7. Equilibria

Question 1

What is a reversible reaction?

Answer 1

• A reaction where the products can react to form the original reactants, which can react again to form the products
• A reversible reaction is denoted by ⇌

Question 2

What is dynamic equilibrium?

Answer 2

• When the rate of the forward reaction equals the rate of the backward reaction in a reversible reaction
• The concentration of reactants and products can be different, but they will not be changing
• Dynamic equilibrium is established after a reaction mixture is left for a certain period of time to give the concentration of products time to increase

If the reactions involve gases, equilibrium can only be reached in a closed system

Question 3

What is Le Chatelier’s principle?

Answer 3

Any change to a chemical system in dynamic equilibrium will be countered by an opposing change

Although the system attempts to counter the change, it will not be entirely undone

Question 4

How does changing the concentration of products or reactants affect the position of dynamic equilibrium?

Answer 4

• If the concentration of reactants increases, equilibrium shifts to the right because the concentration of products increases to counter this change
• The rate of the forward reaction will temporarily increase, followed by an increase in the rate of the backward reaction after products accumulate
• Eventually, a new dynamic equilibrium will be established with a higher concentration of products and reactants, though the calculated K꜀ value will remain constant as both concentrations change
• The reverse happens when an increase in product concentration: equilibrium will shift to the left

Question 5

How does changing the temperature of a chemical system affect the position of dynamic equilibrium?

Answer 5

• If the forward reaction is endothermic, increasing temperature causes equilibrium to shift to the right to absorb more heat
• A new equilibrium will be established, but there will be a higher concentration of products and a lower concentration of reactants
• The reverse happens if the forward reaction is exothermic or if temperature is decreased

If the forward reaction is exothermic, increasing temperature still increases its rate as particles have more kinetic energy, but the rate of the backward reaction will increase more significantly until a new equilibrium is reached

Question 6

How does changing the pressure of a chemical system affect the position of dynamic equilibrium?

Answer 6

• There is only an effect if some of the reactants or products are gases
• If pressure increases, equilibrium will shift toward the side with the fewest moles of gas to offset the increase in pressure
• If pressure decreases, the opposite will occur

Question 7

What is K꜀?

Answer 7

• A constant associated with a chemical system at thermal equilibrium at a given temperature
• It is determined by the proportion of reactant and product concentrations

Question 8

How is K꜀ calculated?

Answer 8

• In a reversible reaction where aA + bB ⇌ cC + dD
• K꜀ = [C]ᶜ x [D]ᵈ/[A]ᵃ x [B]ᵇ
• Square brackets around a species denote the concentration of the species at equilibrium
• The concentrations at equilibrium are raised to the power of the molar coefficient in the balanced chemical equation

Solids are ignored when calculating K꜀

Question 9

Which of changing temperature, changing pressure, adding a catalyst or changing the concentration of reactants or products affects K꜀?

Answer remembering that K꜀ is a ratio of the concentrations of products and reactants at equilibrium

Answer 9

• Only changing temperature affects K꜀
• Changing pressure affects the concentration of reactants and products uniformly (if they are gaseous), so the proportion remains constant
• A catalyst increases the rate of both the forward and backward reaction and simply changes the rate at which equilibrium is established
• Changing the concentration of either the reactants or products shifts equilibrium and changes the amounts of both products and reactants present at equilibrium, but since the concentrations of both reactants and products increase, the calculated K꜀ value does not change

Question 10

How can concentrations of species at equilibrium be deduced?

Answer 10

• Depending on the information you have, you can use the balanced equation and the number of moles of one or a few species at equilibrium to calculate the rest
• The number of moles reactants lose and products gain between the start and equilibrium will be proportional to their coefficients in the balanced equation
• The number of moles of each species is then divided by the total volume of the reaction mixture to calculate the concentrations of each species at equilibrium

Question 11

What is Kₚ?

Answer 11

• A constant associated with a chemical system in dynamic equilibrium that is fixed at a given temperature
• It is often used if the reactants and products are gases (and does not work with species in any other state)
• It is determined by the proportion of the partial pressure exerted by each species at equilibrium instead of concentrations

Question 12

How are the partial pressures of gases at equilibrium calculated?

Answer 12

• The total pressure of the chemical system is found
• The total number of moles of gas present is found
• The number of moles of each gas present at equilibrium is found
• The number of moles of each gas present at equilibrium is divided by the total number of moles to find each gas’ mole fraction
• The mole fraction of each gas is multiplied by the total pressure to find each gas’ partial pressure

Question 13

How is Kₚ calculated?

Answer 13

• In a reversible reaction where aA (g) + bB (g) ⇌ cC (g) + dD (g)
• Kₚ = [Pc]ᶜ x [Pd]ᵈ/[Pa]ᵃ x [Pb]ᵇ
• [Px]ˣ denotes the partial pressure of x (g) at equilibrium
• The partial pressure of each gas is raised to the power of its molar coefficient in the balanced chemical equation

Question 14

How are the units of K꜀ or Kₚ deduced?

Answer 14

• Find the units of each species in the equilibrium expression
• For K꜀, this will be mol dm⁻³ to the power of the molar coefficient
• For Kₚ, the unit is Pa (pascals) or atm to the power of the molar coefficient
• Apply indices rules, multiplying the top units together then dividing them by the bottom units
• Often, there will be no unit as the top and bottom units are the same
• See the example below

Question 15

What are two industrial processes that rely on the principles of dynamic equilibrium, and what are the chemical equations of each of these processes?

Answer 15

• The Haber process
• N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
• The Contact process
• 2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

Both reactions are exothermic

Question 16

How is Le Chatelier’s principle used to maximise the yield of the Haber process?

Answer 16

• A compromise temperature of around 400 ᵒC is used to shift equilibrium to the right as much as possible (as the reaction is exothermic) while still providing sufficient kinetic energy
• The pressure is maximised as the right side has fewer moles, limited only by cost
• Produced ammonia is immediately condensed away and stored at low temperatures
• An iron catalyst is used to speed up the reaction (though it does so in both directions)

Question 17

How is Le Chatelier’s principle used to maximise the yield of the Contact process?

Answer 17

• A high pressure is used to shift equilibrium to the right (as this side has the fewest moles of gas)
• As with the Haber process, a compromise temperature of around 400 ᵒC is used
• SO₃ is readily removed by absorbing it in impure sulfuric acid to form more sulfuric acid
• A vanadium(V) oxide catalyst is used to increase the rate of both the forward and the backward reaction

Question 18

What is the Brønsted–Lowry theory of acids and bases?

Answer 18

• Acids are chemical species capable of donating a proton
• Bases are chemical species capable of accepting a proton using a lone pair of electrons

Question 19

What are two equations that demonstrate water’s amphoteric character?

Answer 19

• H₂O (l) + NH₃ (g) –> OH⁻ (aq) + NH₄⁺ (aq)
• H₂O (l) + HCl (g) –> H₃O⁺ (aq) + Cl⁻ (aq)

• These reactions occur in aqueous acids and alkalis
• That said, Brønsted–Lowry reactions can take place outside of aqueous solutions

Question 20

What is the definition of a strong acid and strong base?

Answer 20

• An acid or a base that fully dissociates in aqueous solution
• Dissociation is reversible, but equilibrium lies so far to the right that it can be represented as irreversible
• They cause significant pH changes as the concentration of H⁺/H₃O⁺ or OH⁻ ions is high

Question 21

What is the definition of a weak acid and weak base?

Answer 21

• An acid or a base that only partially dissociates in aqueous solution
• Equilibrium lies more to the left
• They cause less significant pH changes as the concentration of H⁺/H₃O⁺ or OH⁻ ions is lower after dissociation

Question 22

How can the pH of strong acids and weak acids be calculated?

Answer 22

• pH is –log₁₀ [H⁺ (aq)]
• Therefore, for strong, you substitue the molar concentration of the acid into the equation (as it fully dissociates)
• For weak acids, the acid dissociation constant (Kₐ) must be used

Question 23

What is the pH of water at 298K?

Answer 23

• 7
• It changes slightly with temperature as more H⁺ dissociates, but water remains neutral as the concentration of OH⁻ increases at the same rate

Question 24

How do strong and weak acids differ in terms of conductivity?

Answer 24

• Strong acids have high conductivities as they have a higher concentration of H⁺ ions
• This can be determined using a conductivity metre

Question 25

How do strong and weak acids differ in terms of reactivity with metals?

Answer 25

* Strong acids will react more vigorously as there are more H⁺ ions available to form hydrogen gas and more anions available to form a salt * This can be observed by the difference in effervescence intensity

Question 26

What is the difference between the endpoint and the equivalence point of a titration?

Answer 26

* The equivalence point is the point at which an equal number of moles of the titrant and analyte have reacted and is situated halfway up the vertical section of a pH curve * The endpoint is the point at which a pH indicator changes colour in a titration

Question 27

What does the pH curve for a strong acid-strong base titration look like?

Answer 27

## Footnote * The equivalence point for strong acid-strong base titrations will always be at pH 7 * Strong base-strong acid will be the same but flipped 180 degrees

Question 28

What does the pH curve for a strong acid-weak base titration look like?

Answer 28

## Footnote The equivalence point is around pH 5.5

Question 29

What does the pH curve for a weak acid-strong base titration look like?

Answer 29

## Footnote The equivalence point is around pH 9

Question 30

What does the pH curve for a weak acid-weak base titration look like?

Answer 30

## Footnote The equivalence point is at pH 7

Question 31

How do indicators work to signify pH change?

Answer 31

* They are weak acids that reversibly dissociate to form H⁺ and a conjugate base * The colour changes when the concentration of the conjugate base exceeds the concentration of the acid or vise versa * pH affects the position of equilibrium, so the colour will change at a certain pH (the value of which depends on the indicator) * The endpoint of a titration is when the concentrations of the acid and the conjugate base are equal ## Footnote High pHs tend to push equilibrium to the left and low pHs tend to push equilibrium to the right

Question 32

How can the suitablity of an indicator for a titration be determined?

Answer 32

* The pH range that the indicator changes colour in must fall in the verticle region of the pH curve of the titration * This ensures that the titration reaches its endpoint as the equivalence point is passed * For example, in a strong acid-weak alkali titration, phenolphthalein, with a range of 8.3-10.0, is not suitable, but methyl orange, with a range of 3.1-4.4, is suitable