5 Steps to a 5 AP Chemistry

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:

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

chemical 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 Qcin this case, because molar concentrations are being used. If

this was a gas-phase reaction, gas pressures could be used and it would

become a Qp.

Remember: products over reactants.

Keywords and Equations

Q=reaction quotient

, where aA + bB →cC + dD

Equilibrium Constants:

K=equilibrium constant

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

Kc(molar concentrations)

Kw= [OH−][H+] = 1.0 × 10 −^14 = Ka×Kbat 25°C

pH =−log [H+], pOH =−log [OH−]

14 =pH +pOH

pKa=−log Ka, pKb=−log Kb

Kp= Kc(RT)Δn, where Δn= moles product gas −moles reactant gas

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

pOH = p

HB

b B

+

K +log

[]

[]

pH p

A

a HA]

=+K log

[]

[

−

KKab==

[][ ]+− − +

[]

[][]

[]

HA

HA

OH HB

B

Q=

[][]

[][]

CD

AB

cd

ab

Q=

[C] D]

A] B]

cd

ab

[

[[

212  Step 4. Review the Knowledge You Need to Score High

KEY IDEA

KEY IDEA

KEY IDEA