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Equilibrium Expressions and Calculations – Step-by-Step Guidance

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Q1a. Write the equilibrium constant expression for the following reaction: 2HI(g) ⇌ H2(g) + I2(g)

Background

Topic: Chemical Equilibrium – Equilibrium Constant Expressions

This question tests your ability to write the equilibrium constant expression (K) for a given balanced chemical equation involving gases.

Key Terms and Formulas

  • Equilibrium Constant (K): A ratio of the concentrations (or partial pressures) of products to reactants, each raised to the power of their coefficients in the balanced equation.

  • General Formula:

Step-by-Step Guidance

  1. Identify the products and reactants in the balanced equation: Products are H2 and I2; reactant is HI.

  2. Write the equilibrium expression using concentrations of gases (denoted by square brackets).

  3. Remember to raise each concentration to the power of its coefficient from the balanced equation.

  4. Set up the expression, but do not simplify or calculate any values yet.

Try solving on your own before revealing the answer!

Q1b. Write the chemical reaction for the gases described by the given equilibrium constant expression:

Background

Topic: Chemical Equilibrium – Writing Reactions from K Expressions

This question tests your ability to deduce the balanced chemical equation from a given equilibrium constant expression.

Key Terms and Formulas

  • K Expression: The numerator contains products, the denominator contains reactants, and exponents correspond to coefficients.

Step-by-Step Guidance

  1. Identify which species are products (numerator) and which are reactants (denominator).

  2. Assign coefficients based on exponents in the K expression (e.g., [H2S]^2 means 2 H2S as a reactant).

  3. Write the balanced chemical equation with all species as gases.

  4. Double-check that the K expression matches your written equation.

Try solving on your own before revealing the answer!

Q2. Write an equilibrium constant expression for the reaction below in terms of K1: 2 CO2(g) + 2 H2(g) ⇌ 2 CO(g) + 2 H2O(g)

Background

Topic: Manipulating Equilibrium Constants

This question tests your understanding of how equilibrium constants change when a reaction is reversed or multiplied by a factor.

Key Terms and Formulas

  • K1: The equilibrium constant for the original reaction.

  • Rules:

    • Reversing a reaction:

    • Multiplying coefficients by n:

Step-by-Step Guidance

  1. Compare the new reaction to the original: Is it reversed? Are the coefficients multiplied by a factor?

  2. Apply the rules for manipulating K values based on these changes.

  3. Express the new equilibrium constant (Kc) in terms of K1, using the appropriate mathematical relationship.

Try solving on your own before revealing the answer!

Q2b. For the original reaction (K1), does Kc = Kp?

Background

Topic: Relationship Between Kc and Kp

This question tests your understanding of when the equilibrium constant in terms of concentration (Kc) equals the equilibrium constant in terms of pressure (Kp).

Key Terms and Formulas

  • Kc: Equilibrium constant using concentrations (mol/L).

  • Kp: Equilibrium constant using partial pressures (atm).

  • Relationship: , where is the change in moles of gas (products - reactants).

Step-by-Step Guidance

  1. Calculate for the reaction (difference in moles of gaseous products and reactants).

  2. If , then ; otherwise, they are different.

Try solving on your own before revealing the answer!

Q1a (Quiz 5). Write the equilibrium constant expression for: H2(g) + I2(g) ⇌ 2HI(g)

Background

Topic: Chemical Equilibrium – Writing K Expressions

This question tests your ability to write the equilibrium constant expression for a reaction involving gases.

Key Terms and Formulas

  • Use the general formula for K as above.

Step-by-Step Guidance

  1. Identify products (HI) and reactants (H2, I2).

  2. Write the K expression with concentrations, raising each to the power of its coefficient.

  3. Set up the expression, but do not simplify or calculate any values yet.

Try solving on your own before revealing the answer!

Q1b (Quiz 5). Write the chemical reaction described by the given equilibrium constant expression:

Background

Topic: Writing Reactions from K Expressions

This question tests your ability to reconstruct a balanced chemical equation from a K expression.

Key Terms and Formulas

  • Numerator = products, denominator = reactants; exponents = coefficients.

Step-by-Step Guidance

  1. Identify products and reactants from the K expression.

  2. Assign coefficients based on exponents.

  3. Write the balanced equation with all species as gases.

Try solving on your own before revealing the answer!

Q2 (Quiz 5). For the reaction: 2 HCl(g) ⇌ H2(g) + Cl2(g) (K1), would Kc = Kp?

Background

Topic: Kc vs. Kp

This question tests your understanding of the relationship between Kc and Kp for a reaction involving gases.

Key Terms and Formulas

  • Use the formula as above.

Step-by-Step Guidance

  1. Calculate (moles of gaseous products minus moles of gaseous reactants).

  2. Decide if (then Kc = Kp) or not.

Try solving on your own before revealing the answer!

Q2b (Quiz 5). Write an equilibrium constant expression for: 3 H2(g) + 3 Cl2(g) ⇌ 6 HCl(g) in terms of K1

Background

Topic: Manipulating Equilibrium Constants

This question tests your ability to relate the equilibrium constant for a reaction that is a multiple of another reaction.

Key Terms and Formulas

  • If a reaction is multiplied by n,

Step-by-Step Guidance

  1. Compare the new reaction to the original (K1) and determine the multiplication factor.

  2. Apply the rule for how K changes when the reaction is multiplied.

  3. Express the new Kc in terms of K1.

Try solving on your own before revealing the answer!

Q3. Determine the value of the equilibrium constant (Kc) for: 2 HI(g) + Cl2(g) ⇌ 2 HCl(g) + I2(s), given [HCl]eq = 0.13 M, [HI]eq = 5.6 × 10-16 M, [Cl2]eq = 0.0019 M, [I2]eq = 0.25 M

Background

Topic: Calculating Kc from Equilibrium Concentrations

This question tests your ability to calculate the equilibrium constant using equilibrium concentrations, noting that solids are omitted from the K expression.

Key Terms and Formulas

  • Solids are not included in the K expression.

  • General formula:

Step-by-Step Guidance

  1. Write the Kc expression, omitting the solid (I2).

  2. Plug in the given equilibrium concentrations for each species.

  3. Raise each concentration to the power of its coefficient.

  4. Set up the calculation, but do not compute the final value yet.

Try solving on your own before revealing the answer!

Q4. A 0.500 mol sample of NOCl is placed in a 1.00 L reactor at 227°C and reaches equilibrium with 0.028 mol Cl2. Calculate K at this temperature for: 2 NOCl(g) ⇌ 2 NO(g) + Cl2(g)

Background

Topic: Calculating K from Initial and Equilibrium Amounts

This question tests your ability to use stoichiometry and equilibrium concentrations to calculate the equilibrium constant.

Key Terms and Formulas

  • Set up an ICE table (Initial, Change, Equilibrium) to track moles/concentrations.

  • General formula:

Step-by-Step Guidance

  1. Convert all amounts to concentrations (mol/L).

  2. Set up an ICE table to determine equilibrium concentrations of all species.

  3. Express the changes in terms of x (amount of Cl2 formed).

  4. Plug equilibrium concentrations into the Kc expression, but do not solve for K yet.

Try solving on your own before revealing the answer!

Q3 (repeat). Determine the value of the equilibrium constant (Kc) for: 2 HCl(g) + I2(s) ⇌ 2 HI(g) + Cl2(g), given [HCl]eq = 0.13 M, [HI]eq = 5.6 × 10-16 M, [Cl2]eq = 0.0019 M, [I2]eq = 0.25 M

Background

Topic: Calculating Kc from Equilibrium Concentrations (with a solid)

This question is similar to the previous Kc calculation, but with a different reaction.

Key Terms and Formulas

  • Solids are omitted from the K expression.

  • General formula:

Step-by-Step Guidance

  1. Write the Kc expression, omitting the solid (I2).

  2. Plug in the given equilibrium concentrations for each species.

  3. Raise each concentration to the power of its coefficient.

  4. Set up the calculation, but do not compute the final value yet.

Try solving on your own before revealing the answer!

Q4. The Haber process: 0.1248 M H2 and 0.0416 M N2 are mixed at 500°C. At equilibrium, [NH3] = 0.00272 M. What is K for: N2(g) + 3 H2(g) ⇌ 2 NH3(g)?

Background

Topic: Calculating K from Initial and Equilibrium Concentrations

This question tests your ability to use an ICE table and equilibrium concentrations to calculate the equilibrium constant for a synthesis reaction.

Key Terms and Formulas

  • ICE table: Initial, Change, Equilibrium for each species.

  • General formula:

Step-by-Step Guidance

  1. Set up an ICE table with the initial concentrations of H2 and N2, and the equilibrium concentration of NH3.

  2. Determine the change in concentration for each species based on the stoichiometry.

  3. Calculate the equilibrium concentrations of N2 and H2.

  4. Plug all equilibrium concentrations into the Kc expression, but do not compute the final value yet.

Try solving on your own before revealing the answer!

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