How bicarbonate is formed and its properties

How bicarbonate is formed is a question we often get asked. Bicarbonate is a salt of carbonic acid, which in turn is a weak acid formed when carbon dioxide dissolves in water. It forms part of the dissolved compounds that makes up our oceans and lakes, and is also found inside the human body. In order for bicarbonate to form, we need to combine two other components ;

1. an acidic compound (e.g. hydrochloric acid) and an alkaline compound (e.g. sodium hydroxide).

This reaction is balanced against the dissociation of carbonic acid. It won't proceed unless the reaction is allowed to be as slow as possible, or until equilibrium is reached.

When the acid and alkali are mixed, two things happen; the first being that carbonic acid is formed, and carbon dioxide gas is released. This in turn causes the oceans and lakes to become slightly more acidic. The second effect of this reaction is that sodium bicarbonate (the salt) forms in solution. This can be seen as follows :

HCl + NaOH => H₂CO₃ => CO₂ + NaCl(aq)

The salt of the opposite acids ionise in opposite ways; first we have hydrochloric acid, which produces a hydrogen cation (positively charged hydrogen atom) and an anion (negatively charged chloride ion). Then, the sodium hydroxide is produced, which forms a cation (positively charged sodium ion) and an anion (negatively charged hydroxide ion).

The fact that bicarbonate is formed from the reaction of acid and alkali is important in medicine. The body utilises bicarbonate as a buffer solution to prevent its pH from changing too quickly. For example, it may happen that someone suffers stomach pains and has a poor appetite. This can be caused by either acid or alkali poisoning. Often it's hard to tell which one it was, and treatment will depend on the exact condition this person needs help with. This explains how bicarbonate is formed.

2. a base (e.g. hydroxide ion).

This will take part in the following reaction;

NaOH + C₆H₁₂O₆ => 2NaC₆H₃O₆ => C₆H₁₂O₆^- + H₂O + O₂

The water produced in this process reacts with the other components to form carbon dioxide and sodium bicarbonate. From this we can see that the main factors that contribute to bicarbonate being formed are the dissociation of carbonic acid and its subsequent re-dissociation into molecules of carbon dioxide and sodium bicarbonate.

Properties of bicarbonate

1. bicarbonate is slightly soluble in water.

This is because bicarbonate is almost completely made up of water. If we lose the water component in a compound then a substance that's more soluble will form (e.g. sodium carbonate would form instead of bicarbonate). In terms of solubility the formula for bicarbonate is : HCO₃^-

2. neutralisation takes place when mixed with sodium hydroxide.

This is because bicarbonate easily forms a hydrogen ion and an oxygen ion when mixed with base, which has been shown in previous chemistry notes such as this one on how alkalis behave .This is the same reaction that occurs when limestone is dissolved in water, which forms calcium hydroxide, another base. The difference with bicarbonate is that it's not quite as strong a base as sodium hydroxide.

3. bicarbonate is produced when carbon dioxide reacts with water. Bicarbonate ions are produced and these are amphoteric, which means they can react with an acid or base to form salts, such as sodium bicarbonate (Table salt).

4. only a small amount of carbon dioxide is needed to produce bicarbonate from carbonic acid. What is more, even though carbon dioxide can react with water to produce bicarbonate, carbon dioxide also causes acidity to increase in the atmosphere. Some people are concerned about this so we'll see how bicarbonate compares to other substances and how it fits into the world of chemistry. In particular we'll look at how it fits into the acidity balance of the oceans and lakes.

5. bicarbonate reacts with acids.

This is because its atoms have similar orientations; they have a negative charge on one end, which makes them easier to attract towards an acidic compound. This allows them to react in a way that assists life.

6. the reaction between bicarbonate and acids isn't all plain sailing.

Although bicarbonate does undergo a reaction with acid, it actually favours a particular set of circumstances for this to occur : it needs neutralisation to occur slowly (as if there were two lumps of sugar dissolved in hot water), so that it can react with a strong acid to produce carbon dioxide, as well as a weaker inorganic acid. This is because the reaction between the hydrogen ions and the hydroxide ions creates carbonic acid from carbon dioxide. This means that not all carbon dioxide is converted into bicarbonate; some remains as carbon dioxide gas, which can be seen in exhaled air.

In conclusions, how bicarbonate is formed can be seen to be of great importance in the everyday living of all living things. Although it doesn't exactly fit into our natural environment, we know that the reaction between acids and bases is important in our world. It's also important to remember how bicarbonate can be formed from carbon dioxide reacting with water, and how this reaction allows us to understand a little more about the chemistry of our world.