Category: Pure Chemistry

What are the advantages and disadvantages of the hydrogen fuel cell?

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Advantages:

1. The hydrogen fuel cell produces water as the only waste products. No harmful gases are released to surroundings when it is used.

 2. Hydrogen is an abundant resource, if a cost effective method can be used to extract hydrogen from water.

Disadvantages:

1. There is currently no cost effective method of storing the hydrogen gas.

2. There is currently no cost effective method of extracting hydrogen from seawater.

3. The process to obtain hydrogen, either from electrolysis of water, or from cracking of hydrocarbons, produces waste gases and use up large amount of energy.

Explain why two hydrogen atoms and one oxygen atom are involved in a covalent bond to form a H2O molecule.

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Each oxygen atom needs to share two electrons to achieve a stable electronic structure of a noble gas.

However, one hydrogen atom can only share one electron to achieve its own stable electronic structure.

Therefore, one oxygen atom will share 2 valence electrons with 2 hydrogen atoms to form the H2O molecule.

Explain, using structure and bonding, why graphite can conduct electricity.

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Graphite has a giant molecular structure. It is made up of layers of carbon atoms covalently bonded. Each layer is made up of carbon atoms covalently bonded to one another in hexagonal rings. Each carbon atom is covalently bonded to 3 other carbon atoms, hence each carbon atom has 1 valence electron not involved in bonding. These electrons are delocalised, and can move freely within the layer to conduct electricity.

Explain, using structure and bonding, why carbon dioxide has low melting point and boiling point.

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Carbon dioxide is a simple covalent compound, consisting of discrete molecules. The molecules are held together by weak intermolecular forces of attraction. Very little energy is needed to break these weak bonds, hence carbon dioxide has low melting point and boiling point.

Describe a method of preparing silver chloride from the reaction of silver nitrate and dilute hydrochloric acid.

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The silver chloride salt can be prepared using the precipitation method.

Add equal volumes of dilute hydrochloric acid to a beaker containing aqueous silver nitrate.

Stir the solution. A white precipitate of silver chloride is formed.

Filter the mixture to obtain silver chloride as the residue. Wash the residue with cold distilled water and dry between two pieces of filter paper.

Describe a method of preparing copper(II) sulfate from the reaction of copper(II) oxide and dilute sulfuric acid.

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The copper(II) sulfate salt can be prepared using the filtration method.

Copper(II) oxide is added in excess to a fixed volume of dilute sulfuric acid.

Filter the mixture to remove the excess copper(II) oxide from the copper(II) sulfate solution.

The filtrate of copper(II) sulfate is then heated until saturated.

The saturated solution is allowed to cool so that crystals of copper(II) sulfate can be formed.

Filter the mixture to remove the excess water.

Wash the crystals with a little bit of cold distilled water and dry between two pieces of filter paper.

Explain why calcium oxide has a higher melting point than potassium fluoride.

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The charges of Ca2+ ions and O2- ions in CaO is higher than that of K+ ions and F– ions in KF. The higher the ionic charge, the higher the electrostatic force of attraction between the oppositely charged ions, so more energy is required to overcome the electrostatic force in CaO than in KF.

Hence, CaO would have a higher melting point than KF.

List some examples of reducing agents and oxidizing agents

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Reducing agents:

carbon (C)

carbon monoxide (CO)

hydrogen (H2)

hydrogen sulfide (H2S)

metals

potassium iodide (KI)

sulfur dioxide (SO2)

ammonia (NH3)

hydrogen peroxide (H2O2)
(can act as oxidizing agent as well)

Oxidizing agents:

fluorine (F2)

chlorine (Cl2)

bromine (Br2)

iodine (I2)

concentrated sulfuric acid (H2SO4)

nitric acid (HNO3)

oxygen (O2)

potassium manganate(VII) (KMnO4)

potassium dichromate(VI) (K2Cr2O7)

hydrogen peroxide (H2O2)
(can act as reducing agent as well. It is stronger as an oxidizing agent, especially in acidic solutions)