Equilibrium & The Gaseous State

(12)1, 8

The gas-phase reaction of carbon monoxide with hydrogen forming methanol is an example of an equilibrium. The reaction was investigated by mixing 2.0mol of H2(g) with 1.0mol of CO(g) and allowing equilibrium to be established.

2 H 2 ( g )+CO (g)⇌C H3 OH (g)

Initial moles 2.0 1.0 0

At equilibrium, x mol of H2 had reacted with CO.

What are the amounts, in moles, of each of the components of the equilibrium mixture?

H2(g) CO(g) CH3OH(g)

A 2.0−12

x 1.0−x 12

x

B 2.0−12

x 1.0−2 x x

C 2.0−x 1.0−12

x 12

x

D 2.0−x 1.0−x 12

x

(12)1, 9

Pyruvic acid is a weak acid that is an important intermediate in many biological processes.

CH3COCO2HPyruvic acid

Which graph best represents the change in pH that occurs when a sample of pyruvic acid is titrated with NaOH(aq)?

(12)1, 10

Stomach juices have a pH of 1.0.

Aspirin is a monobasic (monoprotic) acid represented by HA (Ka = 1 x 10-4 moldm-3) which dissociated into ions H+ and A-.

What are the relative concentrations of H+, A- and HA when aspirin from a tablet enters the stomach?

A. [HA] > [H+] = [A-]B. [H+] = [A-] > [HA]C. [H+] > [A-] > [HA]D. [H+] > [HA] > [A-]

(12)1, 11

A 1dm3 solution was made by mixing 0.0040mol of HCl(aq) and 0.0025mol of NaOH(aq).

What was the pH of the resulting solution?

A. 2.19B. 2.40C. 2.60D. 2.82

(11)1, 4

A given mass of ideal gas occupies a volume V and exerts a pressure p at 27°C.

At which temperature will the same mass of the ideal gas occupy the same volume V and exert a pressure 2p?

A. 54°CB. 54KC. 600°CD. 600K

(11)1, 11

Soft drinks often have sodium citrate added to them to act as a buffer.

Which statement about buffer solutions is correct?

A. The pH of a buffer solution changes slightly when very large amounts of acid or base are added.

B. The pH of a buffer solution increases very slightly when small amounts of acid are added.

C. The pH of a buffer solution increases very slightly when small amounts of base are added.

D. The pH of a buffer solution remains unchanged when small amounts of acid or base are added.

(10), 1, 7

The value of pV is plotted against p for three gases, an ideal gas and two non-ideal gases, where p is the pressure and V is the volume of the gas.

Which gas shows the greatest deviation from ideality?

A. AmmoniaB. EtheneC. MethaneD. Nitrogen

(10)1, 13

At a total pressure of 1.0atm, dinitrogen tetraoxide is 50% dissociated at a temperature of 60°C, according to the following equation.

N 2 O4⇌2 N O2

What is the value of the equilibrium constant, Kp, for this reaction at 60°C?

A.13

atm

B.23

atm

C.43

atm

D. 2 atm

(10)1, 36

Two students separately have available equal volumes of 0.1moldm-3 silver nitrate, sodium ethanoate and potassium bromide.

The first student, on mixing the sodium ethanoate and silver nitrate, obtains a white precipitate. On adding potassium bromide to this mixture, the precipitate turns cream.

The second student adds the silver nitrate to the potassium bromide and obtains a cream precipitate. On adding the sodium ehtanoate to this mixture, there is no further change.

Which statements about these observations are correct?

1. Silver ethanoate is insoluble.2. Silver bromide is less soluble than silver ethanoate.3. Ethanoate can oxidise bromide.

(09)1, 7

Use of the Data Booklet is relevant to this question.

Which expression gives the pressure exerted by 1.6 x 10-3 mol of N2 in a container of volume 3.0dm3 at 273°C?

A. 1.6× 10−3 ×8.31 ×2733.0× 10−6 Pa

B.1.6× 10−3 ×8.31 ×(273+273)

3.0 ×1 0−6 Pa

C. 1.6× 10−3 ×8.31 ×2733.0 ×10−3 Pa

D.1.6× 1 0−3 ×8.31 ×(273+273)

3.0 ×1 0−3 Pa

(12)2, 3

Fifty years ago, students of A-level chemistry carried out ‘qualitative inorganic analysis’ in their practical sessions. This method included precipitating cations as chlorides, sulfides, etc., and used the concept of solubility product, Ksp.

a) [5]i. Write an expression for Ksp of PbCl2, stating the units.

ii. The solubility of PbCl2 is 4.7gdm-3.Calculate the solubility product of PbCl2.Give your answer to two significant figures.

When 2.0moldm-3 hydrochloric acid is added to separate samples of aqueous silver nitrate and aqueous lead(II) nitrate, AgCl and PbCl2 are precipitated.

b) [7]i. When the HCl(aq) is added to AgNO3(aq), what will be seen in the container

immediately, and after one hour in daylight?

ii. How would you confirm the original precipitate was AgCl by adding a single reagent to it?Describe what you would see and give an equation with state symbols for the reaction that occurs with the reagent you have chosen.

iii. When concentrated hydrochloric acid is added to the precipitate of PbCl2, the precipitate dissolves.Suggest, with an ionic equation, an explanation for this observation.

Selective precipitation of sulfides may be used to separate the cations present in a solution containing Cu2+, Ni2+ and Zn2+.

The reagent used is hydrogen sulfide which behaves as a dibasic (diprotic) weak acid when in aqueous solution.

H 2 S ( g )+aq⇌H+¿ (aq )+H S−¿(aq) ¿¿ Ka = 9.1 x 10-8 moldm-3

And H S−¿ ( aq )⇌H +¿ (aq ) +S2−¿(aq )¿¿ ¿ Ka = 1.0 x 10-15 moldm-3

Metal sulfides are precipitated by the following reaction.

M 2+¿ (aq )+S2−¿(aq)⟶MS (s )¿ ¿

Relevant Ksp values are given in the table.

salt Ksp /mol2dm-6

CuS 6.3 x 10-36

NiS 4.0 x 10-21

ZnS 1.6 x 10-24

The results of passing hydrogen sulfide through separate solutions of Cu2+, Ni2+, and Zn2+ are shown in the table below.

M2+ being tested H2S passed through an acidic solution of M2+

H2S passed through a neutral solution of M2+

H2S passed through an alkaline solution of

M2+

Cu2+ Black precipitate of CuS

Black precipitate of CuS

Black precipitate of CuS

Ni2+ No precipitate No precipitate Black precipitate of NiS

Zn2+ No precipitate White precipitate of ZnS

White precipitate of ZnS

d) Use the data given above to give an explanation for each of the following questions.You should give equations where appropriate. You are not expected to do any calculations in this part. [6]

i. Why is CuS the only sulfide of these three precipitated in acid solution?

ii. Why is NiS precipitated in alkaline solution and not in neutral or acidic solution?

iii. Why is ZnS precipitated in both neutral and alkaline solution?

(11)2, 2

Phosphorus(V) chloride, PCl5, is a white solid which sublimes at 160°C.

When gaseous phosphorus(V) chloride is heated in a closed container, the following equilibrium is established.

PC l5 ( g )⇌PC l3 (g )+C l2(g)

a) Write an expression for Kc for this equilibrium, stating the units. [2]

Phosphorus(V) chloride dissolves in some polar solvents without reaction and such solutions conduct electricity. This is due to the presence of the two ions [PCl4]+ and [PCl6]-.

b) Draw the structure and suggest the shape of each of these ions. [3]

(11)2, 3

This question refers to the equilibrium previously.

PC l5 ( g )⇌PC l3 (g )+C l2(g)

In an experiment, 1.00mol of PCl5 vapour was heated in a closed 5.00dm3 flask at 500K until equilibrium had been established.The gases in the flask were then rapidly cooled, causing the formation of crystals of PCl5 and liquid PCl3. The chlorine gas present was pumped away and collected.The amount of chlorine collected was x mol.An excess of water was then very carefully added to the two remaining compounds, under suitable conditions, causing the following reactions to occur.

PC l5+4 H 2 O⟶5 HCl+H3 P O4

PC l3+3 H 2O⟶3 HCl+H3 P O3

You may assume that both H3PO4 and H3PO3 behave as dibasic (diprotic) strong acids.

a) Determine, in terms of x, the number of moles of H+ that will be formed after the addition of water to the mixture of PCl3 and PCl5. [4]

The resulting solution was made up to 1.00dm3 in a standard flask. 25.0cm3 of this solution was titrated with 4.00moldm-3 NaOH and required 37.4cm3 for neutralisation.

b) [4]i. Calculate the total amount, in moles, of H+ ions present in the 1.00dm3

standard flask.

ii. Use your answers to 3(a) and 3(b)(i) to calculate the amounts, in moles, of PCl5, PCl3 and Cl2 in the original equilibrium at 500K.

c) Use your answers to question 2(a) and 3(b)(ii) to calculate a value for Kc for the equilibrium. [2]

(11)3, 3

Parts of this question are concerned with the salts of the Group II elements. You may find this following information to be useful.

Salts of Group II elements with divalent anions (i.e. those with a charge of 2-) tend to be insoluble.

These salts become more insoluble down the group. Salts of Group II elements with monovalent anions tend to be soluble. The hydrogencarbonates of Group II elements do not exist in the solid state. Carbon dioxide reacts with solutions of carbonate ions according to the following

equilibriumC O2 (g )+H2O ( l )+C O3

2−¿(aq )⇌2 HC O3−¿( aq)¿ ¿

You should use the above information and the ideas behind Le Chatelier’s principle and solubility product, Ksp, to explain your answers.

a) Mineral waters often contain the chlorides and hydrogencarbonates of Group II metals. Solutions of these salts are formed when rainwater percolates through the roacks of hills and mountains, and emerges as springs in the hillside. [9]

One such mineral water has the following composition.

Ion Concentration /gdm-3

Ca2+ 0.0080Mg2+ 0.0049Cl- 0.0071

HCO3- 0.036

i. Calculate the concentrations of these ions in moldm-3, and hence suggest the formulae of the salts that could exist in the solution, and their relative amounts.

When a sample of the mineral water was partially evaporated, a mixture of white solids was formed, and a solution containing a single salt remained.

ii. Suggest the identity of the first solid to be precipitated. Explain your answer.

iii. Suggest the composition of the rock through which the rainwater had percolated, and explain how the mineral water had been formed.

b) One of the most common phosphorus-containing rocks is fluorapatite, Ca5(PO4)3F. The first step in the production of phosphate as a mineral supplement for animal feed is defluorination, which is achieved by heating the rock in a rotary kiln at 1500°C with silica and steam. [5]

The following equation represents this process.6 C a5 ( P O4 )3 F+a Si O2+b H 2O →c Si F4+d HF+e CaO+ f C a3 ( P O4 )2

i. Complete the balancing of the above equation by deducing values for a, b, c, d, e and f.

ii. Write the expression for the solubility product of calcium phosphate, stating its units.

iii. The value of Ksp for calcium phosphate is 1 x 10-26. Calculate [Ca2+] in a saturated solution of calcium phosphate. Show your working clearly.

c) [6]i. Describe how separate samples of PCl3 and PCl5 can be made in the

laboratory.

When reacted with SO3, PCl3 produces a compound G with Mr = 153.5, and a byproduct H. G is also produced when PCl5 is reacted with a small amount of H2O.

ii. Suggest a formula for G, and describe its shape.

iii. Suggest the identity of the byproduct H and construct equations for the production of G from PCl3 and from PCl5.