Topic 7 Equilibria.

Question 1

What is a reversible reaction?

Answer 1

A reversible reaction is a reaction which can go forwards or baclwards depending on the conditions OR a reaction in which reactants form products and products can reform reactants until an equilibrium is reached.

Question 2

What are the four characteristics of equilibrium?

Answer 2

• The concentrations of reactant and products remain constant at equilibrium.
• The rate of the forward reaction and the rate of the backward reaction are equal.
• It is a closed system.
• It is dynamic.

Question 3

What is a closed system?

Answer 3

A system in which no substances can enter or leave during a reaction.

You cannot establish dynamic equilibrium without a closed system.

Question 4

What is meant by dynamic equilibrium?

Answer 4

For a reversible reaction in a closed system, dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the backward reaction, hence the concentrations of the reactants and products remain constant despite the fact that particles are continually reacting.

Question 5

What is Le Chatelier’s principle?

Answer 5

If dynamic equilibrium is subject to changing conditions, the positions of equilibrium will shift to counteract this change OR if one or more factors that affect a dynamic equilibrium is changed, the position of equilibrium change or move to minimise this change.

Question 6

What are the four factors that affect dynamic equilibrium?

Answer 6

• Concentration.
• Pressure.
• Temperature.
• Catalyst.

Question 7

What is the effect of:
- Increasing the concentration of the reactants.
- Decreasing the concentration of the reactants.

Answer 7

• Increasing the concentration of the reactants:
  Increasing the concentration of the reactants causes the equilibrium to shift right in order to reduce the concentration of reactants and form more products, the forward reaction is favoured.
• Decreasing the concentration of the reactants:
  Decreasing the concentration of reactants causes the position of equilibrium to shift left in order to increase the concentration of reactants and form more reactants, the reverse reaction is favoured.

Question 8

What is the effect of:
- Increasing the concentration of the products.
- Decreasing the concentration of the products.

Answer 8

• Increasing the concentration of the products:
  The reverse reaction is favoured if the concentration of the products is increased so that product is used to make more reactants.
• Decreasing the concentration of the products:
  The forward reaction is favoured if the concentration of the product is decreased, so that more products are formed.

Question 9

What is the effect of:
- Increasing the pressure.
- Decreasing the pressure.
- An equal number of gaseous molecules on both sides of the equation.

For a change in pressure, we consider the number of gaseous molecules only.

Answer 9

• Increasing the pressure:
  Increasing the pressure will cause the position of equilibrium to shift towards the side with the fewest gaseous molecules in order to decrease the pressure.
• Decreasing the pressure:
  Decreasing the pressure will cause the position of equilibrium to shift towards the side with the most gaseous molecules in order to increase the pressure.
• An equal number of gaseous molecules on both sides of the equation:
  If there is an equal number of gaseous molecules on both sides of the equation, changing the pressure will have no effect on the position of equilibrium.

Question 10

What is the effect of:
- Increasing the temperature of an exothermic reaction.

In a chemical reaction, increasing the temperature can affect the position of equilibrium differently depending on whether the reaction is endothermic or exothermic.

Answer 10

• Increasing the temperature of an exothermic reaction:
  If the reaction releases heat (exothermic), increasing the temperature will cause the equilibrium position to shift towards the reactants because the reaction will favour the endothermic direction (the direction that absorbs heat) which is the side with the reactants. Equilibrium constant (K) will decrease because reactants form.

Question 11

What is the effect of:
- Increasing the temperature of an endothermic reaction.

In a chemical reaction, increasing the temperature can affect the position of equilibrium differently depending on whether the reaction is endothermic or exothermic.

Answer 11

• Increasing the temperature of an endothermic reaction:
  If the reaction absorbs heat (endothermic), increasing the temperature will cause the equilibrium position to shift towards the products because the reaction will favour the exothermic direction (the direction that releases heat) which is the side with the products. Equilibrium constant (K) will increase because more products form.

Question 12

What is the effect of:
- Decreasing the temperature of an exothermic reaction.

In a chemical reaction, decreasing the temperature will have the opposite effect of increasing the temperature depending on whether the reaction is exothermic or endothermic.

Answer 12

• Decreasing the temperature of an exothermic reaction:
  For an exothermic reaction (where heat is released) decreasing the temperature will cause the equilibrium position to shift towards the products because the reaction favours the exothermic direction (the direction that releases heat), the equilibrium constant (K) will increase because more products formed.

Question 13

What is the effect of:
- Decreasing the temperature of an endothermic reaction.

In a chemical reaction, decreasing the temperature will have the opposite effect of increasing the temperature depending on whether the reaction is exothermic or endothermic.

Answer 13

• Decreasing the temperature of an endothermic reaction:
  For an endothermic reaction (where heat is absorbed) decreasing the temperature will cause the equilibrium position to shift towards the reactants because the reaction favours the endothermic reaction (the direction that absorbs heat), the equilibrium constant (K) will decrease because more reactants are formed.

Question 14

What is a catalyst?

Answer 14

A catalyst is a substance that facilitates chemical reactions by lowering the activation energy required for the reaction to proceed, while it remains unchanged by the reaction.

Question 15

What effect does adding a catalyst have on the position of equilibrium?

Answer 15

Adding a catalyst does not affect the position of equilibrium. A catalyst speeds up the rate of both the forward and backwards reactions equally, thereby increasing the rate at which equilibrium is reached.

Question 16

What does the equilibrium law state?

Answer 16

The equilibrium law tells us that for any chemical reaction, there is a specific relationship between the amounts of reactants and products when the reaction reaches a balance. This balance is called equilibrium.

Question 17

What is the Haber process? Write an equation for this reaction and state the sources of the two reactants.

Answer 17

The Haber process produces ammonia from nitrogen and hydrogen:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g).
The forwards reaction in the Haber process is exothermic (ΔH = -92 kJ·mol⁻¹).

Nitrogen is obtained from the fractional distillation of air and hydrogen is obtained from natural gas.

Question 18

What are the conditions for the Haber process?

Answer 18

• Temperature: 400 - 450°C.
• Pressure: 200 atm.
• Catalyst: Iron catalyst.

Question 19

What pressure would be expected to be used for the Haber process and why?

Answer 19

According to Le Chatelier’s principle, because there are more molecules on the left hand side of the equation (the reactants side) a high pressure would give the greatest yield of ammonia because a high pressure would cause the position of equilibrium to shift to the right (the products side).
However, high pressure can be dangerous and are expensive to maintain so a lower pressure of 200 atm is used.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g).
4 molecules on the reactants side ⇌ 2 molecules on the products side.

Question 20

What is the Contact process? Write the first equation of the process and state the source of the reactants.

Answer 20

The Contact process produces sulfuric acid:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g).
The forwards reaction in the Contact process is exothermic (ΔH = -196 kJ·mol⁻¹).

Sulfur dioxide is obtained from the heating of sulfur or sulfur ores (e.g. FeS₂) in excess oxygen and oxygen is obtained from air.

Question 21

State the four processes involved in producing sulfuric acid.

Answer 21

• Sulfur dioxide is made from heating sulfur or sulfur ores (e.g. FeS₂) in excess air:
  S (s) + O₂ (g) → SO₂ (g).
• Sulfur dioxide to sulfur trioxide:
  2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g) (ΔH = -196 kJ·mol⁻¹).
• Sulfur trioxide is dissolved in concentrated sulfuric acid (as adding it directly into water would create a fog of sulfuric acid) to make oleum:
  H₂SO₄ (l) + SO₃ (g) → H₂S₂O7 (l)
• The product (oleum) is then dissolved in water to make sulfuric acid:
  H₂S₂O7 (l) + H₂O (l) → 2H₂SO₄ (l)

Question 22

What conditions are used for the Contact process?

Answer 22

• Temperature: 400 - 450°C.
• Pressure: 1- 2 atm.
• Catalyst: Vanadium (V) oxide (V₂O5) catalyst.

Question 23

State 5 common acids.

Answer 23

• Hydrochloric acid: HCl.
• Sulfuric acid: H₂SO₄.
• Nitric acid: HNO₃.
• Ethanoic acid: CH₃COOH.
• Phosphoric acid: H₃PO₄.

Question 24

State 4 common alkalis.

Answer 24

• Sodium hydroxide: NaOH.
• Potassium hydroxide: KOH.
• Ammonia: NH₃.
• Calcium hydroxide - Ca(OH)₂.

Question 25

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

Answer 25

The Brønstead-Lowry theory describes acid and base interactions in terms of proton transfer with bases being proton (H+ ion) acceptors and acids being proton (H+ ion) donors.

Question 26

What is a Brønsted - Lowry acid?

Answer 26

Acids are substances that will lose electrons to form positively charged hydrogen ions (H+ also called protons) when dissolved in water.

An acid is a proton (H+ ion) donor.

Question 27

What is a Brønsted - Lowry base?

Answer 27

Bases (alkalis) are proton acceptors as they ionise in solution producing OH- ions which can accept protons.
A base is a proton (H+ ion) acceptor.

Question 28

What is the pH of:
- Water.
- Acid solutions.
- Alkaline solutions.

Answer 28

• Water: pH 7.
• Acid solutions: pH of below 7.
• Alkaline solutions: pH of above 7.

Question 29

What is pH?

Answer 29

pH is a measure of the concentration of hydrogen ions in a solution.

Question 30

What is a neutralisation reaction?

Answer 30

When a base reacts with an acid to form a salt we call the reaction a neutralization reaction.

Question 31

Describe the neutralization reaction between an acid and an alkali to produce water.

Answer 31

H+ (aq) + OH– (aq) → H2O (l)

Question 32

Describe the meaning of strong acids.

Answer 32

Strong acids ionise completely in water, producing solutions of very low pH for an acid.

Hydrochloric acid is a strong acid:
HCl(aq) → H+ (aq) + Cl– (aq).

Question 33

Describe the meaning of weak acids.

Answer 33

Weak acids partially ionise in water and produce pH values which are closer to the middle of the pH scale.

Ethanoic acid is a weak acid:
CH3COOH(aq) ⇌ H+ (aq) + CH3COO– (aq)

Question 34

Describe the meaning of strong bases.

Answer 34

Strong bases ionise completely in water producing solutions of very high pH for a base.

Question 35

Describe the meaning of weak bases.

Answer 35

Weak bases partially ionise in water and produce pH values which are closer to the middle of the pH scale.

Question 36

What happens when acids react with a metal oxides?

Answer 36

When acids react with metals they form a salt and hydrogen gas:
Acid + Metal oxides → Salt + Hydrogen.

Question 37

What happens when acids react with bases?

Answer 37

In all acid-base neutralisation reactions, salt and water are produced:

Acid + Base → Salt + Water.

Question 38

What happens when acids react with metal carbonates?

Answer 38

Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:

Acid + Metal Carbonate → Salt + Carbon Dioxide + Water.

Question 39

Explain the differences in behaviour between strong and weak acids in a reaction with a reactive metal.

Answer 39

• Strong acids:
  React vigourously with reactive metals like magnesium or zinc producing hydrogen gas and a salt. The reaction is rapid and produces a lot of bubbles.
• Weak acids:
  React more slowly with reactive metals, producing fewer bubbles of hydrogen gas. The reaction is less vigorous.

Question 40

Explain the differences in behaviour between strong and weak acids in pH values by use of a pH meter.

Answer 40

• Strong acids:
  Have very low pH values (typically between 0 and 3) due to the high concentration of hydrogen ions. A pH meter will show a very low pH value due to the high concentration of H⁺ ions.
• Weak acids:
  Have higher pH values (typically between 3 and 7) because they do not release as many hydrogen ions. A pH meter will show a higher pH value compared to strong acids of the same concentration, reflecting the lower concentration of H⁺ ions.

Question 41

Explain the differences in behaviour between strong and weak acids in pH values by use of a universal indicator.

Answer 41

• Strong acids:
  The universal indicator will turn a color indicating a very low pH (red or orange).
• Weak acids:
  The universal indicator will turn a color indicating a higher pH compared to strong acids (yellow or light green).

Question 42

We can tell if an acid is strong or weak by measuring its electrical conductivity, pH and rate of reaction with metals or metal carbonates:

Answer 42

• A strong acid conducts much better than a weak acid of the same concentration.
• A strong acid has a lower pH than weak acid of the same concentration.
• A strong acid reacts faster than a weak acid.

Question 43

Why would does a strong acid conduct better than a weak acid of the same concentration?

Answer 43

Strong acids completely ionise in water, meaning they fully dissociate into their ions. This complete ionisation results in a high concentration of ions in the solution, which enhances the acids ability to conduct electricity.

Question 44

What is an indicator?

Answer 44

A substance that changes color when added to acidic or alkaline solutions.

Question 45

Describe the effect on litmus in acidic and basic conditions.

Answer 45

• In acidic conditions, blue litmus paper turns red.
• In basic conditions, red litmus paper turns blue.

Question 46

Describe the effect of methyl orange in acidic and basic conditions.

What type of titration would methyl orange be a suitable indicator for?

Answer 46

• Methyl orange indicator turns red in acidic conditions.
• Methyl orange indicator turns yellow in basic conditions.

Strong acid and weak base titrations.

Question 47

Describe the effect of bromothymol blue in acidic and basic conditions.

What type of titration would bromothymol blue be a suitable indicator for?

Answer 47

• Bromothymol blue indicator turns yellow in acidic conditions.
• Bromothymol blue indicator turns blue in basic conditions.

Strong acid and strong base titration.

Question 48

Describe the effect of phenolphthalein in acidic and basic conditions.

What type of titration would phenolphthalein be a suitable indicator for?

Answer 48

• Phenolphthalein indicator turns colourless in acidic conditions.
• Phenolphthalein indicator turns pink in basic conditions.

Weak acid and strong base titration.