Back to Student Worksheet
Chemistry Grade 9-12 Answer Key

Chemistry: Chemical Equilibrium and Le Chatelier's Principle

Predicting shifts in reversible reactions

Answer Key
Name:
Date:
Score: / 15

Chemistry: Chemical Equilibrium and Le Chatelier's Principle

Predicting shifts in reversible reactions

Chemistry - Grade 9-12

Instructions: Read each problem carefully. Show your work in the space provided. Include units where needed and explain your reasoning for equilibrium shifts.
  1. 1

    For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), write the equilibrium constant expression, Kc.

    Products go in the numerator and reactants go in the denominator. Coefficients become exponents.

    The equilibrium expression is Kc = [NH3]^2 / ([N2][H2]^3). Pure solids and liquids are not included, but all species here are gases and are included.
  2. 2

    At equilibrium, the concentrations for H2(g) + I2(g) ⇌ 2HI(g) are [H2] = 0.20 M, [I2] = 0.20 M, and [HI] = 1.60 M. Calculate Kc.

    Kc = [HI]^2 / ([H2][I2]) = (1.60)^2 / (0.20 × 0.20) = 2.56 / 0.040 = 64. The equilibrium constant is 64.
  3. 3

    For the equilibrium CO(g) + 2H2(g) ⇌ CH3OH(g), predict the shift if more CO is added to the container.

    A system at equilibrium tends to reduce the effect of an added substance.

    The equilibrium shifts to the right, toward products, because adding CO increases the reactant concentration. The system uses some of the added CO to form more CH3OH.
  4. 4

    For the equilibrium 2SO2(g) + O2(g) ⇌ 2SO3(g), predict the shift if SO3 is removed from the container.

    The equilibrium shifts to the right, toward products, because removing SO3 lowers the product concentration. The system responds by producing more SO3.
  5. 5

    For the equilibrium N2O4(g) ⇌ 2NO2(g), the forward reaction is endothermic. Predict the shift when the temperature is increased.

    For an endothermic forward reaction, write heat on the reactant side.

    The equilibrium shifts to the right, toward NO2, because heat acts like a reactant for an endothermic forward reaction. Increasing temperature favors the endothermic direction.
  6. 6

    For the equilibrium 2NO2(g) ⇌ N2O4(g), the forward reaction is exothermic. Predict the shift when the temperature is decreased.

    The equilibrium shifts to the right, toward N2O4, because decreasing temperature favors the exothermic direction. The system produces heat to oppose the temperature decrease.
  7. 7

    For the equilibrium PCl5(g) ⇌ PCl3(g) + Cl2(g), predict the shift if the pressure is increased by decreasing the volume.

    Compare the total gas coefficients on each side of the balanced equation.

    The equilibrium shifts to the left because the left side has 1 mole of gas and the right side has 2 moles of gas. Increasing pressure favors the side with fewer gas particles.
  8. 8

    For the equilibrium H2(g) + Cl2(g) ⇌ 2HCl(g), predict the shift if the volume is decreased.

    There is no shift due to the volume change because both sides have 2 moles of gas. Changing volume does not favor either side when the number of gas particles is the same.
  9. 9

    For the equilibrium CaCO3(s) ⇌ CaO(s) + CO2(g), write the equilibrium constant expression, Kc.

    Pure solids and pure liquids are left out of K expressions.

    The equilibrium expression is Kc = [CO2]. The solids CaCO3 and CaO are not included because pure solids do not appear in equilibrium expressions.
  10. 10

    For the reaction 2A(g) ⇌ B(g), Kc = 25 at a certain temperature. If [A] = 0.50 M and [B] = 2.0 M, calculate Qc and determine whether the reaction will shift left, shift right, or remain at equilibrium.

    Compare Q to K. If Q < K, more products must form.

    Qc = [B] / [A]^2 = 2.0 / (0.50)^2 = 2.0 / 0.25 = 8.0. Since Qc is less than Kc, the reaction shifts right to form more products.
  11. 11

    For the equilibrium Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq), the product FeSCN2+ is deep red. Predict the color change when more SCN- is added.

    The solution becomes a deeper red because adding SCN- shifts the equilibrium to the right. More FeSCN2+ forms, increasing the red color.
  12. 12

    For the equilibrium 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g), predict the shift if pressure is increased by decreasing volume.

    Add the coefficients of all gaseous substances on each side.

    The equilibrium shifts to the left because the reactant side has 9 moles of gas and the product side has 10 moles of gas. Higher pressure favors the side with fewer gas particles.
  13. 13

    A catalyst is added to a reaction mixture already at equilibrium. Explain what happens to the position of equilibrium and to the rate at which equilibrium is reached.

    The position of equilibrium does not change because a catalyst speeds up the forward and reverse reactions equally. A catalyst helps the system reach equilibrium faster, but it does not change K.
  14. 14

    For the equilibrium CO2(g) + H2(g) ⇌ CO(g) + H2O(g), Kc = 1.0 at a certain temperature. If the mixture contains [CO2] = 0.30 M, [H2] = 0.30 M, [CO] = 0.30 M, and [H2O] = 0.30 M, is the system at equilibrium?

    When Q equals K, the reaction mixture is already at equilibrium.

    Qc = ([CO][H2O]) / ([CO2][H2]) = (0.30 × 0.30) / (0.30 × 0.30) = 1.0. Since Qc equals Kc, the system is at equilibrium.
  15. 15

    For the equilibrium 2NO(g) + O2(g) ⇌ 2NO2(g), predict the shift when O2 is removed and explain how the concentration of NO changes.

    The equilibrium shifts to the left because removing O2 lowers a reactant concentration. As the system shifts left, more NO is produced, so the concentration of NO increases.
LivePhysics™.com Chemistry - Grade 9-12 - Answer Key

Related Cheat Sheets