How Far? The Extent Of Chemical Change

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Apuntes de la lección

Reversible Reactions and Dynamic Equilibrium

Some reactions go to completion, where reactants are used up to form products and the reaction stops.
In reversible reactions, products can react to reform the original reactants; this is shown using two opposing half arrows: ⇌.
In a dynamic equilibrium, the rate of the forward reaction is the same as the rate of the backward reaction in a closed system.
The concentrations of reactants and products remain constant, but they are not necessarily equal.
There is no change in macroscopic properties such as colour and density because these depend on concentration.
Dynamic equilibrium can be reached starting with either reactants or products.

A reversible reaction at equilibrium

Closed and Open Systems

A closed system is one in which none of the reactants or products escape from the reaction mixture.
In an open system, some matter is lost to the surroundings.
When a reaction takes place entirely in solution, equilibrium can be reached in open flasks.
If the reaction involves a gas, equilibrium can only be reached in a closed system.
Dynamic equilibrium can also occur in physical systems, such as the equilibrium between liquid ethanol and ethanol vapour in a sealed bottle.

The Equilibrium Law and the Equilibrium Constant Expression

The equilibrium law explains how the equilibrium constant, K, can be found from the stoichiometry of the reaction.
For the general reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is: K = [C]^c [D]^d / ([A]^a [B]^b).
[A] and [B] are equilibrium reactant concentrations in mol dm⁻³; [C] and [D] are equilibrium product concentrations in mol dm⁻³.
Solids are ignored in equilibrium constant expressions.
The equilibrium constant, K, is specific to a given equation.
In equilibrium constant expressions, square brackets must be used as they represent concentrations.

Interpreting the Magnitude of K

The size of K tells us how the equilibrium mixture is made up with respect to reactants and products.
If K << 1, the reaction hardly proceeds, the equilibrium lies far to the left, and the mixture contains mostly reactants.
If K < 1, the reaction favours the reactants and the equilibrium lies to the left-hand side.
If K = 1, there are significant amounts of both reactants and products, and the equilibrium is balanced between the two sides.
If K > 1, the reaction favours the products and the equilibrium lies to the right-hand side.
If K >> 1, the reaction goes almost to completion, the equilibrium lies far to the right with mostly products present.

The Equilibrium Constant and Temperature

K is a constant at a specified temperature.
Since temperature can affect the position of equilibrium, K is dependent on temperature.
For an endothermic reaction, an increase in temperature increases the value of K.
For an exothermic reaction, an increase in temperature decreases the value of K.
Changes in concentration or pressure do not affect the value of K, provided temperature remains constant.

Relationship Between K Values for Reverse Reactions

The equilibrium constant expression depends on the specific reaction.
For a reaction and its reverse, the equilibrium constants are reciprocals of each other at the same temperature: K' = 1/K.
For example, if the equilibrium constant for a forward reaction is 7.1 × 10³², the equilibrium constant for the reverse reaction is 1.41 × 10⁻³³.

Le Chatelier's Principle: Concentration Changes

Le Chatelier's principle states that if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change.
Increasing the concentration of a reactant shifts the equilibrium to the right, producing more product.
Decreasing the concentration of a reactant shifts the equilibrium to the left.
Increasing the concentration of a product shifts the equilibrium to the left.
Decreasing the concentration of a product shifts the equilibrium to the right.
Changes in concentration do not affect the value of K.

Equilibrium and the Haber process

Le Chatelier's Principle: Pressure Changes

Changes in pressure only affect reactions where reactants or products are gases.
Increasing the pressure shifts the equilibrium in the direction that produces the smaller number of gas molecules.
Decreasing the pressure shifts the equilibrium in the direction that produces the larger number of gas molecules.
If there are the same number of gas molecules on both sides, changes in pressure do not change the position of equilibrium.
Changes in pressure do not affect the value of K.

Le Chatelier's Principle: Temperature Changes

Increasing the temperature shifts the equilibrium in the endothermic direction, absorbing energy.
Decreasing the temperature shifts the equilibrium in the exothermic direction, releasing energy.
Changes in temperature change the value of K.
For an endothermic reaction, increasing temperature increases K.
For an exothermic reaction, increasing temperature decreases K.

Catalysts and Heterogeneous Equilibria

A catalyst increases the rate of a chemical reaction by increasing the rate of the forward and reverse reactions equally.
Catalysts only cause a reaction to reach equilibrium faster; they have no effect on the position of equilibrium or on the value of K.
Le Chatelier's principle can also be applied to heterogeneous equilibria.
In a fizzy drink bottle, an equilibrium exists between dissolved CO₂ and gaseous CO₂: CO₂(g) ⇌ CO₂(aq).
When the bottle is opened, some CO₂(g) escapes, the equilibrium shifts to the left, and bubbles of CO₂(g) are observed.

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Preguntas de práctica

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1.Which statement describes a closed system?
Easy
- AOnly matter can be transferred across the boundary
- BOnly energy can be transferred across the boundary
- CEnergy and matter can be transferred across the boundary
- DEnergy and matter cannot be transferred across the boundary
2.Dinitrogen tetraoxide, N2O4, and nitrogen dioxide exist in equilibrium according to the equation: 2NO2(g) ⇌ N2O4(g) ΔH = –57 kJ mol–1. Which conditions give the greatest percentage of NO2 at equilibrium?
Medium
- Ahigh pressure and high temperature
- Blow pressure and high temperature
- Chigh pressure and low temperature
- Dlow pressure and low temperature
3.Which statement below best describes a dynamic equilibrium?
Easy
- Athe rate of the forward reaction is equal to the backwards reaction, and the concentrations of reactants and products is constant
- Bthe rate of the forward reaction is equal to the backwards reaction in a closed system, and the concentrations of the reactants and products are equal
- Cthe rate of the forward reaction is equal to the backwards reaction in a closed system, and the concentrations of the reactants and products is constant
- Dthe rate of reaction changes in either direction to counteract a change in conditions
4.Ethanol is manufactured by reacting steam with ethene: C2H4(g) + H2O(g) ⇌ C2H5OH(g) ΔH = –45 kJ mol–1. Which changes would increase the equilibrium yield of ethanol? (select all that apply)
Medium
- AAdding a catalyst
- BIncreasing the pressure
- CDecreasing the temperature
- DIncreasing the temperature
- ERemoving ethanol as it forms
5.Propyl ethanoate is formed in an esterification reaction: CH3CH2CH2OH(l) + CH3COOH(l) ⇌ CH3COOCH2CH2CH3(l) + H2O(l) ΔH = –10 kJ mol–1. How can the value of the equilibrium constant Kc be increased?
Medium
- Aincreasing the temperature
- Badding a catalyst
- Cincreasing the pressure
- Dlowering the temperature
6.An equilibrium is established at 450 °C when gaseous, purple iodine reacts with colourless hydrogen to form colourless hydrogen iodide: H2(g) + I2(g) ⇌ 2HI(g). The reaction is exothermic. Which change in conditions will cause the purple colour of the equilibrium mixture to become paler?
Medium
- Adecrease in pressure
- Bdecrease in temperature
- Cincrease in pressure
- Dincrease in temperature
7.Nitrosyl chloride decomposes into nitrogen monoxide and chlorine according to the following equation: 2NOCl(g) ⇌ 2NO(g) + Cl2(g). What is the correct expression for Kc?
Medium
- A[NOCl]2 / ([NO]2 [Cl2])
- B[NO] [Cl2] / [NOCl]
- C2[NO] [Cl2] / 2[NOCl]
- D[NO]2 [Cl2] / [NOCl]2
8.An equilibrium is established in the reaction: AB(aq) + CD(aq) ⇌ AC(aq) + BD(aq) ΔH = +180 kJ mol–1. Which factors would affect the value of Kc in this equilibrium?
Medium
- Achange in temperature in the absence of a catalyst
- Bchange in pressure in the presence of a catalyst
- Cincreasing the concentration of AB
- Dincreasing the concentration of AC

Más preguntas

9.What is the equilibrium constant expression for the following equation? 3Cl2(g) + I2(g) ⇌ 2ICl3(g)
Medium
- A2[ICl3] / (3[Cl2] + [I2])
- B2[ICl3] / (3[Cl2] [I2])
- C([Cl2]3 [I2]) / [ICl3]2
- D[ICl3]2 / ([Cl2]3 [I2])
10.Hydrogen iodide can be made by the direct combination of hydrogen and iodine in the following equilibrium reaction: H2(g) + I2(g) ⇌ 2HI(g). What is the expression for the equilibrium constant?
Medium
- A[2HI] / ([H2] [I2])
- B2[HI]2 / ([H2] + [I2])
- C[2HI] / ([H2] + [I2])
- D[HI]2 / ([H2] [I2])
11.What is the significance of the value of Kc ≫ 1 in a reversible reaction?
Medium
- AThe reaction is almost complete
- BVery little reaction occurs
- CEquilibrium is established very quickly
- DThe rate of the forward reaction is larger than the rate of the backward reaction
12.Which is the correct Kc expression for the reaction between hydrogen and fluorine? H2(g) + F2(g) ⇌ 2HF(g)
Medium
- A[HF]2 / ([H2] [F2])
- B[HF] / ([H2] [F2])
- C2[HF] / ([H2] [F2])
- D([H2] [F2]) / [HF]2
13.Nitrosyl chloride decomposes into nitrogen monoxide and chlorine according to the following equation. The forward reaction is endothermic: 2NOCl(g) ⇌ 2NO(g) + Cl2(g). Which change in condition would change the value for Kc?
Medium
- ADecreasing the pressure
- BAdding a catalyst
- CIncreasing the temperature
- DIncreasing the pressure
14.Which systems are correctly described? (select all that apply)
Medium
- AMatter and energy can be transferred across the boundary of an open system
- BOnly matter can be transferred across the boundary of a closed system
- CMatter and energy cannot be transferred across the boundary of an isolated system
- DOnly energy can be transferred across the boundary of a closed system
15.Ethyne and hydrogen are formed from methane and a dynamic equilibrium is established: 2CH4(g) ⇌ 3H2(g) + C2H2(g). What are the units for Kc?
Medium
- Amol dm–3
- Bmol2 dm–6
- Cmol3 dm–9
- Dmol4 dm–12
16.The dissociation of gas X2Y4 into XY is represented in the equation below: X2Y4(g) ⇌ 2XY(g) ΔHθ = +60 kJ mol–1. At constant pressure, if the temperature of the equilibrium mixture of the gases is increased, will the volume of the mixture increase or decrease and why?
Hard
- Athe volume will increase, but only because of a shift of equilibrium towards the right
- Bthe volume will increase, both because of a shift of equilibrium towards the right and also because of thermal expansion
- Cthe volume will stay the same because any thermal expansion could be exactly counteracted by a shift of equilibrium towards the left
- Dthe volume will decrease because a shift of equilibrium towards the left would more than counteract any thermal expansion
17.The reaction between nitrogen and hydrogen produces ammonia gas: N2(g) + 3H2(g) ⇌ 2NH3(g). If the equilibrium constant for the reaction is x, what is the equilibrium constant for the following reaction? 2N2(g) + 6H2(g) ⇌ 4NH3(g)
Hard
- Ax
- B2x
- Cx2
- D2x2
18.When a chemical reaction reaches equilibrium which of the following can be said about the concentrations of the substances present, and the rates of reaction?
Medium
- AConcentrations continue to change; rates of forward and backward reactions remain the same
- BConcentrations remain constant; rates of forward and backward reactions remain the same
- CConcentrations continue to change; rates of forward and backward reactions are different
- DConcentrations remain constant; rates of forward and backward reactions are different
19.The Haber process is used to produce ammonia industrially by the following reaction: N2(g) + 3H2(g) ⇌ 2NH3(g). Which statements explain why a catalyst is used? (select all that apply)
Medium
- AA catalyst lowers the activation energy
- BA catalyst moves the position of equilibrium towards the product
- CA catalyst allows the same rate to be achieved at a lower temperature
- DA catalyst increases the value of Kc
20.Study the following equilibrium reaction and determine which of the changes listed below will shift the equilibrium to the right hand side: 2CO2(g) ⇌ 2CO(g) + O2(g). (select all that apply)
Medium
- Ausing a catalyst
- Breducing the oxygen concentration
- Cincreasing the volume of the container
- Dincreasing the pressure
21.Which of the following conclusions can be made about this reaction? H2O(g) ⇌ H2(g) + ½O2(g) Kc = 8 × 10–41 at 25 °C
Medium
- AThe reaction does not proceed
- BThe reaction goes almost to completion
- CThe products have a higher concentration than the reactants
- DThe concentrations of reactants and products are the same
22.In a dynamic equilibrium, the concentrations of reactants and products are equal.
Easy
True or false?
23.A catalyst increases the rate of both the forward and reverse reactions equally and therefore does not change the position of equilibrium.
Easy
True or false?
24.For the reaction N2O4(g) ⇌ 2NO2(g), increasing the pressure shifts the equilibrium to the right.
Medium
True or false?
25.If Kc for a reaction is 1, there are significant amounts of both reactants and products at equilibrium.
Easy
True or false?
26.Match each term with its correct description.
Medium
- Open system
- Closed system
- Isolated system
- Dynamic equilibrium
- Matter and energy can be transferred across the boundary
- Only energy can be transferred across the boundary
- Neither matter nor energy can be transferred across the boundary
- Forward and backward reaction rates are equal and concentrations are constant
27.Arrange the steps to deduce the equilibrium constant expression for a reaction in the correct order.
Medium
- Write the balanced chemical equation
- Identify the products and their coefficients
- Write the product concentrations raised to their coefficients in the numerator
- Write the reactant concentrations raised to their coefficients in the denominator
- Combine numerator and denominator to form the expression
28.Match each type of system with the correct description of its boundary.
Easy
- Open system
- Closed system
- Isolated system
- Both matter and energy can be transferred across the boundary
- Only energy can be transferred across the boundary
- Neither matter nor energy can be transferred across the boundary
29.Which of the following statements about a system at dynamic equilibrium are correct? (select all that apply)
Medium
- AThe rate of the forward reaction equals the rate of the reverse reaction.
- BThe concentrations of reactants and products are equal.
- CThe concentrations of reactants and products remain constant.
- DThe system must be closed so that no reactants or products escape.
- EThe reaction has stopped completely.
30.Arrange the steps in the correct order to write the equilibrium constant expression for the reaction N2(g) + 3H2(g) ⇌ 2NH3(g).
Medium
- Write the product concentration in the numerator, raised to its stoichiometric coefficient.
- Write the reactant concentrations in the denominator, each raised to its stoichiometric coefficient.
- Omit any pure solids or liquids from the expression.
- Combine into a single expression: K = [NH3]2 / ([N2][H2]3).
31.For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), which expression is the correct equilibrium constant expression?
Medium
- AK = [SO3]2 / ([SO2]2[O2])
- BK = [SO2]2[O2] / [SO3]2
- CK = [SO3] / ([SO2][O2])
- DK = [SO3]2 / ([SO2][O2]2)
32.Adding a catalyst to a reaction at equilibrium increases the value of the equilibrium constant, K.
Easy
True or false?
33.For the exothermic reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), which changes would increase the equilibrium yield of SO3? (select all that apply)
Medium
- AIncreasing the pressure
- BDecreasing the temperature
- CAdding a catalyst
- DIncreasing the temperature
- ERemoving SO3 as it forms
34.Which statements about the equilibrium constant K are correct? (select all that apply)
Medium
- AK is constant at a given temperature.
- BIf K ≫ 1, the equilibrium mixture contains mostly products.
- CK changes when the pressure is increased.
- DIf K < 1, the equilibrium lies to the left.
- EK for the reverse reaction is the same as K for the forward reaction.
35.For the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), K = 2.6 × 10⁻¹⁸ at a certain temperature. What does this value indicate about the equilibrium mixture?
Medium
- AThe mixture contains mostly products.
- BThe mixture contains mostly reactants.
- CThe reaction goes almost to completion.
- DThe concentrations of reactants and products are equal.
36.For the reaction 2NOCl(g) ⇌ 2NO(g) + Cl2(g), which expression is the correct equilibrium constant expression?
Medium
- AK = [NO]2[Cl2] / [NOCl]2
- BK = [NOCl]2 / ([NO]2[Cl2])
- CK = [NO]2[Cl2] / (2[NOCl])
- DK = [NO][Cl2] / [NOCl]
37.Nitrogen monoxide reacts with fluorine according to the equation 2NO(g) + F2(g) ⇌ 2NOF(g). At a given temperature, K = 7.1 × 10³². What is the value of the equilibrium constant for the reverse reaction, 2NOF(g) ⇌ 2NO(g) + F2(g), at the same temperature?
Medium
- A1.4 × 10⁻³³
- B7.1 × 10³²
- C–7.1 × 10³²
- D1.4 × 10⁻³¹
38.Hydrogen iodide decomposes at equilibrium: 2HI(g) ⇌ H2(g) + I2(g), with K = 6.25 × 10⁻³ at a particular temperature. Extra HI is then added to the mixture at constant temperature. Which statement is correct once equilibrium has been re-established?
Medium
- AK is unchanged and the ratio [H2][I2]/[HI]2 returns to 6.25 × 10⁻³
- BK increases because more HI has been added
- CK decreases because the equilibrium shifts to the left
- DK is unchanged but the ratio [H2][I2]/[HI]2 becomes larger than 6.25 × 10⁻³
39.Carbon dioxide dissolves in water in a sealed fizzy drink bottle: CO2(g) ⇌ CO2(aq). The bottle is opened and some CO2(g) escapes. What is observed and why?
Medium
- ABubbles of CO2 form as the equilibrium shifts to the left
- BBubbles of CO2 form as the equilibrium shifts to the right
- CNo change occurs because K has not changed
- DThe drink becomes flat instantly because the equilibrium shifts to the right
40.For the reaction 2CH4(g) ⇌ 3H2(g) + C2H2(g), what are the units of Kc?
Medium
- Amol² dm⁻⁶
- Bmol dm⁻³
- Cmol³ dm⁻⁹
- Dmol⁴ dm⁻¹²
41.The Kc expression for the reaction H2(g) + I2(g) ⇌ 2HI(g) is Kc = [HI]2 / ([H2] [I2]). At equilibrium there were 0.234 moles of HI, 0.150 moles of H2 and 0.025 moles of I2 in a 1.0 dm3 container. What is the value of the equilibrium constant, Kc, under these conditions?
Medium
42.Complete the statement about dynamic equilibrium.
Easy
In a dynamic equilibrium, the rate of the forward reaction is ____ to the rate of the backward reaction.
43.Complete the equilibrium constant expression for the reaction: aA + bB ⇌ cC + dD.
Medium
K = [C]^c [D]^d / ([A]^a [B]^____)
44.What is the name of the principle that predicts the direction of shift when a change is made to a system at dynamic equilibrium?
Easy
45.What is the term for a system in which none of the reactants or products escape from the reaction mixture?
Easy
46.For the reaction H2(g) + I2(g) ⇌ 2HI(g) at equilibrium, [HI] = 0.234 mol dm⁻³, [H2] = 0.150 mol dm⁻³ and [I2] = 0.025 mol dm⁻³. Calculate the value of Kc. Give your answer to three significant figures.
Medium
47.Complete the sentence about a closed system.
Easy
A closed system is one in which none of the ____ or ____ escape from the reaction mixture.
48.At equilibrium, a mixture contains [HI] = 0.234 mol dm⁻³, [H2] = 0.150 mol dm⁻³ and [I2] = 0.025 mol dm⁻³ for the reaction H2(g) + I2(g) ⇌ 2HI(g). Calculate the value of Kc. Give your answer to 3 significant figures.
Medium
49.Complete the expression for the equilibrium constant of the reaction aA + bB ⇌ cC + dD.
Medium
K = [C]^____ [D]^____ / ([A]^____ [B]^____)
50.Explain what is meant by a dynamic equilibrium.
Hard
51.Explain the effect of increasing the temperature on the position of equilibrium and on the value of Kc for an exothermic reaction.
Hard
52.The reaction Ag2CO3(s) ⇌ Ag2O(s) + CO2(g) is at equilibrium. Increasing the temperature at constant pressure increases the amount of CO2(g) present. State whether the forward reaction is exothermic or endothermic and explain your reasoning.
Hard
53.Ethanoic acid reacts with ethanol: CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l). Explain what happens to the position of equilibrium when some C2H5OH(l) is removed from the mixture.
Hard
54.The Kc expression for the reaction H2(g) + I2(g) ⇌ 2HI(g) is Kc = [HI]2 / ([H2] [I2]). At equilibrium there were 0.234 moles of HI, 0.150 moles of H2 and 0.025 moles of I2 in a 1.0 dm3 container. What is the value of the equilibrium constant, Kc, under these conditions?
Medium
- A13.6
- B15.6
- C16.6
- D14.6
55.For the reaction H2(g) + I2(g) ⇌ 2HI(g) at equilibrium, [HI] = 0.234 mol dm⁻³, [H2] = 0.150 mol dm⁻³ and [I2] = 0.025 mol dm⁻³. Calculate the value of Kc. Give your answer to three significant figures.
Medium
- A16.6 no unit
- B14.6 no unit
- C13.6 no unit
- D15.6 no unit
56.At equilibrium, a mixture contains [HI] = 0.234 mol dm⁻³, [H2] = 0.150 mol dm⁻³ and [I2] = 0.025 mol dm⁻³ for the reaction H2(g) + I2(g) ⇌ 2HI(g). Calculate the value of Kc. Give your answer to 3 significant figures.
Medium
- A14.6
- B15.6
- C16.6
- D13.6

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