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Master ICE tables to solve equilibrium problems and calculate equilibrium concentrations from initial conditions.
Learn step-by-step with practice exercises built right in.
ICE = Initial, Change, Equilibrium
Organized method to track concentrations:
| A | + | B | โ | C | + | D | |
|---|---|---|---|---|---|---|---|
| Initial | [A]โ | [B]โ | [C]โ |
For Hโ(g) + Iโ(g) โ 2HI(g), K_c = 50.0 at 500 K. If 1.00 mol Hโ and 1.00 mol Iโ are placed in a 1.00 L container, calculate equilibrium concentrations.
Given:
Set up ICE table:
| Hโ |
|---|
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | โ |
| Free Response (Long) | FRQ | 3 | 69 min | 30% | โ |
| Free Response (Short) | FRQ | 4 | 36 min | 20% | โ |
Avoid these 3 frequent errors
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mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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| [D]โ |
| Change | -ax | -bx | +cx | +dx |
| Equilibrium | [A]โ-ax | [B]โ-bx | [C]โ+cx | [D]โ+dx |
Key points:
Steps:
For reaction: aA + bB โ cC + dD
If A changes by amount ax:
Ratio of changes = ratio of coefficients
Example: Nโ + 3Hโ โ 2NHโ
If Nโ changes by -x:
Given: All equilibrium concentrations Find: K
Method:
Given: Initial concentrations and K Find: Equilibrium concentrations
Method:
Given: Some reactants, some products initially Find: Equilibrium concentrations
Method:
When K is very small (< 10โปยณ):
If [initial] >> K:
Check validity:
Example:
K = 1.0 ร 10โปโต, [initial] = 0.10 M
With approximation: 0.10 - x โ 0.10
Check: x/0.10 < 5%? If yes, good!
When approximation invalid:
Standard form: axยฒ + bx + c = 0
Quadratic formula:
Use (+) or (-) based on physical meaning:
Reaction: Hโ(g) + Iโ(g) โ 2HI(g)
Given: [Hโ]โ = 0.50 M, [Iโ]โ = 0.50 M, [HI]โ = 0
ICE Table:
| Hโ | + | Iโ | โ | 2HI | |
|---|---|---|---|---|---|
| I | 0.50 | 0.50 | 0 | ||
| C | -x | -x | +2x | ||
| E | 0.50-x | 0.50-x | 2x |
K expression:
Step 1: Write balanced equation
Step 2: Organize data
Step 3: Set up ICE table
Step 4: Write K expression
Step 5: Solve for x
Step 6: Calculate final answer
โ Wrong stoichiometry in change row โ Use coefficients from balanced equation
โ Forgetting to square/cube in K expression โ Exponents = coefficients
โ Using initial concentrations in K โ Use equilibrium (E row)
โ Taking wrong quadratic root โ Concentrations must be positive
โ Invalid small x approximation โ Check x < 5% of initial
| Iโ |
|---|
| 2HI |
|---|
| I | 1.00 | 1.00 | 0 |
| C | -x | -x | +2x |
| E | 1.00-x | 1.00-x | 2x |
Write K expression:
Solve for x:
Take square root of both sides:
Cross multiply:
Calculate equilibrium concentrations:
[Hโ]: 1.00 - x = 1.00 - 0.780 = 0.22 M
[Iโ]: 1.00 - x = 1.00 - 0.780 = 0.22 M
[HI]: 2x = 2(0.780) = 1.56 M
Check answer:
โ
Answers:
For the reaction: 2NO(g) + Brโ(g) โ 2NOBr(g), K_c = 1.3 ร 10โด at 300 K. If [NO]โ = 0.020 M and [Brโ]โ = 0.025 M, find equilibrium concentrations.
Given:
Set up ICE table:
| 2NO | Brโ | 2NOBr | |
|---|---|---|---|
| I | 0.020 | 0.025 | 0 |
| C | -2x | -x | +2x |
| E | 0.020-2x | 0.025-x | 2x |
Note stoichiometry:
Write K expression:
Large K analysis:
K = 1.3 ร 10โด is VERY large โ reaction goes nearly to completion
Limiting reactant:
Assume reaction goes to completion:
Then small amount comes back:
| 2NO | Brโ | 2NOBr | |
|---|---|---|---|
| After completion | 0 | 0.015 | 0.020 |
| C | +2y | +y | -2y |
| E | 2y | 0.015+y | 0.020-2y |
With small y:
Assume y << 0.015:
Equilibrium concentrations:
[NO]: 2y = 2(7.2 ร 10โปโด) = 1.4 ร 10โปยณ M
[Brโ]: 0.015 + y โ 0.015 M = 0.015 M
[NOBr]: 0.020 - 2y = 0.020 - 0.0014 = 0.019 M
Answers:
For NโOโ(g) โ 2NOโ(g), K_c = 4.6 ร 10โปยณ at 298 K. Initially, [NโOโ] = 0.050 M and [NOโ] = 0. Calculate equilibrium concentrations.
Given:
Set up ICE table:
| NโOโ | 2NOโ | |
|---|---|---|
| I | 0.050 | 0 |
| C | -x | +2x |
| E | 0.050-x | 2x |
Write K expression:
Try small x approximation:
Since K is small (4.6 ร 10โปยณ):
Check approximation:
15.2% > 5% โ approximation INVALID!
Must use quadratic equation.
Quadratic solution:
Quadratic formula:
Take positive root:
Calculate equilibrium concentrations:
[NโOโ]: 0.050 - x = 0.050 - 0.00703 = 0.043 M
[NOโ]: 2x = 2(0.00703) = 0.014 M
Verify:
โ
Answers: