15.1b Equilibrium Constant, Kc

created in 1864 by Guldberg and Waage (Norweigen)

for a reaction: aA + bB cC + dD⇄equilibrium constant: Kc

[ ] [ ]

[ ] [ ]

c d

ca b

C DK

A B

always use a balanced equation with whole number coefficients when using equilibrium law

the greater the Kc value the more the products are favoured at equilibrium

both temperature and concentration changes can effect the value of Kc

Kc only provides a measure of equilibrium position and not info on reaction rates

will always have the same value at a certain temperatureno matter what amounts are added ratio at equilibrium will always be same

Since equilibrium depends on concentrations, these substances must be expressed as they existions in solution are written as net ionic

equations

each set of equilibrium concentrationsdepends on initial concentrations

involves more than one phase

position of heterogeneous equilibrium does NOT depend on amounts of:pure solidspure liquids

because their concentrations stay constant (since they are PURE)

do not include liquids or solids in equilibrium expression

only include gases and solutions (aq)

Write the equilibrium expression for:2H2O (l) 2H⇄ 2 (g) + O2 (g)

Write the equilibrium expression for:2H2O (g) 2H⇄ 2 (g) + O2 (g)

22 2[ ] [ ]cK H O

22 2

22

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[ ]c

H OK

H O

Write the equilibrium expression for:4NH3 (g) + 7O2 (g) 4NO2 (g) + 6H2O (g)

What would it be for the reverse reaction?

4 62 2

4 73 2

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[ ] [ ]c

NO H OK

NH O

Text p688 #1-4LSM 15.1C

summary