The most important factor in determining the relative acid strengths of these molecules is the nature of the ions formed. The bonds won't be identically strong, because what's around them in the molecule isn't the same in each case. Note: You've got to be a bit careful about this. In these cases, you seem to be breaking the same oxygen-hydrogen bond each time, and so you might expect the strengths to be similar. Two of the factors which influence the ionisation of an acid are: if the same bond is being broken in each case, why do these three compounds have such widely different acid strengths?ĭifferences in acid strengths between carboxylic acids, phenols and alcohols Use the BACK button on your browser to return to this page later. It isn't, however, particularly important to the rest of the current page. Note: If you aren't sure about coordinate covalent (dative covalent) bonding, you might like to follow this link. In each case, the same bond gets broken - the bond between the hydrogen and oxygen in an -OH group.
Comparing the other two to ethanoic acid, you will see that phenol is very much weaker with a pK a of 10.00, and ethanol is so weak with a pK a of about 16 that it hardly counts as acidic at all! Remember - the smaller the number the stronger the acid. Three of the compounds we shall be looking at, together with their pK a values are: The smaller the number on this scale, the stronger the acid is. The strengths of weak acids are measured on the pK a scale. The position of equilibrium therefore lies well to the left. For example, in the case of dilute ethanoic acid, the solution contains about 99% of ethanoic acid molecules - at any instant, only about 1% have actually ionised. At any one time, most of the acid will be present in the solution as un-ionised molecules. The organic acids are weak in the sense that this ionisation is very incomplete. Hydrogen ions are always attached to something during chemical reactions.
Writing H + (aq) implies that the hydrogen ion is attached to a water molecule as H 3O +. If you write it like this, you must include the state symbols - "(aq)". This equilibrium is sometimes simplified by leaving out the water to emphasise the ionisation of the acid. Note: We are writing the acid as AH rather than HA, because, in all the cases we shall be looking at, the hydrogen we are interested in is at the right-hand end of a molecule.Ī hydroxonium ion is formed together with the anion (negative ion) from the acid. We are going to get a measure of this by looking at how easily the acids release hydrogen ions to water molecules when they are in solution in water.Īn acid in solution sets up this equilibrium: This page explains the acidity of simple organic acids and looks at the factors which affect their relative strengths.įor the purposes of this topic, we are going to take the definition of an acid as "a substance which donates hydrogen ions (protons) to other things".
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