CHEMISTRY STPM : SEMESTER 2 (U2 2017)

The following equations represent both standard enthalpies of combustion and formation

except

• C(s) + O₂(g) -------> CO₂(g)

• H₂(g) + 1/2O₂(g) ------------> H₂O(l)

• S(s) + O₂(g) ----> SO₂(g)

• Mg(s) + 1/2O₂(g) -------> MgO(s)

Answer explanation

The standard enthalpy of combustion is the enthalpy change when one mole of a substance is burned in excess oxygen under standard conditions.

The standard enthalpy of formation is when one mole of a substance is formed from its element under standard conditions.

Under standard conditions, sulphur exist as S_8 molecules and not S atoms. The correct equation should be:

1/8 S₈ (s) + O₂(g) ---> SO₂(g)

Tags

Chemical Energetic

Standard enthalpies of formation of SO₂, CO₂, and CS₂ are -298 kJ mol^-1, -395 kJ moI^-1 and +119 kJ mol^-1 respectively. What can be deduced from the thermochemical data?

• Standard enthalpy of combustion of SO₂ is -298 kJ mol^-1.

• Standard enthalpy of combustion of CS₂ is -812 kJ mol^-1

• CO₂ is more stable than CS₂ under standard conditions.

• Combustion of sulphur is more exothermic than that of carbon under standard conditions.

Answer explanation

Enthalpy change of formation for SO₂ is the same as the enthalpy change of combustion of S (s) and not SO₂ (A wrong).

Standard enthalpy change of combustion for CS₂ (s) + 3 O₂ (g) --> CO₂ (g) + 2 SO2 (g) ; ΔHc = [(-298 x 2) + (-395)] - (+119) = -1110 kJ mol^-1 (B wrong).

ΔH_f CO₂ = -395 kJ mol^-1 is more exothermic than ΔH_f CS₂ = +119 kJ mol^-1 , hence CO₂ is more stable.

ΔHc of S (-298 kJ mol^-1) = ΔH_f of S is less exothermic than ΔHc of C (-395 kJ mol^-1) = ΔH_f of C

Tags

Chemi

The standard reduction potentials of several species are shown below.

Cr₂O₇^2- (aq) + 14H⁺(aq) + 6e --->2Cr³⁺(aq) + 2H₂O(l) E° = +1.33 V

Cu²⁺(aq) + 2e ---> Cu(s) E° = +0.34 V

Mg²⁺(aq) + 2e ---> Mg(s) E° = -2.37 V

Br₂(l) + 2e --> 2Br^- (aq) E° = +1.07 V

Which statement is true based on the data above?

• Mg(s) is a stronger reducing agent than Cu(s).

• Cu(s) acts as the anode in Cu/Mg electrochemical cell.

• Cu²⁺(aq) ion reacts with Cr³⁺(aq) ion to form an orange solution.

• NaBr reacts spontaneously with acidifled Cr₂O₇^2- (aq), Cu²⁺(aq) and Mg²⁺ (aq) ions.

Answer explanation

E° value of Mg²⁺/Mg is more negative than Cu²⁺/Cu, therefore Mg is stronger reducing agent than Cu (A correct)

Hence Mg act as anode of the cell (B wrong)

E°_cell when Cu²⁺ react with Cr³⁺ is negative

+0.34 + (–1.33V) = -0.99V (C wrong) therefore reaction is non-spontaneous ;

Br^- react spontaneously with only

Cr₂O₇^2- but not Cu²⁺ and Mg²⁺ (D wrong)

Tags

Electrochemistry

The cell diagram of an electrochemical cell is shown below.

Pb(s)|Pb²⁺(0.10 mol dm^-3)||H⁺ (0.050 mol dm^-3)|H₂(1 atm)|Pt(s)

The standard reduction potential of lead is -0.13 V. What is the e.m.f. of the cell at 298 K?

• 0.024 V

• 0.083 V

• 0.12 V

• 0.18 V

A mass of 2.0 g of silver is deposited when a quantitv of electric current passes through an

aqueous solution of silver nitrate. In the anodisation of an aluminium object, aluminium is oxidised

to aluminium(Ill) oxide, Al₂O₃. What is the mass of Al₂O₃ formed if the same quantity of electricity

is used in the process?

• 0.31 g

• 0.47 g

• 0.63 g

• 0.93 g

Answer explanation

Mol of Ag = 2.0 / 108 = 0.01852 mol ;

Ag⁺ + e^- --> Ag ;

since 1 mol Ag = 1 mol e^- ;

mol of e- = 0.01852 mol ;

In Al₂O₃, 6e- is required to form 1 mol of Al₂O₃ ; mol of Al₂O₃ = 0.01852 /6 = 0.003086 mol ;

mass of Al₂O₃

= 0.003086 x 102 = 0.31g

Sulphur and phosphorus are simple molecules. Which statement is true about both molecules?

• Phosphorus has a larger bond angle compared to sulphur.

• Sulphur has a lower melting point compared to phosphorus

• Phosphorus has weaker covalent bonds compared to sulphur.

• Sulphur has stronger van der Waals forces compared to phosphorus.

Answer explanation

Since sulphur and phosphorus exist naturally as S₈ and P₄ ; therefore molecular size of sulphur is greater than

that of phosphorous, therefore a stronger Van Der Waals’ forces

Which oxide is not hydrolysed in an aqueous solution?

• Na₂O₂

• Al₂O₃

• SO₂

• Cl₂O₇

Answer explanation

MgO will hydrolysed with steam to form Mg(OH)₂ ; while SO₂ forms H₂SO₃ with water, and Cl₂O₇ forms HClO₄ in water. Al₂O₃ is insoluble in water hence does not hydrolysed.

Element M has the following propeties.

* Reacts vigorously with water

* Forms peroxide, MO₂, when heated in excess oxygen

* Carbonate of M decomposes upon heating at high temperature

Based on the information above, the valence electronic configuration of element M most probably is

• 2s²

• 3s²

• 4s²

• 5s²

Answer explanation

Since metal M react vigorously with water, and form metal peroxide when burnt, with high thermal stability,

hence M comes from Group 2 at lower position in Periodic Table

Which statement is true about beryllium or its compounds?

• Beryllium chloride is ionic.

• Beryllium reacts with water.

• Beryllium oxide dissolves in alkali.

• Beryllium sulphate is insoluble in water.

Answer explanation

BeCl₂ is a covalent compound (A wrong) ; Be does not dissolve in water due to formation of oxide layer (B wrong) ; Since BeO is amphoteric oxide, hence it can dissolve in both acid and alkali (C correct) ; BeSO₄ is soluble in water due to small Be²⁺

Which is a common property for germanium and lead compounds?

• Stability of +2 oxidation state of both compounds is due to the inert pair effect.

• Tetrachlorides of both compounds are thermally stable.

• Dioxides of both compounds are amphoteric.

• Monoxides of both compounds are basic.

Answer explanation

Ge is more stable as +4 while Pb is more stable as +2 due to inert pair effect (A wrong)

GeCl₄ is thermally stable but PbCl₄ decomposed even at room temperature (B wrong)

Both monoxide & dioxide of Ge and Pb are

amphoteric (C right) (D wrong)