Model 1. What we perceive as the taste of sour is actually our tongues detecting the presence of H+ cations. H+ cations are special cations called protons. When they are released by molecules into water they make the solution acidic, and that acidity is what we taste as sour. The term acid comes from the Latin acidus, meaning sour or tart. And it is that same Latin word that is the origin for the word acetus more commonly known as vinegar. What is it in vinegar that creates acidity and conveys the taste of sour?Figure 8.1. Acetic acid dissociating into ions If acetic acid (the sour taste of vinegar) is added to water, then the carboxylic acid group will give up a proton, leaving behind an anion.Why dont the proton and the anion just immediately recombine to make acetic acid again? Because the anion is quite stable. When the anion is stable, the acid is more acidic. To put this another way, the more stable the anion produced, the more likely the acid is to release a proton. Chemists measure how readily an acid releases a proton using a number called the pKa; the lower the pKa, the more acidic the acid and the more stable the anion produced.Table 8.1. Acids found in typical foods.Acid Structure pKa of acid Food sourceAcetic AcidVinegarCitric Acid3.15, 4.77, 5.19 Lemon juiceMalic Acid3.40, 5.11 Apple juiceLactic Acid3.88 YogurtFigure 8.2. The dissociation of malic acid into ions. Hydrogens (a) and (d) are lost as protons. De-protonation (i.e. loss of a proton) does not occur for hydrogens (b) and (c).Questions1. What is the significance of the red bond on the left of Figure 8.1 and the two red electrons on the oxygen (on the right side of the figure)?2. As shown in Table 8.1., citric, malic and lactic acids all have hydrogen atoms that are not part of carboxylic acids. Considering the information provided for malic acid in Figure 8.2, why do you think that de-protonation (i.e. removal of a proton) does not occur at positions (b) and (c)?3. In Table 8.1, why does citric acid have three pKa measurements listed, while malic acid has two and lactic and acetic acids each have one?4. Vitamin C is a molecule with a pKa of 4.1. Is Vitamin C an acid or base? Explain.
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