Chemical Equilibrium and Le Chatelier's Principle
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Questions and Answers

What does Le Chatelier's Principle state?

  • If stress is applied to a system at equilibrium, it will adjust to relieve the stress. (correct)
  • Equilibrium cannot be disturbed once established.
  • Increased temperature favors the forward reaction in all cases.
  • The forward reaction will always favor the reactants.

A catalyst changes the position of equilibrium in a reaction.

False (B)

Define chemical equilibrium.

A dynamic state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction.

According to Le Chatelier's Principle, an increase in pressure will favor the side with the ______ number of molecules.

<p>smaller</p> Signup and view all the answers

What is the effect of an increase in temperature on an exothermic reaction?

<p>It favors the backward reaction. (A)</p> Signup and view all the answers

Match the following reactions with their type of heat change:

<p>Forward reaction of 2NO2 to N2O4 = Exothermic Backward reaction of N2O4 to 2NO2 = Endothermic</p> Signup and view all the answers

Chemical equilibrium can occur only in reactions that are irreversible.

<p>False (B)</p> Signup and view all the answers

What does the symbol ΔH signify in a chemical reaction?

<p>The heat change associated with the reaction, with - indicating exothermic and + indicating endothermic.</p> Signup and view all the answers

What is the primary industrial use of ammonia?

<p>Fertilizers (A)</p> Signup and view all the answers

The production of ammonia is favored by low pressures.

<p>False (B)</p> Signup and view all the answers

What is the equilibrium constant expression for the reaction of ammonia production?

<p>K_c = [NH3]^2 / [N2][H2]^3</p> Signup and view all the answers

Sulfuric Acid is made by dissolving __________ in water.

<p>sulfur trioxide</p> Signup and view all the answers

Match the following production processes with their associated conditions:

<p>Ammonia Production = 200 atmospheres and 200°C Sulfuric Acid Production = Just above atmospheric pressure and 450°C Haber Process = High pressure and low temperature Contact Process = Moderate pressure with a hot catalyst</p> Signup and view all the answers

Which of the following describes the Haber process?

<p>Exothermic with high pressure (B)</p> Signup and view all the answers

What will happen to the concentration of PCl5 if more PCl3 is introduced?

<p>The concentration will increase (A)</p> Signup and view all the answers

Calculate the value of K_c for the reaction at the specified temperature if 13.6 moles of N_2, 1.0 mole of H_2, and 1.5 moles of NH_3 are present.

<p>K_c = 0.037</p> Signup and view all the answers

Increasing temperature in an exothermic reaction will shift the equilibrium to the right.

<p>False (B)</p> Signup and view all the answers

The reaction producing sulfur trioxide in the Contact Process is endothermic.

<p>False (B)</p> Signup and view all the answers

What is the color change observed when concentrated hydrochloric acid is added to the iron (III) chloride and potassium thiocyanate mixture?

<p>The red color disappears.</p> Signup and view all the answers

If the pressure is increased in the reaction involving PCl3, Cl2, and PCl5, the equilibrium will shift to the __.

<p>right</p> Signup and view all the answers

What happens to the equilibrium of the red complex when potassium thiocyanate solution is added to the solution?

<p>The red complex reforms (B)</p> Signup and view all the answers

Le Chatelier's Principle states that if a change is applied to a system at equilibrium, the system will adjust to counteract that change.

<p>True (A)</p> Signup and view all the answers

In the Haber process, one of the most important chemicals produced is ______.

<p>ammonia</p> Signup and view all the answers

Match the substances with their role in the experiment:

<p>Fe^3+ = Forms a red complex with CNS^- CNS^- = Reactant that combines with Fe^3+ HCl = Used to shift the equilibrium left water = Used for comparison in the experiment</p> Signup and view all the answers

What happens to the concentration of Fe^3+ ions when KCNS is added to the equilibrium mixture?

<p>It decreases (A)</p> Signup and view all the answers

Changing the pressure of a solution at equilibrium will always affect the equilibrium position.

<p>False (B)</p> Signup and view all the answers

What is the value of the equilibrium constant (K_c) for the given reaction?

<p>138.87</p> Signup and view all the answers

The reaction between Fe^3+ and CNS^- leads to the formation of __________.

<p>Fe(CNS)2+</p> Signup and view all the answers

Match the temperature change to its effect on the equilibrium and the type of reaction:

<p>Cooling = Shifts equilibrium forward, Exothermic Heating = Shifts equilibrium backward, Endothermic</p> Signup and view all the answers

What is the equilibrium constant expression for the reaction 2NO (g) ↔ N2 (g) + O2 (g)?

<p>$\frac{{[NO]^2}}{{[N_2][O_2]}}$ (C)</p> Signup and view all the answers

An increase in pressure will affect the value of Kc for the reaction 2NO (g) ↔ N2 (g) + O2 (g).

<p>False (B)</p> Signup and view all the answers

State Le Châtelier's principle.

<p>If a system at equilibrium is subjected to a stress, it tends to oppose the stress.</p> Signup and view all the answers

In the reaction 2NO (g) ↔ N2 (g) + O2 (g), when 2 moles of NO decompose, the number of moles of N2 at equilibrium is ______.

<p>0.9 moles</p> Signup and view all the answers

Match the following catalyst metals to their usage in catalytic converters:

<p>Platinum = Catalyst in automotive exhaust systems Palladium = Reduces harmful emissions Rhodium = Converts nitrogen oxides to nitrogen Lead = Catalyst poison</p> Signup and view all the answers

What happens to the reaction when the temperature is increased in an exothermic reaction?

<p>The reaction shifts to the left (A), Kc decreases (B)</p> Signup and view all the answers

The reaction 2NO (g) ↔ N2 (g) + O2 (g) is an example of a dynamic equilibrium.

<p>True (A)</p> Signup and view all the answers

Name one way that catalysts increase the rate of reaction.

<p>Lower activation energy</p> Signup and view all the answers

Flashcards

Chemical Equilibrium

A state where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products.

Reversible Reaction

A reaction where the products can react to reform the reactants, meaning the reaction proceeds in both directions.

Le Chatelier's Principle

If a stress is applied to a system in equilibrium, the system shifts to relieve the stress.

Effect of Pressure Change (Gases)

In gaseous reactions, increasing pressure favors the side with fewer gas molecules to reduce volume.

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Stress (in Equilibrium)

A change in reaction conditions, such as concentration, pressure, or temperature.

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Exothermic Reaction

A reaction that releases heat.

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Endothermic Reaction

A reaction that absorbs heat.

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Catalyst Effect on Equilibrium

A catalyst speeds up the rate to reach equilibrium, but does not change the position of equilibrium.

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Haber process

A method for manufacturing ammonia by reacting nitrogen and hydrogen, using a catalyst.

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Ammonia Production conditions

High pressure (approx. 200 atm) and low temperature (approx. 200°C) are ideal for maximizing ammonia yield in the Haber process.

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Equilibrium Constant (Kc)

A constant value that represents the relationship between the concentrations of reactants and products at equilibrium for a reversible reaction.

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Kc Formula

Kc = [products]^coefficient / [reactants]^coefficient. Using concentrations.

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Contact Process

A method for manufacturing sulfuric acid by reacting sulfur dioxide with oxygen in the presence of a catalyst.

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Contact Process conditions

A temperature of 450°C is optimal for the catalyst and a pressure slightly above atmospheric are used in the contact process.

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Equilibrium Concentration calculation

Given the concentrations of reactants and products at equilibrium, you can calculate the equilibrium constant.

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Effect of adding PCl3 on PCl5 concentration

Increasing PCl3 concentration shifts the equilibrium to the right, increasing PCl5 concentration.

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Effect of pressure on PCl5 concentration

Increasing pressure shifts equilibrium to the side with fewer gas molecules (right for PCl3+Cl2 ↔ PCl5), increasing PCl5 concentration.

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Effect of temperature on PCl5 concentration

Increasing temperature in an exothermic reaction shifts equilibrium to the left, decreasing PCl5 concentration.

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Fe3+ + CNS- ↔ Fe(CNS)2+

Equilibrium reaction between iron(III) ions and thiocyanate ions forming a thiocyanate iron(III) complex.

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Effect of adding HCl on Fe(CNS)2+ colour

Adding HCl shifts the equilibrium left, decreasing the red complex concentration and making the solution paler.

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Effect of adding KSCN on Fe(CNS)2+ colour

Adding KSCN shifts the equilibrium right, increasing the red complex concentration and making the solution redder.

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Adding KCNS to Fe^3+^

Adding KCNS to a solution containing Fe^3+^ will cause the equilibrium to shift to the right, favoring the formation of Fe(CNS)^2+^, resulting in a decrease in the concentration of Fe^3+^.

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Effect of Pressure on Liquid Equilibrium

Changing the pressure has no effect on the equilibrium position of reactions in solutions because the volume of liquids is relatively constant.

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Exothermic vs. Endothermic

An exothermic reaction releases heat, causing the temperature to increase, while an endothermic reaction absorbs heat, causing the temperature to decrease.

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How to Calculate Kc

The equilibrium constant, Kc, is calculated using the concentrations of reactants and products at equilibrium. The expression for Kc is determined by the stoichiometry of the balanced chemical equation.

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Effect of Temperature on Kc

Increasing temperature favors the endothermic direction of a reversible reaction, leading to a change in the value of Kc.

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Effect of Pressure on Kc (Gases)

Changing pressure in a reaction involving gases does NOT affect the value of Kc. Kc is only affected by temperature changes.

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Catalyst's Role in Equilibrium

A catalyst speeds up the rate at which equilibrium is reached, but it does not change the position of equilibrium or the value of Kc.

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What is Chemical Equilibrium?

A state in which the rates of the forward and reverse reactions are equal. The concentrations of reactants and products remain constant.

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Why is Equilibrium Dynamic?

Even though concentrations appear constant at equilibrium, the forward and reverse reactions are still occurring, just at equal rates.

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Study Notes

Chemical Equilibrium

  • Chemical equilibrium is a state of dynamic balance in a reversible reaction, where the rate of the forward reaction equals the rate of the backward reaction.
  • A reversible reaction is one in which the products react to form the reactants. The reactants continuously form products and the products continuously form reactants.
  • Le Chatelier's Principle states that if a stress is applied to a system at equilibrium, the system readjusts to relieve the stress. Stress can be a change in concentration, pressure, or temperature.
  • In an exothermic reaction, heat is given out (ΔH-). In an endothermic reaction, heat is taken in (ΔH+).
  • An increase in pressure in a reaction involving gases favors the reaction that produces fewer molecules, as this reduces volume and relieves the pressure increase.
  • Catalysts speed up the rate of a reaction reaching equilibrium but do not change the position of equilibrium.

Le Chatelier's Principle Example: PCI3(g) + Cl2(g) → PCI5(g)

  • Introducing more PCl3 will shift the equilibrium to the right, increasing the concentration of PCI5.
  • Increasing pressure will shift the equilibrium to the right (fewer gas molecules on the right hand side).
  • Increasing temperature will shift the equilibrium to the left (the forward reaction is exothermic).

Mandatory Experiment: Iron (III) Chloride and Potassium Thiocyanate

  • Mixing FeCl3 and KCNS solutions creates a red complex (Fe(CNS)2+) according to the equation.
  • Adding HCl to the mixture reduces the red color, shifting equilibrium to the left.
  • Adding KCNS to the equilibrium solution increases the concentration of red complex.

Industrial Applications of Le Chatelier's Principle

  • Haber Process: Manufacturing Ammonia (N2 + 3H2 ⇌ 2NH3) is optimized for high pressure and low temperature to maximize ammonia yield
  • Contact Process (2SO2(g) + O2(g) ⇌ 2SO3 ΔH = -196 kJ/mol ) is best at 450°C because it's high temperature to improve catalyst efficiency.

Equilibrium Constant (Kc)

  • Kc is a constant that represents the relationship between the concentrations of products and reactants in an equilibrium system.

Exam Question Examples:

  • Consider the reversible reaction 2NO(g) ⇌ N2(g) + O2(g). ΔH = -183 kJ
  • Calculate the number of moles of nitrogen gas at equilibrium if 2 moles of nitrogen monoxide decompose to nitrogen gas and oxygen gas in a closed container, and the equilibrium constant (Kc) is 20.25.
  • Determining the equilibrium constant (Kc) for a given reaction with concentrations of reactants and products at equilibrium.

Stress and Equilibrium Shift

  • A stress on the equilibrium will be counteracted by a shift in the reaction direction, whether forward or backward, to alleviate the stress.

Catalysts in Catalytic Converters

  • The catalytic converters in cars use metals like platinum, palladium, and rhodium as catalysts.
  • These catalysts increase reaction rates—primarily heterogenous catalysis—by lowering activation energy in car exhausts.

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Description

Explore the concepts of chemical equilibrium and Le Chatelier's Principle in this quiz. Understand how reversible reactions function and the effects of stress on a system at equilibrium. Test your knowledge of exothermic and endothermic reactions, as well as the role of catalysts.

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