Understanding Chemical Equilibrium

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Questions and Answers

For a reaction at equilibrium, if the rate of the forward reaction increases, what change must occur in the reverse reaction?

  • The rate of the reverse reaction must decrease until it ceases completely.
  • The rate of the reverse reaction must also increase until it equals the rate of the forward reaction. (correct)
  • The rate of the reverse reaction will fluctuate randomly, with no predictable pattern.
  • The rate of the reverse reaction will remain constant, unaffected by the change in the forward reaction.

Consider the reaction $aA + bB \rightleftharpoons cC + dD$. How is the equilibrium constant expression, $K_c$, formulated?

  • $K_c = \frac{[cC]^c[dD]^d}{[aA]^a[bB]^b}$ (correct)
  • $K_c = \frac{[aA]^a[bB]^b}{[cC]^c[dD]^d}$
  • $K_c = [aA]^a[bB]^b[cC]^c[dD]^d$
  • $K_c = \frac{[aA][bB]}{[cC][dD]}$

What condition is necessary to calculate the value of $K_c$ from known equilibrium amounts?

  • The volume of the reaction vessel must be exactly 1 liter.
  • The equilibrium amounts must be expressed in molar concentrations. (correct)
  • The reaction must be at standard temperature and pressure.
  • The reaction must involve only gaseous reactants and products.

Consider the reversible reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$. If, at equilibrium, the partial pressures are $P_{N_2} = 2 atm$, $P_{H_2} = 3 atm$, and $P_{NH_3} = 4 atm$, what is the value of $K_p$?

<p>0.74 (D)</p> Signup and view all the answers

At 700 K, the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ has an equilibrium constant $K_c = 4.2 \times 10^5$. If the reaction mixture contains [SO₂] = 0.20 M, [O₂] = 0.25 M, and [SO₃] = 85 M, which direction will the reaction proceed to reach equilibrium?

<p>The reaction will proceed in the reverse direction. (D)</p> Signup and view all the answers

In the reaction $CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)$, what change will occur if the volume of the container is decreased?

<p>The equilibrium will shift to the side with fewer moles of gas. (C)</p> Signup and view all the answers

Given the endothermic reaction $N_2O_4(g) \rightleftharpoons 2NO_2(g)$, what effect will increasing the temperature have on the value of $K_c$?

<p>The value of $K_c$ will increase. (D)</p> Signup and view all the answers

Consider the equilibrium: $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ If at equilibrium, some $O_2$ is added to the system, what will happen to the amount of $SO_3$?

<p>The amount of $SO_3$ will increase. (A)</p> Signup and view all the answers

Given the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, with $K_c = 50$ at a certain temperature. If a reaction vessel initially contains [Hâ‚‚] = 0.2 M, [Iâ‚‚] = 0.2 M, and [HI] = 0.0 M, what will happen initially in order for said reaction to obtain equilibrium?

<p>More HI will be produced and more $H_2$ and $I_2$ will be consumed. (C)</p> Signup and view all the answers

For the reaction $A(g) + B(g) \rightleftharpoons C(g)$, the equilibrium constant $K_c$ is 4. If the initial concentrations are [A] = 2 M, [B] = 2 M, and [C] = 0 M, what is the equilibrium concentration of C?

<p>2.13 M (B)</p> Signup and view all the answers

How does a catalyst affect a reaction at equilibrium?

<p>It increases the rate of both forward and reverse reactions equally, so it does not affect the equilibrium. (A)</p> Signup and view all the answers

What is the molar composition of the equilibrium mixture? $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$. Initially 1.00 mol $PCl_5$ in a 1.00-L container, and at equilibrium 0.135 mol $PCl_3$.

<p>$PCl_5$ = 0.865 mol, $PCl_3$ = 0.135 mol, $Cl_2$ = 0.135 mol (C)</p> Signup and view all the answers

Methanol is commercially made by $2H_2(g) + CO(g) \rightleftharpoons CH_3OH(g)$. What is the $K_c$ expression for this reaction?

<p>$K_c = \frac{[CH_3OH]}{[H_2]^2[CO]}$ (C)</p> Signup and view all the answers

Carbon dioxide decomposes at elevated temperature by $2CO_2(g) \rightleftharpoons 2CO(g) + O_2(g)$. At 3000 K, 2.00 mol $CO_2$ placed in 1.00-L container, at equilibrium, 0.90 mol $CO_2$ remains. What is the value of $K_c$ at this temperature?

<p>0.82 (A)</p> Signup and view all the answers

Write the $K_c$ expression for the following heterogeneous reaction: $H_2O(g) + C(s) \rightleftharpoons CO(g) + H_2(g)$

<p>$K_c = \frac{[CO][H_2]}{[H_2O]}$ (A)</p> Signup and view all the answers

Given: $aA + bB \rightleftharpoons cC + dD; K_1$. What is the $K_2$ if the reaction is reversed?

<p>$K_2 = \frac{1}{K_1}$ (A)</p> Signup and view all the answers

Given: $aA + bB \rightleftharpoons cC + dD; K_1$. What is the $K_2$ if the reaction is doubled?

<p>$K_2 = K_1^2$ (B)</p> Signup and view all the answers

How are $K_p$ and $K_c$ related? $aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)$

<p>$K_p = K_c(RT)^{c+d-a-b}$ (D)</p> Signup and view all the answers

If the value of $K_c$ at $227^\degree C$ is 0.0952 for the following reaction: $CH_3OH(g) \rightleftharpoons CO(g) + 2H_2(g)$, what is $K_p$ at this temperature?

<p>1.60 x $10^2$ (C)</p> Signup and view all the answers

For $K_c$ = 0.82 for a reaction, which statement best describes the composition of the equilibrium mixture?

<p>The equilibrium mixture contains substantial amounts of both reactants and products. (A)</p> Signup and view all the answers

Nickel(II) oxide reduced to the metal by $CO(g) + NiO(s) \rightleftharpoons CO_2(g) + Ni(s)$. If the partial pressure of CO is 100. mmHg and the total pressure of CO and $CO_2$ does not exceed 1.0 atm, will this reaction occur at 1500 K at equilibrium? ($K_p$ = 700. at 1500 K)

<p>Yes, the reaction will proceed in the forward direction. (C)</p> Signup and view all the answers

Nitrogen and oxygen form nitric oxide by $N_2(g) + O_2(g) \rightleftharpoons 2NO(g)$. If an equilibrium mixture at $25^\circ C$ contains 0.040 M $N_2$ and 0.010 M $O_2$, what is the concentration of NO in this mixture? The equilibrium constant at $25^\circ C$ is $1.0 \times 10^{-30}$

<p>2.0 x $10^{-17}$ M (D)</p> Signup and view all the answers

Hydrogen iodide decomposes by $2HI(g) \rightleftharpoons H_2(g) + I_2(g)$. At 800 K, the equilibrium constant, $K_c$, for this reaction is 0.016. IF 0.50 mol HI placed in a 5.0-L flask, what is the composition of the equilibrium mixture in molarities?

<p>[HI] = 0.08 M, [$H_2$] = 0.01 M , [$I_2$] = 0.01 M (C)</p> Signup and view all the answers

$N_2O_4$ decomposes by $N_2O_4(g) \rightleftharpoons 2NO_2(g)$. At $100^\circ C$, $K_c$ = 0.36. If a 1.00-L flask initially contains 0.100 M $N_2O_4$, what will be the equilibrium concentration of $NO_2$?

<p>0.12 M (D)</p> Signup and view all the answers

Initial conditions: $H_2(g) + F_2(g) \rightleftharpoons 2HF(g)$; $K_c = 1.15 \times 10^2$. 3.000 mol of each species is put in 1.500-L vessel. What is the equilibrium concentration of each species?

<p>[HF] = 5.06 M, [$H_2$] = [$F_2$] = 0.47 M (B)</p> Signup and view all the answers

The following reaction is at equilibrium: $COCl_2(g) \rightleftharpoons CO(g) + Cl_2(g)$. If chlorine gas is added to the reaction mixture what direction will the reaction shift?

<p>Left (D)</p> Signup and view all the answers

The following reaction is at equilibrium: $COCl_2(g) \rightleftharpoons CO(g) + Cl_2(g)$. If carbon monoxide gas is removed from the mixture what direction will the direction shift?

<p>Right (C)</p> Signup and view all the answers

In which direction will each reaction shift when the volume of the reaction container is increased?

<p>$COCl_2(g) \rightleftharpoons CO(g) + Cl_2(g)$ (C)</p> Signup and view all the answers

If $\Delta H^\degree$ = 484 kJ for the reaction $2H_2O(g) \rightleftharpoons 2H_2(g) + O_2(g)$, would you expect this reaction to be favorable at high or low temperatures?

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

The Fischer-Tropsch process involves $8CO(g) + 17H_2(g) \rightleftharpoons C_8H_{18}(g) + 8H_2O(g)$. Suppose the reaction is at equilibrium at 200°C, then cooled to condense the octane, and then the remaining gases are reheated to 200°C. In which direction will the equilibrium shift?

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

A typical reaction that occurs in the Fischer–Tropsch process is $8CO(g) + 17H_2(g) \rightleftharpoons C_8H_{18}(g) + 8H_2O(g)$. In which direction will the equilibrium shift when the pressure is increased?

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

Which statement accurately describes dynamic equilibrium in a reversible chemical reaction?

<p>The rate of the forward reaction equals the rate of the reverse reaction. (D)</p> Signup and view all the answers

For the synthesis of ammonia, $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, indicate how $K_p$ and $K_c$ are related.

<p>$K_p = K_c(RT)^{-2}$ (C)</p> Signup and view all the answers

For a reaction where solids or pure liquids are involved, what is their contribution to the $K_c$ expression?

<p>Both C and D (D)</p> Signup and view all the answers

Flashcards

Chemical equilibrium

State where forward and reverse reactions occur at equal rates.

Stoichiometry at equilibrium

Apply stoichiometry to determine mixture composition at equilibrium.

Equilibrium constant (Kc)

A ratio of products to reactants at equilibrium.

Equilibrium-constant expression

Expression derived from balanced equation using coefficients as exponents.

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Law of mass action

Rate of reaction is proportional to the active masses of reactants.

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Equilibrium constant Kp

Equilibrium constant using partial pressures for gases.

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Combining equilibrium constants

Kc for combined reactions equals the product of individual Kc.

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Homogeneous equilibrium

Equilibrium with all species in the same phase.

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Heterogeneous equilibrium

Equilibrium with species in multiple phases.

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Solids/liquids in equilibrium

Concentrations of solids and liquids are constant.

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Reaction quotient (Q)

Ratio of products to reactants used to predict reaction direction.

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Comparing Q to K

If Q > K, reaction shifts left; if Q < K, shifts right.

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ICE tables

ICE tables can be used to solve for equilibrium.

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Le Châtelier's principle

Disturbance shifts equilibrium to relieve stress.

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Adding/removing reactants/products.

Adding a gas shifts equilibrium to reduce the total moles of gas.

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Pressure effect

System shifts to fewer molecules for high pressure and greater for low pressure

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Temperature effect

Affects equilibrium constant, shifts reaction to relieve.

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Effect of a catalyst

Lowers activation energy to change reaction rate.

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Study Notes

Describing Chemical Equilibrium

  • Chemical reactions often appear to stop before completion, actually, they are reversible with original reactants forming products, and products reacting to reform original reactants.
  • Chemical equilibrium exists when the forward and reverse reactions occur at the same rate.
  • Stoichiometry applies to compute the content of the reaction mixture at equilibrium.

Learning Objectives for Chemical Equilibrium

  • Define dynamic equilibrium and chemical equilibrium.
  • Apply stoichiometry to an equilibrium mixture.

The Equilibrium Constant

  • The equilibrium constant expression involves multiplying product concentrations, dividing by reactant concentrations, and raising each term to the power of its coefficient.
  • The equilibrium constant is the value of the expression when equilibrium concentrations are substituted.
  • Equilibrium-constant expression and equilibrium constant should be defined.
  • The law of mass action should be stated.
  • Equilibrium-constant expressions should be written.
  • Kinetics argument for the approach to chemical equilibrium should be described.
  • Equilibrium constant should be obtained from reaction composition.

The Equilibrium Constant: Kp and Kc

  • Kp represents the equilibrium constant described in terms of partial pressures.
  • Kp and Kc are related and the formula for their relationship should be stated.
  • The law of mass action should be stated.
  • Kc can be obtained for a reaction written as a sum of other reactions with known Kc values.

Homogeneous vs. Heterogeneous Equilibria

  • Homogeneous equilibrium consists of reactions where all reactants and products are in the same phase.
  • Heterogeneous equilibrium consists of reactions where reactants and products are in more than one phase.
  • Kc can be written for a reaction containing pure solids or liquids; the concentrations of pure solids and liquids are not included in the equilibrium expression.

Interpreting and Using the Equilibrium Constant

  • A qualitative interpretation of the equilibrium constant can be made bases on its value.
  • The reaction quotient, Q, is defined.
  • The direction of a reaction can be described by comparing Q with Kc.
  • The reaction quotient should be used.
  • Equilibrium concentrations can be calculated.
  • One equilibrium concentration can be obtained from others.
  • Equilibrium problems involving linear equations in 'x' can be solved.
  • Equilibrium problems involving quadratic equations in 'x' can be solved.

Predicting Reaction Direction

  • The equilibrium constant value can qualitatively describe the reaction mixture content. K is very large (>10^2), the equilibrium mixture is mostly products.
  • If K is very small (10^2), the equilibrium mixture is mostly products.
  • If K approaches 1, the equilibrium mixture contains appreciable amounts of both reactants and products.

Reaction Quotient, Q

  • Q has the same form as the equilibrium constant but uses initial concentrations; it is used to predict reaction direction.
  • If Kc > Q, the reaction proceeds to the right.
  • If Kc < Q, the reaction proceeds to the left.
  • If Kc = Q, the reaction is at equilibrium.
  • Qc must move toward Kc.

Le Châtelier’s Principle on Reactants/Products

  • When a reactant or product is added or removed, it alters the equilibrium.
  • Le Châtelier’s principle states that if a system in chemical equilibrium is disturbed by a change in concentration, temperature, or pressure, the system shifts to counteract the change.
  • If a substance is added to the mixture, the equilibrium shifts to use it.
  • If a substance is removed from the mixture, the equilibrium shifts to produce it.
  • Changes in partial pressure affect equilibrium like adding/removing substances.

Le Châtelier’s Principle on Pressure

  • A change in pressure occurs due to a change in the volume of the reaction container.
  • Decreasing the container size increases pressure; the reaction shifts to reduce pressure, favoring the side with fewer gas molecules.
  • Increasing the container size decreases pressure; the reaction shifts to increase pressure, favoring the side with more gas molecules.
  • Pressure has no effect on equilibrium if both sides have equal gas moles.

Le Châtelier’s Principle on Temperature

  • Changing temperature impacts the equilibrium constant value and causes a shift in equilibrium.
  • For endothermic reactions (∆H° > 0), heat is considered a reactant.
  • For exothermic reactions (∆H° < 0), heat is considered a product.
  • Increasing temperature increases Kc for endothermic reactions, and decreases it for exothermic reactions. Decreasing temperature has the opposite effect.
  • Adding heat shifts the reaction to use heat and removing heat shifts the reaction to produce heat.

Effect of a Catalyst

  • Define catalyst.
  • A catalyst accelerates the rate of reaction but does not alter the equilibrium.
  • Describe how a catalyst can affect the product formed.

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