Sunday, September 13, 2009

Chemistry - Sec 3 Chapter 11 - Acids and Bases (Part 1)

The chemistry question are from the Chemistry Workbook "Worksheet 7 for Chapter 11", page 62.

1. An element, X, burns in oxygen to form an oxide that is soluble in water. The solution of this oxide has a pH greater than 7. What is X?
A Carbon
B Copper
C Sodium
D Sulphur

Solution: C, Sodium
When burned in air, sodium forms sodium peroxide Na2O2(s), or with limited oxygen, the oxide Na2O(s).
If burned in oxygen under pressure, sodium superoxide NaO2(s) will be produced.

Sodium oxide, Na2O(s), dissolves in water to form Sodium Hydroxide, NaOH (aq),

Sodium oxide + water → Sodium hydroxide
Na2O(s) + H2O(l) → 2NaOH(aq).

Sodium Hydroxide, NaOH (aq), has a pH value of 14
(From CHEMISTRY Matters, Page 180, Fig 11.11, pH values of common substances).


Sodium in metal form
Picture is obtained from http://upload.wikimedia.org/wikipedia/commons/c/ca/Nametal.JPG.jpg



4. Methylbenzene is an organic solvent. Which statement is true about a solution formed by dissolving ammonia gas in methylbenzene?
A It contains NH ions.
B It does not conduct electricity.
C It has a pH value of less than 7.
D It turns Universal Indicator solution from green to yellow.

Solution: B, It does not conduct electricity. ?
Methylbenzene is also known as toluene is an organic solvent that react with ammonia gas to form organic compound which is not an electrolyte.
From http://dwb4.unl.edu/chemistry/smallscale/SS035c.html



Methylbenzene (known as toluene is flammable) with Silicon
Picture is obtained from http://www.cap.ca/cap/Gifs/CAP/2003c2n02.jpg




7. Which substance produces hydrogen ions when dissolved in water?
A Carbon monoxide
B Nitrogen
C Oxygen
D Sulphur dioxide

Solution: D, Sulphur dioxide
Sulphur dioxide dissolved in water to form sulphurous acid, H2SO3(aq)

sulphur dioxide + water → sulphurous acid
SO2(q) + H2O(l) → H2SO3(aq).
(from CHEMISTRY Matters, Page 364, Acid Rain).

Acid, for example, Sulphurous acid, H2SO3(aq), produces hydrogen ions, H+.
Carbon monoxide, Nitrogen and Oxygen does not form acid when dissolved in water.



The sulphur dioxide Mount
Picture is obtained from http://www.nature.com/nature/journal/v447/n7141/images/447132a-i2.0.jpg




8a. Ammonia gas can be produced by heating calcium hydroxide with ammonium chloride. Write the chemical equation for the reaction.

Heat the chemicals:
ammonium chloride + calcium hydroxide → ammonia gas + water + calcium chloride
2NH4Cl(s) + Ca(OH)2(aq) → 2NH3(g) + 2H2O(l) + CaCl2(aq)

Whenever an ammonium salt is heated with an alkali (for example, Sodium Hydroxide or Calcium Hydroxide), ammonia is displaced from its salt.
(from CHEMISTRY Matters, Page 354, Displacement of Ammonia from its Salts).



Ammonia gas leaked into the air from Nov. 12 to Nov. 14 after a 16-year-old boy drilled into an exposed portion of a 30-mile-long anhydrous pipeline that extends over the Alafia River. About 300 people were evacuated and schools were closed down while the pipe was repaired.
Picture is obtained from http://www.sptimes.com/2008/01/17/images/tb_Ammonia_450.jpg


8b. The ammonia produced in (8a) was used to produced a concentrated solution of a weak alkali. Explain the meaning of
i) Concentrated solution:
It is a solution that contains a large amount of solute relative to the amount that could dissolve.

ii) Weak alkali:
A weak alkali is an alkali that does not fully dissociates when dissolved in water.

(From CHEMISTRY Matters, Page 180, Concentration and Strength
or http://www.ktf-split.hr/glossary/en_o.php?def=concentrated%20solution).




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