Acid-base equilibria
Read our general notes on Risk Assessment
Why do weak acids/bases turn into their conjugate bases/acids? I don't really understand what makes them do this. I understand what happens as this affects the amount of acid/alkali needed to titrate a solution.
Igloo replies ...
Acids turn into their conjugate bases when they give away protons.
Acids cannot be converted into their conjugate bases unless they meet up with a proton acceptor, i.e. a base. In other words, this process cannot occur in isolation.
Take, for example, ethanoic acid. Pure (concentrated) ethanoic acid molecules cannot form ethanoate ions by themselves. If, however, a proton acceptor, such as water or ammonia, is present, then proton transfer between these two molecules can take place and an equilibrium is set up:
equation 1: CH3COOH + H2O <=> CH3COO- + H3O+
Likewise, bases cannot behave as such unless they meet up with a “willing” proton donor. For example, ammonia (NH3) only behaves a base when it meets up with an acid.
equation 2: NH3 + H2O <=> NH4+ + OH-
Notice that in the two cases above, the water molecule has behaved as a base (equation 1) and as an acid (equation 2). The water molecule can either give or receive protons, depending on the circumstances.
In a titration it is important to emphasise that the number of moles of base/acid needed to neutralize one mole of another acid/base does not depend on whether it is strong or weak. It depends only on the total number of protons transferred between the species.
To give an example:
1 mole of sodium hydroxide (strong base) and 1 mole of ammonia (weak base) are each neutralized by 1 mole of hydrochloric acid (strong acid) or 1 mole of ethanoic acid (weak acid).
1 mole of nitric acid (strong acid) and 1 mole of ethanoic acid (weak acid) are each neutralized by 1 mole of sodium hydroxide (strong base) or 1 mole of ammonia (weak base).
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 23 February 2004
