5 Steps to a 5 AP Chemistry 2019

Equilibrium ❮ 213

KEY IDEA

It is important to remember that at equilibrium the concentrations of the

chemical species are constant, not necessarily equal. There may be a lot of C and

D and a little A and B, or vice versa. The concentrations are constant, unchanging,

but not necessarily equal.

At any point during the preceding reaction, a relationship may be defined called

the reaction quotient, Q. It has the following form:

Q

[C][D]

[A][B]

cd

= ab

The reaction quotient is a fraction. In the numerator is the product of the chemi-

cal species on the right-hand side of the equilibrium arrow, each raised to the power

of that species’ coefficient in the balanced chemical equation. It is called the Qc in

this case, because molar concentrations are being used. If this was a gas-phase reac-

tion, gas pressures could be used and it would become a Qp.

Remember: products over reactants.

Keywords and Equations

Q = reaction quotient

Q [C][D]

[A][B]

Cd

= ab, where aA + bB → cC + dD

Equilibrium Constants:

K = equilibrium constant

Ka (weak acid) Kb (weak base) Kw (water) Kp (gas pressure)

Kc (molar concentrations)

KK==

[H+−][A] −+

[HA]

[OH][HB]

ab[B]

Kw = [OH-] [H+] = 1.0 × 10 -^14 = Ka × Kb at 25°C

pH = - log [H+], pOH = - log [OH-]

14 = pH + pOH

pH plog

[A]

a [HA]

=+

−

K

pOH=plog

[HB]

b [B]

+

K +

pKa = - log Ka, pKb = - log Kb

Kp = Kc(RT)Dn, where Dn = moles product gas - moles reactant gas

Gas constant, R = 0.0821 L atm mol-^1 K-^1

KEY IDEA

KEY IDEA