Can some one help with my science sheet?

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?

H

O

C

O
C
and
aproton
aspecialCATion
theanion
O
CH
3
CH
3
O
H
aspecialgroupofatoms
calledacarboxylicacid
aceticacid(i.e.vinegar)
inwater
thisparticularanioncalled
acetateisquitestable
thepKaisameasureofhowwellthisreactionoccurs.Strong
acids(withlowpKa’s)generateH
+
ions(andthe
correspondinganion)easily.
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.

H
O
C
O
C

CHO

O
O
H
H
O
C
O
C
CHO
O
O
H

2H

+
and

HH

pKa~40

pKa~14

pKa3.4

pKa5.1

HH

(a)

(b)

(c)

(d)

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).

Questions

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.

H
N
H
O
H
thehydroxideANion
H
H
O
H
H
N
H
HH
ammonia
(i.e.householdbleach)ammonium
H
N
H
HH
releaseofaprotonbyammonium
isnotafavorablereaction…
verysmall
concentrationof
protons
basesproducemany
hydroxideionswhen
mixedwithwater
instead,theNH
3
base
consumesanyprotons
Basescanalso
reactwith(and
therefore
decreasethe
concentrationof)
protons

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. IncreasingAcidity Neutral Decreasingacidity (increasingalkalinity) 1 2 3 4 5 6 7 8 9 10 11 12 humangastricjuice(pH1.3-3.0) lemonjuice(pH2.1) distilledwhitevinegar(pH2.4) orangejuice(pH3.0) yogurt(pH4.5) blackcoffee(pH5.0) milk(pH6.9) eggwhite(pH7.6-9.5) Bakingsodainwater(pH8.4) householdammonia(pH11.9) Figure 8.4. pH values of common foods. H O H OH and aproton aspecialCATion thehydroxideANion H equalamounts Figure 8.5. The dissociation of water. Water has a neutral pH - so the amounts of H+ and –OH ions produced are equal. Questions: 1. Based on the pH of milk in Figure 8.4, what would you predict about the relative concentration of H+ and –OH ? 2. 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? 3. 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. 4. 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. Copyright © 2016 Wiley, Inc. Page 1 _1466880596.cdx _1466880625.cdx _1466880643.cdx _1466881150.cdx _1466880603.cdx _1466854953.cdx _1466855119.cdx _1466880188.cdx _1466852750.cdx

PAGE

O

il and Water Don’t Mix

Model 1. When chemists think of fats they think of a large class of molecules called lipids. The word lipids comes from the Greek “lipos” for fat. Natural fats and oils are made mostly of molecules called triglycerides. Fats are solid triglycerides, while oils are liquid triglycerides.

A triglyceride is made from the combination of fatty acids and glycerol; it has the basic structure shown in Figure 4.1. Fatty acids can be from 4 to

3

5 carbons long, but 14-

2

0 carbon fatty acids are most common in food.

O

C

C

H

2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2

CH

3
O
O
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O
O
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O
H
2
C

HC

H
2
C
O
O
O
H
2
C
HC
H
2
C

HO

C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O
HO
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O
HO
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O

+

3H

2
O

Glycerol

Fatty Acids

Triglyceride

H
C
H

really looks like…

H
H
H

Figure 9.1. Glycerol and fatty acids combine to make a triglyceride.

When fatty acids and glycerol combine, bonds are broken and formed in a chemical reaction to produce a triglyceride and 3 molecules of water. In that process, a new group of atoms is formed called an ester. The properties of a given triglyceride depend upon the chemical structure of the three fatty acids it contains, and the properties of a lipid depend upon the particular mixture of triglycerides it contains.

Table 9.1. The composition of mono-, di- and triglycerides

RO
C
R
O
anesterfunctional group
R = the “rest” of the molecule

Figure 9.2. An ester functional group
By definition, Lipids are insoluble in water, so that means triglycerides are insoluble in water. To be soluble means that two molecules will dissolve in one another to form a homogeneous mixture. When compounds are insoluble, the combination forms a heterogenous mixture. When a lipid (e.g. oil) is mixed with water, you will see boundaries form between the two phases – literally, the two cannot mix.

Key Concept
The polarity of a molecule is determined by the separation of charge between
its atoms. In polar molecules most atoms are connected polar bonds. In non-
polar molecules most atoms are connected by non-polar bonds.

OH
NH
CO
-
-
-
+
+
+
CH
CC
CC
Polarbonds
Non-polarbonds

Figure 9.3. Non-polar versus polar bonds

Polar compounds can mix with or dissolve /are soluble in water (hydrophilic) to form homogeneous mixtures (i.e. sugar dissolving in water, lemon juice dissolving in water, vinegar dissolving in water). Non-polar compounds can mix with or dissolve /are soluble in oils (hydrophobic) to form homogenous mixtures (e.g. vanilla extract dissolving in oil, melted butter mixing with olive oil). These facts are described by the principle
like dissolves like
.

BUT oil (non-polar, hydrophobic) and water (polar, hydrophilic) don’t mix or dissolve in one another.
1. Why is a fatty acid called a fatty acid?
2. How many ester bonds are formed when a triglyceride is made?
3. On Figure 9.1, use the Draw tool or insert a shape to box the atoms that become the water molecule.
4. How many water molecules are produced when a monoglyceride is made?
5. A triglyceride has some polar and some non-polar bonds – and yet the molecule as a whole is very hydrophobic (i.e. water hating).
a. Why is the tryiglyceride – as a whole – water hating?
b. Explain why the non-polar carbon chain is unable to interact with water.
6. Define the phrase non-polar bond. Use the word charge in your answer.
7. The words hydrophobic and hydrophilic are derived from the Greek: hydro = water, phobos = fear, and phileo = love. Explain why these words are consistent with the chemical properties of a triglyceride.
Saturated and Unsaturated Fats
Model 2. Fatty acids come in different forms.

HO
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
CH
3
O
HO
C
C
H
2
C
C
H
2
C
CH
2
H
2
C
CH
2
H
2
C
CH
2
H
2
C
CH
2
CH
3
OH
H
HO
C
C
H
2
C
C
H
2
C
CH
2
H
2
C
C
C
H
2
C
CH
2
H
2
C
CH
3
OH
H
H
H
saturatedfattyacidchain
monounsaturatedfattyacidchain
polyunsaturatedfattyacidchain
makingthedoublebond
requiredthelossof2H
atoms.TwoHatoms=a
singleunitofunsaturation

Figure 9.4. Saturated, Unsaturated and polyunsaturated Fatty Acids
Saturated, monounsaturated and polyunsaturated fatty acids can all be used to make triglycerides. A single triglyceride can be made of fatty acid chains of all of one type (e.g. all saturated) or a mixture of types, for example one saturated, one monounsaturated and one polyunsaturated fatty acid chain.

C
O
C
C
C
O
O
O
C
O
C
O
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
CH
3
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
C
C
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
CH
3
H
H
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
C
C
C
C
H
2
C
C
H
2
H
2
C
H
3
C
H
H
H
H
H
H
H
H
H

Figure 9.5. A triglyceride made with different types of fatty acids
The properties of a given triglyceride molecule depend on the structure (i.e. type) of the three fatty acids that make up the triglyceride, and their relative position on the glycerol backbone.

C
O
OH
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
C
C
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
CH
3
H
H
C
O
OH
C
H
2
H
2
C
C
H
2
H
2
C
C
H
2
C
C
H
2
C
C
H
2
H
2
C
C
H
2
H
2
C
CH
3
H
H
type 1
type 2

Figure. 9.7. On a nutrition label, Total fat includes Saturated, Polyunsaturated and Monounsaturated fats.

Figure 9.6. Different types of double bonds.
Fatty acids can have double bonds between carbon atoms. Double bonds come in two main types, as shown in Figure 9.6. The double bond between the carbons “fixes” all the atoms in place – in effect, the 4 atoms attached to the doubly bonded carbons (shown in boldface in figure 9.6) are “stuck there”. Since these atoms are fixed in space, we can think of the double bond as having two “sides”. This is in contrast to singly bonded atoms, which are able to rotate freely.

The generic food term “fat” refers largely to triglycerides. Therefore, for example, saturated fat is referring to the composition of the fatty acid chains that make up the triglycerides.

8. Using the information in Figure 9.4,
a. What does it mean when a fatty acid is unsaturated?
b. What do the prefixes mono- and poly- tell you about the unsaturation?
c. If some fatty acids are saturated, what are they saturated with?
9. Looking at Figure 9.6,
a. What is the difference between the two types of double bonds?
b. Chemists refer to these two types of double bonds as cis and trans. Considering that trans is Latin for “on the opposite side” and cis is Latin for “on the same side”, which type of double bond (Figure 9.6, type 1 or 2) is trans and which type is cis?
Putting it all together:
10. It is possible to cook garlic in olive oil and transfer the flavors of the garlic to the oil. Since the flavors of garlic can all be attributed to molecules, what does this process tell you about the polarity of the garlic flavor molecules?
11. Many salad dressings are a mixture of oil and vinegar, like the image shown below.

Photo by Bill Keller

Please explain why the mixture on the left is apparently separated into two phases.

12. Some vitamins like vitamins A and D are fat soluble. What does that tell you about their structure?
Copyright © 2016 Wiley, Inc.
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