change concentration of Reactants Increase concentration of reactants

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Res ources ~\Nelson Chemistry 12 Textbook\Chem_1 2\Attachments\d)_Animations\16M11AN1.mov Decrease concentration of reactants F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Res ources ~\Nelson Chemistry 12 Textbook\Chem_1 2\Attachments\d)_Animations\16M11AN2.mov Video: F:\Courses\SCH4U1 - Chemistry Gr12 Univ\ ~ Resources ~\McGrawHill Chemistry 12 Te xtbook\elp\unit4\4_1.mov

Change concentration of Products Increase concentration of products F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Res ources ~\Nelson Chemistry 12 Textbook\Chem_1 2\Attachments\d)_Animations\16M11AN3.mov Decrease concentration of products F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Res ources ~\Nelson Chemistry 12 Textbook\Chem_1 2\Attachments\d)_Animations\16M11AN4.mov Effect of Ions in Aqueous Equilibrium

Systems Common ion effect Involves adding an ion to a solution in which the ion is already present in solution (concentration effect) Equilibrium shifts away from the added ion Video: F:\Courses\SCH4U1 - Chemistr y Gr12 Univ\~ Resources ~\Mc GrawHill Chemistry 12 Textbo ok\elp\unit4\4_3a.avi Effect of Temperature on Equilibrium

The value of the equilibrium constant changes with temperature, because the forward and reverse reactions are affected Le Chteliers Principle still holds. The sign of the enthalpy change for the reaction is important. Endothermic Change (H >0)H >0) temperature, shift equilibrium to right, forming products & K c

increases temperature, shift equilibrium to left, forming reactants, & K c decreases Exothermic Change (H >0)H <0) Video: Video: temperature, shiftF:\Courses\SCH4U1 equilibrium to left, formingF:\Courses\SCH4U1 reactants & K c - Chemistr - Chemistr y Gr12 Univ\~ Resources ~\Mc y Gr12 Univ\~ Resources ~\Mc

decreases GrawHill Chemistry 12 Textbo GrawHill Chemistry 12 Textbo temperature, shiftok\elp\unit4\4_3b.avi equilibrium to right, forming products, & K c ok\elp\unit4\4_3c.avi increases Effect of Volume & Pressure Changes on Equilibrium Pressure and Volume are inversely related at a constant temperature.

Pressure is caused by gas molecules striking the walls of a container, so you change pressure by changing the number of gas molecules. if volume, pressure so, shift to more gas molecules if volume, pressure so, shift to less gas molecules What if same number of gas molecules on both sides of equation? Volume & pressure have no effect on the position of equilibrium.

Changes that do not affect the position of Equilibrium Systems Catalyst A catalyst speeds up the rate of a reaction. Overall effect is to lower the activation energy, which increases the rate of reaction, of both the forward and reverse reactions. Catalysts do not affect the position of equilibrium, only the time taken.

Changes that do not affect the position of Equilibrium Systems Inert Gases (N2 & Noble Gases) Inert gases do not react with other gases. This will not be part of the equilibrium system. The equilibrium position of the system will not change. The presence of the inert gas changes the probability of successful collisions for both the reactants and products

equally, resulting in no shift in the equilibrium system. Video: F:\Courses\SCH4U1 - Chemis try Gr12 Univ\~ Resources ~ \McGrawHill Chemistry 12 Te xtbook\elp\unit4\4_2.avi Chickens and Equilibrium

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