Equilibria

Question 1

Sulfuric acid is manufactured by the Contact Process.In this process, sulfur dioxide reacts with oxygen. The equation for the equilibrium that is established isSO2(g) + 1/2O2(g) SO3(g) ΔH = –98kJmolState and explain the effect of a decrease in temperature on the equilibrium yield of SO3and why?

Answer 1

(Yield) increases / goes up / gets moreThe (forward) reaction / to the right is exothermic or gives out / releases heatOR The reverse reaction / to the left is endothermic or takes in /absorbs heatThe (position of ) equilibrium shifts / moves left to right to oppose the decrease in temperature

Question 2

Give two features of a reaction at equilibrium

Answer 2

1. Concentration(s) (of reactants and products) remain or stay constant / the same2. Forward rate = reverse / backward rate

Question 3

Write an equation for the reaction of concentrated sulfuric acid with potassium bromide to form potassium hydrogensulfate and hydrogen bromide.

Answer 3

KBr + H2SO4 ——>KHSO4 + HBr

Question 4

Bromine is one of the products formed when concentrated sulfuric acid reacts with hydrogen bromide.Write an equation for this reaction.State the role of sulfuric acid in this reaction.

Answer 4

2HBr + H2SO4 ——>Br2 + SO2 + 2H2OOxiding agent or electron acceptor or oxidanti.e. to oxidise the bromide ion

Question 5

Concentrated sulfuric acid is used in a two-stage process to convert 2-methylpropene into 2-methylpropan-2-ol.Stage 1 (CH3)2C CH2 + H2SO4 —–>(CH3)2C(OSO2OH)CH3 Stage 2 (CH3)2C(OSO2OH)CH£ + H2O —–>(CH3)2C(OH)CH3 + H2SO4Name this type of conversion in stage 1 and 2State the overall role of sulfuric acid in this conversion.

Answer 5

1. Electrophilic addition2.Hydrolysis3. Catalyst

Question 6

State the meaning of the term rate of reaction.

Answer 6

Change in concentration of a substance / reactant / product in unit time

Question 7

Hydrogen gas is collected when a sample of magnesium reacted with an excess of dilute hydrochloric acid.The rate of this reaction can be studied by measuring the time it takes for a given volume of hydrogen to be collected. In terms of collision theory explain why, at a fixed temperature, the rate of this reaction doubles when the concentration of the hydrochloric acid doubles.

Answer 7

Twice / double as many particles / hydrogen ions (in a given volume) causes twice / double as many effective / successful collisions (in a given time)

Question 8

State the meaning of the term activation energy.

Answer 8

The activation energy is the minimum energy for a reaction to start

Question 9

A study of equilibrium is important for understanding chemical reactions.State Le Chatelier’s principle.

Answer 9

(When a system / reaction in equilibrium is disturbed), the (position of) equilibrium shifts / moves in a direction which tends to reduce the disturbance

Question 10

State the meaning of the term catalyst. Explain, in general terms, how catalysts work.

Answer 10

A substance that speeds up the reaction / alters the rate but is chemically unchanged at the end / not used upCatalysts provide an alternative route / alternative pathway / different mechanismthat has a lower activation energy / Ea ORlowers the activation energy / Ea

Question 11

State the effect, if any, of a catalyst on the time taken to reach equilibrium.

Answer 11

(Time is) less / shorter / decreases / reduces

Question 12

State the effect, if any, of a catalyst on the position of an equilibrium.

Answer 12

None

Question 13

Ammonia is manufactured by the Haber process in which the following equilibrium isestablished.N2(g) + 3H2(g) 2NH3(g)Give two features of a reaction at equilibrium.

Answer 13

Concentrations (of reactants and products) remain or stayconstant / the same Forward rate = Reverse / backward rate

Question 14

Explain why a catalyst has no effect on the position of an equilibrium.

Answer 14

Catalysts increase rate of / speed up both forward andreverse / backward reactionsincrease in rate / affect on rate / speed is equal / the same

Question 15

A sample of nitrogen dioxide gas (NO2) was prepared by the reaction of copper with concentrated nitric acid.Balance the equation for the reaction of copper with concentrated nitric acid. Cu + …….. HNO3 ———->Cu(NO3)2 + …….. NO2 + …….. H2O

Answer 15

Cu + 4HNO3 ———->Cu(NO3)2 + 2NO2 + 2H2O

Question 16

Give the oxidation state of nitrogen in each of the following compounds. HNO3NO2

Answer 16

HNO3 (+) 5NO2 (+) 4

Question 17

Deduce the half-equation for the conversion of HNO3 into NO2 in this reaction.

Answer 17

HNO3 + H+ + e- ———>NO2 + H2O

Question 18

The following equilibrium is established between colourless dinitrogen tetraoxide gas (N2O4) and dark brown nitrogen dioxide gas.N2O4(g)  2NO2(g) ΔH = + 58 kJ mol–1 Give two features of a reaction at equilibrium

Answer 18

*Concentration(s) (of reactants and products) remain(s) constant *Forward rate = Reverse / backward rate

Question 19

Use Le Chatelier’s principle to explain why the mixture of gases becomes darker in colour when the mixture is heated at constant pressure.

Answer 19

1.The (forward) reaction / to the right is endothermic or takes in i.e. absorbs heatORThe reverse reaction / to the left is exothermic or gives out / releases heat2.The equilibrium shifts / moves left to right to oppose the increase in temperature

Question 20

Use Le Chatelier’s principle to explain why the amount of NO2 decreases when the pressure is increased at constant temperature.

Answer 20

1.There are fewer moles (of gas) on the left OR more moles (of gas) on the right.OR there is one mole (of gas) on the left and 2 moles on the right.2. The equilibrium shifts / moves right to left to oppose the increase in pressure

Question 21

An equilibrium is established between oxygen and ozone molecules as shown below. 3O2(g) 2O3(g) ΔH = +284 kJ mol–1State Le Chatelier’s principleUse Le Chatelier’s principle to explain how an increase in temperature causes an increase in the equilibrium yield of ozone (2)

Answer 21

(When a system / reaction in equilibrium is disturbed), the equilibrium shifts/moves in a direction which tends to reduce the disturbancethe (forward) reaction / to the right is endothermic or takes in heatorthe reverse reaction / to the left is exothermic or gives out heatANDThe equilibrium moves / shifts to oppose the increase in temperature

Question 22

State and explain the effect of an increase in total pressure on the rate of attainment of this equilibrium (3)

Answer 22

Increase / speed up / faster (rate of attainment of equilibrium)More particles / molecules in a given volume / space OR the particles / molecules are closer togetherOR an increase in concentration.*More / higher chance of successful / effective / productive collisions (between particles)OR more collisions / higher chance of collisions (of particles) with E>EAct

Question 23

Hydrogen reacts with nitrogen in the Haber Process. The equation for the equilibrium that is established is shown below.N2(g) + 3H2(g)  2NH3(g)Use Le Chatelier’s principle to explain why an increase in the total pressure of this equilibrium results in an increase in the equilibrium yield of ammonia.

Answer 23

(If any factor is changed which affects an equilibrium), the position of equilibrium will shift / move / change/ respond / act so as to oppose the change.