EQUILIBRIUM BASICSChapter 14.1-14.3

Lesson ObjectivesKnow -Factors that affect/don’t affect a reaction reaching equilibrium

-K is equilibrium constant for any reversible reaction-Kc and Kp are related to each other through Kp = Kc(RT)Dng

-Coefficient Rule-Reciprocal Rule-Rule of Multiple Equilibria

Understand -Equilibrium is based on when RATES of forward and reverse reactions are equal, not when concentrations of reactants and products are equal-The equilibrium constant K has its roots in the kinetics rate law constant k-The size of the equilibrium constant tells us which side (reactants or products) is favored in an equilibrium reaction-K may only be calculated with concentration or pressure values when the system is at EQUILIBRIUM-Solids and liquids are not used in equilibrium constant expressions

Do -Write an equilibrium constant expression for a given reaction-Calculate K based on equilibrium concentrations-Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple Equilibria to calculate a new K from a known K-Calculate K when given initial concentration or pressure values (ICE Chart)

Which of the following is NOT true about a reaction at equilibrium?N2O4 2NO2

A. [N2O4] = 2 [NO2]

B. The rate of the forward and reverse reactions are the same

C. The equilibrium constant will describe whether the process favors reactants or products

D. [N2O4] and [NO2] remain constant.

E. For every 2 molecules of NO2 reacted, 1 molecule of N2O4 is produced

AT EQUILIBRIUM

Two reactions are occurring (forward and reverse) indicated by double arrow.

Equilibrium definition: a reaction has reached equilibrium (balance) when the forward and reverse reactions occur at the same RATE.

This means that concentrations of reactants and products stay CONSTANT over time

An Equilibrium Example

See this interactive tutorial here

Equilibrium factors:

Equilibrium IS dependent on◦TEMPERATURE (affects the equilibrium

constant, K)

It is NOT dependent on◦Original concentrations◦Volume of the container◦Total pressure of the system

Remember – the partial pressure of a gas in a mixture is proportional to the mole fraction of that gas in the mixture!

The following data are for the system A(g) 2B(g)

Time (s) 0 20 40 60 80 100

PA (atm) 1.00 0.83 0.72 0.65 0.62 0.62

PB (atm) 0.00 0.34 0.56 0.70 0.76 0.76

How long does it take the system to reach equilibrium?

K: equilibrium constantTells us the extent to which the reaction will go

until it reaches equilibrium◦Large K means the reaction goes mostly to product◦Small K means the reaction stays mostly as reactant

For a reaction2A(g) + B(g) 2C(g) K = 1 x 1083

which of the following can be concluded?

A. At equilibrium, [A] = ½[B] = [C]B. At equilibrium, the largest concentration is [A]C. At equilibrium, the largest concentration is [C]D. This reaction favors the reactantsE. This reaction happens very quickly

Writing K expression (equilibrium expression)

Uses only gaseous or aqueous products and reactants (no solids or liquids; their concentrations do not change during reactions)

Coefficients become exponentsProducts over reactantsFor aA + bB cC + dD

Kp = or Kc =

Relate to each other: Kp = Kc(RT)Δng

◦R = gas law constant (0.0821 atm·L/mol·K)◦Δng = change in moles of gas (products – reactants)

(PC)c x (PD)d

(PA)a x (PB)b

[C]c [D]d

[A]a [B]b

Which of the following species should NOT be included in the equilibrium constant expression for the reaction?2A(s) + B(aq) 2C(l) + D(g)

A. A, B, CB. B, CC. A, CD. A, C, DE. B, D

For a reaction 2A + B 2CWhen equilibrium is established, the ratio of products to reactants (K) = 0.5. Which of the following initial conditions could be changed to cause a different equilibrium constant value?

A. Start with more AB. Dilute the reaction by adding waterC. Increase the temperature of the reactionD. Add a catalyst

To calculate Kp from Kc for the reactionA(g) + 2B(g) C(g)what would be the exponent to which Kc(RT) is raised?

A. -2B. -1C. 0D. 1E. 2

Write the equilibrium constant (K) expressions

for the following reactions:

I2(g) + 5F2(g) 2 IF5(g)

SnO2(s) + 2H2(g) Sn(s) + 2H2O(l)

For the reaction2 NO(g) + O2(g) 2 NO2(g)determine the Kp at 298K if Kc is 4.67 x 1013

True or False?Given the equation below,

N2 + 3H2 2 NH3

if one mole of N2 is mixed with 3 moles of H2, 2 moles of NH3 will form.

True or False?Given the equation below,

N2 + 3H2 2 NH3

for every 1 mole of N2 that reacts, 3 moles of H2 will also react and 2 moles of NH3 will form.

Calculate K for the reactionNH4CO2NH2(s) 2 NH3(g) + CO2(g) if at equilibrium there are 0.159 g of NH4CO2NH2 and pressures of 0.0451 atm of CO2 and 0.0961 atm NH3

K depends on the form (coefficients) of the balanced equation! This means that K changes when: ◦The coefficients are multiplied◦Reactions are reversed◦Reactions are added together (mechanism)

Coefficient rule: If coefficients are multiplied by a factor (n),

then K is RAISED TO THE (n) EXPONENTA2 (g) + 2B (g) 2AB (g)

K =

3A2 (g) + 6B (g) 6AB (g)

K =

PAB2

PA x PB2

PAB6

PA3 x PB

6

Calculate K for the formation of 2 moles of ICl(g)ICl (g) ½I2 (g) + ½Cl2 (g) K = 2.2x10-3

Reciprocal rule: If the reaction is reversed, K of the forward

and K of the reverse are reciprocals of each other (new K = 1/old K)

A2 (g) + 2B (g) 2AB (g)

K =

2AB (g) A2 (g) + 2B (g)

K =

PAB2

PA x PB2

PAB2

PA x PB2

Rule of multiple equilibria

If multiple reactions are added together to get an overall reaction, the overall K is the PRODUCT of the individual K’s

A2 (g) + 2B (g) 2AB (g)

2AB (g) + C (g) A2B2C (g)

A2 (g) + 2B (g) + C (g) A2B2C (g)

Find overall K it by multiplying individual K’s:

Given the reactions below and their constants, calculate K for the rxn:Fe(s) + H2O(g) FeO(s) + H2(g)

H2O(g) + CO(g) H2(g) + CO2(g) K = 1.6

FeO(s) + CO(g) Fe(s) + CO2(g) K = 0.67

Determining KK must use values AT EQUILIBRIUM!

When given initial concentrations or pressures in a problem , you must determine what the values will be at equilibrium before you can calculate K

Use an ICE Chart to determine how the initial values will increase or decrease to get to equilibrium, then solve for K.◦ Solids and liquids have no effect on equilibrium, so they

don’t need values in your ICE chart.

The ICE ChartInitial/Change/EquilibriumGiven initial pressures, find equilibrium constant

A2 = 0.1 atmB = 0.2 atm

A2(g) + 2B(g) 2AB(g)ICE

0.1 atm 0.2 atm 0.0 atm-x -2x +2x

0.1 - x 0.2 - 2x 2x

For the decomposition reactionNH4HS(s) NH3(g) + H2S(g)

In a sealed flask are 10.0 g of NH4HS, NH3 with a partial pressure of 0.692 atm and H2S with a partial pressure of 0.0532 atm. When equilibrium is established, it is found that the partial pressure of NH3 has increased by 12.4%. Calculate K for this reaction.

Lesson ObjectivesKnow -Factors that affect/don’t affect a reaction reaching equilibrium

-K is equilibrium constant for any reversible reaction-Kc and Kp are related to each other through Kp = Kc(RT)Dng

-Coefficient Rule-Reciprocal Rule-Rule of Multiple Equilibria

Understand -Equilibrium is based on when RATES of forward and reverse reactions are equal, not when concentrations of reactants and products are equal-The equilibrium constant K has its roots in the kinetics rate law constant k-The size of the equilibrium constant tells us which side (reactants or products) is favored in an equilibrium reaction-K may only be calculated with concentration or pressure values when the system is at EQUILIBRIUM-Solids and liquids are not used in equilibrium constant expressions

Do -Write an equilibrium constant expression for a given reaction-Calculate K based on equilibrium concentrations-Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple Equilibria to calculate a new K from a known K-Calculate K when given initial concentration or pressure values (ICE Chart)