Category: Pure Chemistry

What is an ionic equation? How do you write these equations?

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An ionic equation is a balanced equation in which the substances are expressed as dissociated ions in aqueous solution. Always make sure the type of elements and the number of each type of elements are balanced on both sides of the equation. Equally important, but quite often left unchecked by students, is to make sure the net charge of the ions is balanced on both sides of the equation.

You can start off an ionic equation by first writing down the full chemical equation, or if it is a simple and common reaction, you can write down the ionic equation directly. (For simplicity, we assume an ionic equation means a net ionic equation, with the spectator ions omitted)

There are only 3 general rules to follow:

– If the substance is an ionic compound in the aqueous state, split it into its ions.

– If the substance is a strong alkali or a strong acid, split it into its ions.

– If it does not fall in categories 1 or 2, do not change anything. Leave it as it is.

Examples:

Precipitation Reaction:

AgNO3 (aq) + NaCl (aq) –> AgCl (s) + NaNO3 (aq)

Splitting into ions: Ag+ + NO3– + Na+ + Cl– –> AgCl + Na+ + NO3–

Cancel away the spectator ions: Ag+ + NO3– + Na+ + Cl– –> AgCl + Na+ + NO3–

Final ionic equation: Ag+ (aq) + Cl– (aq) –> AgCl (s)

Displacement Reaction:

Ca (s) + Cu(NO3)2 (aq) –> Ca(NO3)2 (aq) + Cu (s)

Splitting into ions: Ca + Cu2+ + 2NO3– –> Ca2+ + 2NO3– + Cu

Cancel away the spectator ions: Ca + Cu2+ + 2NO3– –> Ca2+ + 2NO3– + Cu

Final ionic equation: Ca (s) + Cu2+ (aq) –> Ca2+ (aq) + Cu (s)

Neutralisation Reaction:

2NaOH (aq) + H2SO4 (aq) –> Na2SO4 (aq) + 2H2O (l)

Splitting into ions: 2Na+ + 2OH– + 2H+ + SO42- –> 2Na+ + SO42- + 2H2O

Cancel away the spectator ions: 2Na+ + 2OH– + 2H+ + SO42- –> 2Na+ + SO42- + 2H2O

Final ionic equation, in simplest ratio: OH– (aq) + H+ (aq) –> H2O (l)

Describe the general trend in the melting points of the elements in Period 3. Explain the trends in the melting points, in terms of bonding and structure of the elements.

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Melting points of the elements in Period 3

From Na to Si, the melting points increase, with a sharp increase from Na to Mg, and Al to Si, reaching a maximum for Si.

From Si to P, there is a sharp decrease in melting point, followed by a small increase in melting point from P to S, and a decrease in melting point from S to Ar.

Na, Mg, Al all have metallic structure. Moving from Na to Mg to Al, the number of protons and electrons increase, so there is an increasing electrostatic force of attraction between the positive ions and delocalised electrons, pulling the electrons closer to the nucleus, making the metallic bonding stronger. An increasing amount of energy is needed to break the increasing strength of the metallic bonds during melting. This results in the increasing trend of melting points from Na to Al.

Silicon exists as a giant molecular structure. A lot more energy is needed to break the large number of covalent bonds in the giant molecular structure, resulting in Silicon’s very high melting point.

Phosphorus, sulfur, chlorine and argon are non-metals which have simple covalent structures, with weak intermolecular forces of attraction. These forces require less energy to break during melting, resulting in their relatively lower melting points.

Phosphorus exists as P4 molecules, sulfur exists as S8 molecules, chlorine exists as Cl2 molecules and argon exists as individual atoms. The strength of the intermolecular forces of attraction decreases as the size of the molecule decreases, so melting points decrease from S to P to Cl to Ar. This explains the slight increase in melting point from phosphorus to sulfur, as sulfur molecules are bigger in mass than phosphorus molecules.

Explain why, going down Group VII, the reactivity of the element decreases.

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Going down Group VII, the number of electronic shells increases, and atomic radius increases. As the valence electrons get further from the nucleus, the attractive force between the positive protons in the nucleus and the negative valence electrons decreases.

Hence, going down Group VII, it becomes more difficult for the atom to gain a valence electron to form an ion, causing reactivity to decrease.

Burning biodiesel as a fuel is considered to be a “carbon neutral” activity. Explain what is meant by “carbon neutral” in this context.

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The source of biodiesel is from plant parts. During the growth and development of these plants, they absorb carbon dioxide from the environment for photosynthesis. During the combustion of biodiesel, the same amount of carbon dioxide is released to the environment.

The net amount of carbon added to the environment from the burning of biodiesel is considered to be zero, hence “carbon neutral”.

Explain, using structure and bonding, why alloys are stronger and harder than pure metals.

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Pure metals consist of a lattice of positive ions, arranged in regular pattern, surrounded by a sea of delocalised electrons. When a force is applied, the layers of positive ions slide over one another easily, without disrupting the metallic bonding. This makes the pure metal soft and malleable.

An alloy is a mixture of a pure metal with other elements. The atoms of the added element have a different size from those of the pure metal. This breaks up the regular arrangement of atoms in the pure metal. When a force is applied, the atoms of different sizes cannot slide over one another easily. This makes the alloy stronger and harder.

With reference to the graph, explain the progress of the reaction.

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At the start, the gradient of the curve is the steepest. This shows that speed of reaction is the highest at the start. The concentration of the reacting particles is the highest at the start, resulting in highest speed of reaction.

As the reaction progresses, the gradient of the curve decreases. This shows that speed of reaction is slowing down. As the reactants are used up in the reaction, the concentration of reacting particles decreases, resulting in a decrease in speed of reaction.

When the gradient of the curve becomes horizontal, it shows that the reaction has stopped. Either one or all of the reactants have been completely used up, so reaction stopped.

What are the harmful effects of acid rain?

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Acid rain speeds up the corrosion of buildings and structures made of limestone or reactive metals, like steel.

Acid rain makes water too acidic, which can be harmful to aquatic organisms.

Acid rain removes essential metal ions needed by plants, such as calcium, potassium, magnesium ions from the soil, affecting the health of plants.

Explain why it is not advisable for farmers to add slaked lime and ammonium-based fertilizers to their crop soil at the same time.

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When both substances are added at the same time, they will react to produce a salt, ammonia gas and water.

Slaked lime will be used up in the reaction, hence the acidity of the soil will not be reduced.

Fertilizers will be used up in the reaction. Ammonia gas produced will escape to the surroundings, so nitrogen element, which is needed by the plants for healthy growth, will not be absorbed by the plants.