Questions I Got Wrong

Question 1

What periodic groups can change their oxidation number?

Answer 1

• group 1 and group 2 metals can’t change oxidation states. - group 7 halides can change oxidation states

Question 2

How many moles are there in 2 grams of iodine?

Answer 2

2/126.9x2 = 0.015 Remember you divide it by the mass twice because it’s 2 atoms per molecule.

Question 3

What is the volume of one mole of gas?

Answer 3

1mol=24dm^3

Question 4

A chemist carries out a reaction between hydrogen and nitrogen monoxide at a higher pressure.

State what happens to the rate constant.

Answer 4

The rate constant does not change.

Question 5

Platin, Pt(NH₃)₂Cl_2, is a neutral complex.

Explain why platin is neutral.

Answer 5

Pt2+/Pt is +2/2+, 2 x Cl– –2

Hence that charges cancel out and the platin has no overall charge(hence neutral.)

Question 6

State one property of Fe²⁺, other than the ability to form complex ions, which is typical of an ion of a transition element.

Answer 6

• coloured (compound/complex/precipitate/ions) OR catalyst
• IGNORE ‘variable oxidation states’ …. but ALLOW the idea that Fe2+ can react to form an ion with a different charge/oxidation state

Question 7

Ammonium thioglycolate is used by hairdressers to perm hair.

Hair is a protein and its shape is largely the result of cross-linked disulfide bonds, -S-S-. The formula of the protein in hair can be represented as R-S-S-R.

Perming of hair involves two stages.

Stage 1:

• Hair is first wound around curlers and a solution of ammonium thioglycolate is applied to the ahir.
• In this process, each disulfide bond is broken by two thioglycolate ions to form two molecules containing thiol groups, -S-H, and one other product.

Stage 2:

• After 15-30 minutes, the hair is rinsed with a weak solution of hydrogen peroxide, H₂O₂.
• The hydrogen peroxide reforms disulfide bonds that lock the hair in the shape of the curlers. The hair is now ‘permed’.

Suggest equations for the two processes that take place during perming.

In your equations, use R-S-S-R to represent the protein in hair.

Ammonium thioflycolate = HSCH₂COONH₄

Answer 7

2HSCH₂COO^- + R-S-S-R → ^-OOCCH₂S-SCH₂COO^- + 2R-SH

2R-SH + H₂O₂ → R-S-S-R + 2H₂O

Question 8

Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for vehicles.

Answer 8

• methanol is a liquid AND methanol is easier to store/transport

Question 9

Many experimental measurements use published data, such as K_w, measured at 25*C. Often these measurements have been taken at different temperatures, especially in experimental work carried out at body temperature.

What is the consequence of this published scientific work?

Answer 9

(Work is) inaccurate OR invalid because K_w varies with temperature.

ALLOW incorrect with reason

IGNORE unreliable

Question 10

When dissolved in water, the enthalpy change of solution of the salt potassium fluoride, KF, is -15kJmol^-1.

The salt rubidium fluoride, RbF, has an enthalpy change of solution in water of -24kJmol^-1.

Suggest reasons for the difference between enthalpy changes of solution of KF and RbF.[4]

Answer 10

• Ionic radius Potassium ion OR K+ OR K ion is smaller OR K+ has greater charge density.
• Lattice enthalpy of KF is more negative than RbF(or ionic bonds are stronger.)
• H(hydration) of K+ is more negative than Rb+(or K+ has greater attraction for H2O)
• Idea that ∆H(solution) is affected more by lattice enthalpy than enthalpy change of hydration.

Question 11

The cold pack is made of two seperated compartments, one containing ammonium nitrate crystals, NH₄NO₃, and the other water. The pack is activated by breaking the barrier between the two compartments. The crystals dissolve spontaneously in the water causing the temperature of the pack to drop.

Explain why ammonium nitrate in the cold pack dissolves spontaneously in water even though this process is endothermic.

Answer 11

-During dissolving entropy/disorder increases.

TΠS > ΠH

OR T∆S is more positive than ∆H

• or ∆H – T∆S is negative

Question 12

Explain wether the change in entropy is positive or negative when water melts.

Answer 12

∆S: + AND more random/more disorder/more ways of arranging energy

Question 13

The electrolyte in NiCd cells is aqueous KOH. The standard electrode potentials for the redox systems that take place in NiCd cells are shown below.

Cd(OH)₂ + 2e^- ⇔ Cd + 2OH^- E= -0.80V

NiO(OH) + H₂O + e^- ⇔ Ni(OH)₂ + OH^- E = +0.45V

As the cell approaches full charge, the aqueous KOH electrolyte starts to decompose, forming hydrogen gas at one electrode and oxygen gas at the other electrode.

Answer 13

4OH^- ⎯→ O₂ + 2H₂O + 4e^-

2H₂O + 2e^- ⎯→ H₂ + 2OH^-

Question 14

Write the reaction between Cu and I^-.

Answer 14

2Cu₂+ + 4I^- ⎯→ 2CuI + I₂

Question 15

A student wanted to determine the lattice enthalpy of sodium carbonate, Na₂CO₃. Unfortunately this is very difficult to do using a similar Born-Haber cycle to that used for sodium oxide.

Answer 15

Cycle needs formation of CO₃^2- ions (from C and O).

or

carbonate ion contains 2 elements

Question 16

The student thought that he could determine the lattice enthalpy of Na₂CO₃ using a Born-Haber cycle that links lattice enthalpy with enthalpy change of solution. The enthalpy change of solution of Na₂CO₃ is exothermic.

Answer 16

Question 17

Aqueous cobalt(II) sulfate, CoSO₄(aq), reacts with aqueous sodium hydroxide.

What is the transition element species formed?

What is the type of reaction?

Answer 17

• transition element species formed:Co(OH)₂
• Type of reaction: Precipitation reaction

Question 18

Dinitrogen tetroxide, N₂O₄(g), and nitrogen dioxide, NO₂(g), coexist in the following equilibrium.

2NO₂(g) ⇔ N₂O₄(g) πH = -57KJ mol^-1.

A value of Kc is obtained from this equilibrium.

The experiment is repeated but the pressure in the container is doubled.

Explain in terms of Kc, the effect on the concentrations of NO₂ and N₂O₄ when the mixture has reached equilibrium.

Answer 18

Effect on Kc Kc does not change (with pressure)  Comparison of conc terms after increase in pressure [NO₂]² increases more than [N₂O₄] OR concentration (term) on bottom (of Kc) increases more that concentration (term) on top (of Kc)  Changes in concentrations linked to Kc (amount /concentration of) N₂O₄ increases AND (amount /concentration of) NO₂ decreases AND to maintain/restore Kc 

Question 19

Aqueous potassium manganate is not suitable for titrating against a solution of FeCl₂ but potassium dichromate is.

Explain why.

E^* = +0.77v for Fe³⁺/Fe²⁺

E* = +1.33V for Cr₂O₇^2-/Cr³⁺

E* = +1.51v

E* = +1.36V for Cl_2/Cl^-

Answer 19

E* for MnO4 – is more positive/greater than Cl₂ OR E* for Cr₂O_7²– is less positive/smaller than Cl₂  MnO4 – reacts with Cl– OR HCl (forming Cl₂ gas) OR Cr₂O₇²– does not react with Cl– ions .

Question 20

A chemistry carries out an investigation on the equilibrium system shown below.

2CO(g) + 2NO(g) ⇔ 2CO₂(g) + N₂(g) ΠH = -788

Explain why it is difficult to predict what would happen to the position of equilibrium after these changes in temperature and pressure?

Answer 20

Effect of T and P on equilibrium

(increased) temperature shifts equilibrium to left AND (increased) pressure shifts equilibrium to right AND fewer (gaseous) moles on right-hand side 

Overall effect on equilibrium Difficult to predict relative contributions of two opposing factors

Question 21

The student adds 50cm³ of 0.250 mol dm^-3 butanoic acid to 50cm³ of 0.0500mol dm^-3 sodium hydroxide. A buffer solution forms.

Explain why a buffer solution forms.

Answer 21

CH3(CH2)2COONa OR CH3(CH2)2COO– forms OR CH3(CH2)2COOH + OH^- → CH3(CH2)2COO^- + H2O

CH3(CH2)2COOH is in excess OR acid is in excess OR some acid remains .

Question 22

Define the term transition element.

Answer 22

(A transition element) has (at least) one ion with a partially filled d sub-shell/ d orbital.

Question 23

Draw the structure of ethanedioate.

Answer 23

• Remember negative charge on the oxygens!

Question 24

A student added some chromium metal to an acidified solution containing copper(II) ions. A reaction took place. The student concluded that “chromium is more reactive than copper” in this reaction.

Cu²⁺(aq) + 2e^- ⇔ Cu(s) E*=+0.34

Cr³⁺(aq) + 3e^- ⇔ Cr(s) E*=-0.74

Explain, in terms of their electrode potentials, why ‘chromium is more reactive than copper’ in this reaction.

Answer 24

E* for chromium (redox system) is more negative/lower/less (than copper redox system) ORA

chromium system shifts to the left / Cr(s) ⇒ Cr³⁺(aq) + 3e^-

AND copper system shifts to the right / Cu²⁺(aq) + 2e^- ⇒ Cu(s)

Question 25

Suggest two advantages of using methanoic acid as the fuel in a fuel cell rather than hydrogen.

Answer 25

1. Methanoic acid is a liquid AND easier to store/transport

OR hydrogen is a gas AND harder to store/transport

OR hydrogen as a liquid is stored under pressure

2. Hydrogen is explosive/more flammable
3. HCOOH gives a greater cell potential/voltage
4. HCOOH has more public/political acceptance than hydrogen as a fuel

Question 26

This question refers to chemistry of d-block elements in Period 4 (Sc-Zn).

Give two elements that have atoms with unpaired d electrons.

Answer 26

Titanium and nickel

Question 27

When should the vertical section in a pH curve occur?

Answer 27

When all the alkali has reacted with all the acid.

Question 28

Define the enthalpy change of hydration.

Answer 28

hydration: (enthalpy change for)

1 mole of gaseous ions OR 1 mole of hydrated/aqueous ions 

gaseous ions forming aqueous/hydrated ions 

Question 29

Define enthalpy change of solution

Answer 29

solution: (enthalpy change for)

1 mole of a compound/substance/solid/solute dissolving

Question 30

Ca(HSO₃)₂ can be made by reacting an excess of sulfur dioxide gas with suspension of calcium carbonate in water.

Write the equation for this reaction

Answer 30

CaCO₃ + 2SO₂ + H₂O → Ca(HSO₃)₂ + CO₂