CHM2046 Lecture 8/25

What is K?

The equilibrium constant; a number with no units that depends on the relative amounts of products and reactants at equilibrium

How do we determine K?

From rate constants (k) → Kc = kfwd/krev

Where does Kc = kfwd/krev come from?

{products] / [reactants] review this!!

What does high K mean? Rationalize.

High K means the reaction is product-favored. Equilibrium lies right. This comes from a high [products] compared to [reactants].

What does K ~ 1 mean?

At equilibrium, there is a mixture of products and reactants.

Kc =

Rule #1 for writing equilibrium constant, K, expressions

Heterogeneous reactions: Pure solids and pure liquids WILL NOT appear in K, so do not include their concentrations in K. Why? Pure solids and liquids have standard-state quantity of 1.

Heterogeneous reactions

K is a defined value for ___ and ___.

A given temperature and specific balanced equation.

Rule #2 for writing K expressions

Rule #3

Koverall =

K1 x K2 x K3…

Rule #4

K refers to a balanced qeuation as written. K changes if multiplied by a common factor.

ex. A→B; 1/2A→1/2B. → Kc → K^1/2

Kp =

Why can we use the same rules for Kc as Kp?

Every concentration can be rewritten as a pressure using a factor of RT. → P=(n/v)RT → M=P/RT

How do we express K in terms of Kp?

delta n = change in moles of gas = moles gas products - moles gas reactants

Always check that units given match ___!

units of K given

Do the rules for K apply to Q?

Yes

Q

The reaction quotient; Q is at any time in the reaction, any set of conditions; Q COULD be at equilibrium

If Q is at equilibrium, ___.

Q = K

K < Q

Reaction proceeds from right to left to reach equilibrium; goes to reactants. This is because we have “too much” product.

K > Q

Reaction proceeds from left to right to reach equilibrium; goes to products. This is because we have “too much” reactant.

K = Q

Reaction at equilibrium. No change in concentrations occurs.