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 don’t 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 source
3.15, 4.77, 5.19 Lemon juice
3.40, 5.11 Apple juice
Figure 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).
1. 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.
Model 2. Some molecules are the opposite of acidic; these molecules don’t release protons, instead they take protons from other molecules. Taking a proton from water creates an anion with a special name – hydroxide. Molecules that produce hydroxide ions when mixed with water are alkaline, they are also called bases or basic molecules.
Figure 8.3. The base ammonia takes a proton from water to make the hydroxide anion.
Table 8.2. Relative concentrations of protons and hydroxide ions in acidic, neutral and basic solutions
Concentration H+ (protons) Concentration of –OH (hydroxide)
Acidic pH High Low H+ > –OH
Neutral pH Equal Equal H+ = –OH
Basic pH (Alkaline) Low High H+ < –OH
The pH is a different number used to measure the concentration or the amount of H+ ions in solution. The more protons (H+) there are, the lower the pH. Alkaline or basic molecules produce very few H+ ions, and they can also consume H+ ions – both effects lower the H+ concentration and raise the pH.
Figure 8.4. pH values of common foods.
Figure 8.5. The dissociation of water. Water has a neutral pH – so the amounts of H+ and –OH ions produced are equal.
5. Based on the pH of milk in Figure 8.4, what would you predict about the relative concentration of H+ and –OH ?
6. Water has a neutral pH. If you squirt some lemon juice into water, the pH changes.
a) What do you expect will happen to the pH of the mixture of water and lemon juice? Will the number increase or decrease?
b) What do you expect about the relative concentrations of H+ and –OH in the lemon juice and water mixture?
7. If you measure carefully, it is possible to take some vinegar (acidic) and mix it with baking soda dissolved in water (basic/alkaline)- the mixture will get warm (evidence of a chemical reaction occurring), but the final mixture has a neutral pH. How can this be? Talk about the relative concentration of H+ and –OH ions.
8. Natural unsweetened cocoa powder has a pH of about 5 (slightly acidic). Dutch processed cocoa is made by treating natural unsweetened cocoa powder with a base (an alkaline substance). The resulting Dutch processed cocoa is darker in color and milder in flavor. Should the pH of Dutch processed cocoa be lower or higher than natural unsweetened cocoa powder? Explain.
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