Topic 4: Acid-Base reactions and the alkaline-earth elements

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I’m going to begin with the characteristics of Acids. These characteristics are usually only available when there is water present (i.e. the acid is aqueous). Here are general characteristics:

 

·        A pH lower than 7

 

·        Reacts with metals to form a salt and hydrogen.

 

·        Neutralisation reaction when reacted with metal oxide or hydroxide to produce salt and water

 

·        Reacts with carbonate to form salt, water and carbon dioxide gas.

 

But how can we prove that these characteristics are absent from acids with the absence of water?

 

By dissolving an acid (e.g. HCl) in a hydrocarbon solvent, we can eliminate use of water in reactions.

 

1)     Addition of a metal to the dry acid result in no reaction. Litmus paper (NOT DAMP) gives no colour change.

 

2)     Addition of base, still no reaction

 

Now, these two reactions prove that characteristics of acids are only available with presence of water.. BUT THERE IS AN EXCEPTION, and this exception comes after I discuss the Arrhenius theory of Acids and bases.

 

-Arrhenius Theory

 

Arrhenius theory states that:

 

-An acid is a compound containing hydrogen which will form hydrogen ions in water.

 

-A base is compound that will form hydroxide ions in water.

 

The problem with this theory is that it is restricted to acids and bases IN WATER. The problem is, as I mentioned above, there is an exception, and the following acid-base reaction does occur without water

 

NH3 (g) + HCl (g) --> NH4Cl (s)

 

This is still an acid-base reaction, but without the presence of water This led to the introduction of the Bronsted-Lowry theory.

 

-Bronsted-Lowry Theory

 

This theory defines acids and bases as follows:-

 

-          An acid is a proton donor.

 

-          A base is a proton acceptor.

 

What we mean by proton is the hydrogen ion. Since it only composes of a hydrogen nucleus, which in effect has no neutrons, thus, is only a proton.

 

When an acid dissolves in water, it dissociates to form ions (H+ and Cl- for example). The acid donates its ‘proton’ to a water molecule to form an oxonium ion (H3O+) . Thus, since the water molecule has accepted a proton, it is a base.

 

HCl + H20 --> Cl- + H3O+

 

In another scenario, water can act as an acid whilst reacting with a base:

 

NH3 + H20 --> NH4+ + OH-

 

It donates a proton to ammonia to form hydroxide ions.

 

A strong acid is one that completely ionises in water and donates all its protons to form oxonium ions. A weak acid is one that partially dissociates into ions and donates very little of its protons to form oxonium ions.

 

A strong Base is one that completely ionises in water and accepts protons from it to form hydroxide ions. A weak base is one that partially dissociates in water to accept little protons from water to form hydroxide ions.

 

It is the magnitude of Hydrogen ions that defines the pH of a solution.

 

Group 2: Alkaline Earth Metals

 

The following physical trends occur down the group:

 

-          Melting & Boiling points increase

 

-          Density decreases

 

-          Atomic & ionic radii increase

 

-          Reactivity increases

 

-          Readiness to form hydrated salts decreases

 

-          All their chlorides are very soluble, but solubility decreases.

 

-          All sulphates are slightly soluble, but solubility decreases

 

-          All hydroxides are soluble, and solubility increases.

 

-          All carbonates are barely soluble, and solubility decreases.

 

 

But why?

 

-          The increase in overall charge causes more attraction in a giant metallic structure, which results in higher melting and boiling points.

 

-          The increase on Molar volume (Increase in atomic volume) means density becomes less. (Less mass occupying same volume)

 

-          Going down the group, we are adding shells on the atom, increasing its atomic radius.

 

-          Outer electrons become further from the nucleus as you go down the group. They are also shielded by inner electrons. This means there is a lower Em so reactions happen easier.

 

-          Lower Em means lower hydration enthalpy (You don’t need to know this)

 

-          The last 4 are important precipitate-wise (if that makes any sense). When u try and dissolve a group 2 hydroxide, it may appear cloudy because not all has dissolved. But with the addition of HCl, it becomes colourless. This is because the chloride formed is very soluble. If I used sulphuric acid instead, that wouldn’t be the case since sulphates aren’t very soluble and we are left with a white precipitate. The same goes for carbonic acid.