Chapter 1 Part B: Structure and Bonding, acids and bases

Summary

This educational script delves into the nature of polar covalent bonds, emphasizing the unequal electron distribution between atoms of differing electronegativity. It contrasts polar covalent bonds with nonpolar and ionic bonds, using examples like carbon-oxygen and carbon-lithium bonds. The script also covers the concept of electronegativity, highlighting fluorine as the most electronegative element and the trend's increase from cesium. It explores inductive effects, acid-base reactions with Bronsted-Lowry theory, and the significance of pKa values in determining acid strength. The discussion extends to organic acids and bases, including their structures and roles in reactions, and touches on Lewis acids and bases. Lastly, it addresses the practical aspects of organic chemistry in food production, discussing the use of pesticides like atrazine, their benefits, and the risk-benefit analysis in agriculture.

Takeaways

• 🔬 Polar covalent bonds occur when there's an unequal sharing of electrons due to a difference in electronegativity between the bonded atoms.
• 📉 The electronegativity scale ranges from cesium (0.7) being the least electronegative to fluorine being the most electronegative.
• 🔋 Nonpolar covalent bonds are formed between atoms of similar electronegativity, while polar covalent bonds form between atoms with significant electronegativity differences.
• 🌐 The inductive effect describes how the electron distribution in a molecule is influenced by the electronegativity of nearby atoms.
• 🧪 Bronsted-Lowry acid-base theory defines acids as proton (H+) donors and bases as proton acceptors, with pKa values indicating acid strength.
• 📊 A lower pKa value corresponds to a stronger acid, as it indicates a greater tendency of the acid to donate a proton.
• 🍶 Organic acids often contain polarized hydrogen atoms that can be lost, typically found next to oxygen or a carbonyl group.
• 🧪 Organic bases are compounds with atoms that have lone pairs of electrons available to bond with H+ ions, often nitrogen-containing.
• 🔬 Lewis acids and bases differ from Bronsted-Lowry definitions, focusing on electron pair acceptance and donation, respectively.
• 🌱 The term 'organic' in food does not necessarily imply a lack of synthetic chemicals or pesticides; it's a label that can be misleading without regulation.

Q & A

What is a polar covalent bond?

A polar covalent bond is a type of covalent bond where the electron distribution is not equal due to a difference in electronegativity between the two bonded atoms. The more electronegative atom attracts the bonding electrons more strongly, creating a dipole.

How does electronegativity affect the polarity of a bond?

Electronegativity is the intrinsic ability of an atom to attract electrons. The difference in electronegativity between two atoms in a bond produces bond polarity, with the more electronegative atom pulling the electron density towards itself.

What is the difference between a nonpolar covalent bond and a polar covalent bond?

In a nonpolar covalent bond, the electronegativities of the two atoms are similar, resulting in an even distribution of electron density. In contrast, a polar covalent bond has a difference in electronegativity, leading to an uneven distribution of electron density and the formation of a dipole.

What is the most electronegative element?

Fluorine is the most electronegative element, with the highest tendency to attract electrons in a chemical bond.

How does the inductive effect influence the polarity of a molecule?

The inductive effect is the shifting of electrons within a molecule in response to the electronegativity of nearby atoms. This can alter the distribution of electron density, affecting the overall polarity of the molecule.

What is the difference between a Bronsted acid and a Bronsted base?

A Bronsted acid is a substance that donates a hydrogen cation (proton), while a Bronsted base is one that accepts a hydrogen cation. The reaction between them results in the formation of a conjugate acid and a conjugate base.

What is the significance of pKa values in acid-base chemistry?

pKa values, which are the negative logarithms of the acid dissociation constant (Ka), measure the strength of an acid. A smaller pKa value indicates a stronger acid, as it means the acid is more likely to donate a proton.

How can you predict the direction of an acid-base reaction using pKa values?

You can predict the direction of an acid-base reaction by comparing the pKa values of the acids involved. The reaction will favor the formation of the conjugate base from the weaker acid, as it is less likely to accept a proton.

What is a Lewis acid and how does it differ from a Bronsted acid?

A Lewis acid is an electron pair acceptor, which can accept electrons directly from a Lewis base, an electron pair donor. Unlike Bronsted acids, which donate protons, Lewis acids do not necessarily involve proton transfer.

Why might the FDA allow the use of certain pesticides like atrazine despite potential health risks?

The FDA may allow the use of certain pesticides, such as atrazine, because the benefits of increased crop yield and reduced food costs can outweigh the potential health risks. The decision is often based on a risk-benefit analysis that considers both short-term and long-term exposure risks.

Outlines

00:00

🔬 Understanding Polar Covalent Bonds

This paragraph delves into the nature of polar covalent bonds, explaining how they form when there is an unequal attraction of bonding electrons by the atoms involved. It contrasts this with nonpolar covalent bonds, where the electron distribution is even due to similar electronegativities of the atoms. The concept of electronegativity is introduced as the intrinsic ability of an atom to attract electrons, with fluorine being the most electronegative element. The paragraph also discusses how differences in electronegativity can lead to bond polarity and eventually ionic bonds if the difference is significant. Examples of polar covalent bonds are given, such as carbon to oxygen or oxygen to hydrogen. The inductive effect, which is the shifting of electron density in response to nearby atoms' electronegativity, is also mentioned, along with a brief explanation of Bronsted-Lowry acid-base concepts.

05:01

🌡 Acid-Base Reactions and pKa Values

The second paragraph focuses on acid-base chemistry, specifically the Bronsted-Lowry definition where acids donate protons (H+) and bases accept them. It explains the concept of conjugate acids and bases using the example of HCl reacting with water. The paragraph introduces pKa and pKb values, which are measures of acid and base strength, respectively, with lower values indicating stronger acids or bases. The pKa of water is given as a reference point, and the paragraph explains how these values can be used to predict the direction of acid-base reactions. It also discusses the relative strengths of different acids and bases, including organic compounds like ethanol and HCl, and how their pKa values relate to their strength.

10:03

🍇 Organic Chemistry in Food Production

This paragraph explores the application of organic chemistry in food production, addressing misconceptions about what 'organic' means in the context of food. It clarifies that organic farming still involves the use of chemicals and pesticides, albeit often natural ones, and that the FDA does not strictly regulate the term 'organic.' The discussion then turns to the use of pesticides like atrazine, which is widely used to control weeds in crops but can have environmental and health impacts. The paragraph presents a risk-benefit analysis, comparing atrazine's benefits in increasing crop yields with its potential risks, and explains why it is still allowed for use despite these concerns. It also touches on the use of other chemicals in farming, such as herbicides, insecticides, and fungicides, and the challenges of maintaining crop production without them.

Keywords

💡Polar Covalent Bonds

Polar covalent bonds are a type of chemical bond that forms when there is an unequal sharing of electrons between two atoms, due to a difference in their electronegativity. This results in an uneven distribution of electron density, creating a dipole. In the video, this concept is central to understanding molecular polarity, as it explains how certain bonds within molecules can lead to overall molecular polarity. For example, a bond between carbon and oxygen in a molecule like ethanol is described as polar covalent due to oxygen's higher electronegativity.

💡Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It is a fundamental concept in the video that helps explain the polarity of bonds and molecules. The video mentions that fluorine is the most electronegative element, and there is a trend of increasing electronegativity in a given direction across the periodic table. This property influences the formation of polar covalent bonds, as more electronegative atoms draw electron density towards themselves.

💡Dipole Arrow

A dipole arrow is a representation used in chemistry to indicate the direction of electron flow in a polar bond, pointing from the partial positive charge towards the partial negative charge. The video uses this concept to illustrate how electron density is not evenly distributed in polar covalent bonds, which is crucial for understanding molecular polarity. For instance, in a carbon-oxygen bond, the dipole arrow points towards the oxygen atom, indicating its greater electronegativity.

💡Nonpolar Covalent Bonds

Nonpolar covalent bonds occur between two atoms of the same element, where the electronegativity is equal, leading to an even distribution of electron density. The video contrasts this with polar covalent bonds to highlight how molecular polarity arises from differences in electronegativity. An example given is the C-C bond in ethane, where the electron density is shared equally, resulting in a nonpolar molecule.

💡Ionic Bonds

Ionic bonds result from the complete transfer of electrons from one atom to another, typically between a metal and a nonmetal. The video explains that when the difference in electronegativity is significant, the bond can become ionic, with one atom taking on a full negative charge and the other a full positive charge. This is exemplified by the bond between carbon and lithium in methyl lithium, where lithium takes on a positive charge and carbon a negative one.

💡Inductive Effect

The inductive effect refers to the influence of electron-withdrawing or electron-donating groups on the electron distribution within a molecule. The video discusses how this effect can alter the polarity of bonds in a molecule, affecting its overall reactivity and properties. For example, the presence of electronegative atoms like chlorine in carbon tetrachloride (CCl4) influences the polarity of the molecule despite individual polar C-Cl bonds.

💡Bronsted-Lowry Acids and Bases

The Bronsted-Lowry definition of acids and bases is introduced in the video as a way to classify substances based on their ability to donate or accept protons (H+ ions). The video explains that a Bronsted acid is a substance that donates a proton, while a base is one that accepts a proton. This concept is used to understand acid-base reactions, such as the reaction between hydrochloric acid (HCl) and water (H2O), where HCl acts as an acid by donating a proton to water, which acts as a base.

💡pKa

pKa is the negative logarithm of the acid dissociation constant (Ka), and it is a measure of the strength of an acid in a solution. The video emphasizes the importance of pKa values in determining the extent of proton donation in acid-base reactions. A lower pKa value indicates a stronger acid, as it means the acid is more likely to donate a proton. The video provides examples of pKa values for different substances, such as water, acetic acid, and ethanol, to illustrate their relative acid strengths.

💡Lewis Acids and Bases

Lewis acids and bases are defined in the video as substances that accept or donate electron pairs, respectively. This is a broader definition than the Bronsted-Lowry concept, as it includes reactions involving electron pairs rather than just protons. The video explains that Lewis acids can be metals, nonmetals, or molecules with empty orbitals that can accept electron pairs, while Lewis bases are typically molecules with lone pairs of electrons. Examples given include boron trifluoride as a Lewis acid and water or ammonia as Lewis bases.

💡Organic Chemistry in Food

The video touches on the application of organic chemistry in the food industry, discussing the use of pesticides and the concept of 'organic' food. It explains that organic food is not necessarily free from chemicals or pesticides, as natural pesticides are still used. The video also discusses the use of atrazine, a pesticide used on crops, and its potential health risks versus its benefits in increasing crop yields and reducing food costs. This section of the video connects the chemical concepts discussed to real-world applications and their implications.

Highlights

Polar covalent bonds form when there's an unequal attraction of bonding electrons by the bonded atoms.

Covalent bonds between two identical atoms have equal electron distribution, resulting in nonpolar bonds.

Electronegativity differences between atoms in a bond lead to polar covalent bonds.

Electronegativity is the ability of an atom to attract electrons, with fluorine being the most electronegative element.

Nonpolar covalent bonds occur between atoms with similar electronegativity, such as carbon and hydrogen.

Polar covalent bonds are exemplified by carbon to oxygen, oxygen to hydrogen, and carbon to halogen bonds.

Ionic bonds typically form between metals and nonmetals, with the electronegative atom gaining all electron density.

Methanol and methyl lithium illustrate the difference between polar covalent and ionic bonds through electron density maps.

The inductive effect describes the shift of electrons in a bond due to the electronegativity of nearby atoms.

Ethanol is a polar compound due to the uneven distribution of electron density, as shown by its molecular structure.

CCL4 has polar bonds but is a nonpolar molecule because the polarities of its bonds cancel each other out.

Bronsted-Lowry acid-base theory defines acids as proton donors and bases as proton acceptors.

pKa values, the negative logarithm of the acid dissociation constant (Ka), measure the strength of acids.

A smaller pKa indicates a stronger acid, with water's pKa being 15.74, making it a weak acid.

Organic acids are characterized by the presence of polarized hydrogen atoms that can be lost as protons.

Organic bases have atoms with lone pairs of electrons that can bond to H+, with nitrogen-containing compounds being the most common.

Lewis acids accept electron pairs and are different from Bronsted acids, which do not accept electrons directly.

Most oxygen and nitrogen compounds are Lewis bases due to their lone pairs of electrons.

The food industry's use of the term 'organic' can be misleading as it does not necessarily mean absence of synthetic chemicals or pesticides.

Atrazine, a widely used herbicide, is an example of a chemical allowed in organic farming despite potential health risks.

Risk-benefit analysis is used to justify the continued use of atrazine in agriculture due to its benefits outweighing the risks.

Transcripts

polar covalent bonds are bonds that are

present when the bonding electrons are

attracted

by one atom than by the other so it just

means that the electron distribution is

not the same if we look at a covalent

bond between two of the same atoms

they're going to have similar polarities

if you have one that has this slightly

more electronegative

then the way that your electron density

is going to be weighted is slightly more

on the electronegative atom versus this

one over here so if you have a

difference of electronegativity between

two atoms in a bond the one that is more

electronegative is more likely to carry

more of the electron density than the

other so this would be a covalent bond

where it's shared equally this is a

polar covalent bond where you can draw a

dipole arrow if there's enough of an

electron difference it becomes an ionic

bond where the electronegative atom is

carrying all of the electron density and

this is taking none of that

so the electronegativity is the

intrinsic ability of an atom to attract

electrons the differences in

electronegativity between two atoms in a

bond produces bond polarity or fluorine

is the most electronegative one and the

trend increases as you move in this

direction so the lowest electronegative

atom is cesium at 0.7 and halogens and

nitrogen oxygen tend to be more

electronegative so with non covalent

bonds you have atoms with similar

electronegativity

en is abbreviation for electronegativity

CH is an example of two atoms that have

nonpolar covalent bonds polar covalent

bonds have a difference in

electronegativity of atoms

so the examples of these would be carbon

to oxygen oxygen to hydrogen or carbon

to a halogen ionic bonds generally occur

between a metal and a nonmetal

so if we look at these two examples here

we have methanol which is a carbon bond

between carbon and oxygen and that

electron density you can see her from

this electron density map the red

indicates that it's negative this is

going to be positive so we have the

dipole arrow here with the electron

density on the oxygen and the partial

positive on the carbon with methyl

lithium this is more of an ionic bond

between carbon and lithium you can see

this is very blue versus this was a

slight green color and this is very red

so the dipole arrow is drawn in the

opposite direction where carbon now has

that negative charge lithium is positive

what an inductive effect is is a

shifting of electrons in a bond in

response to the electronegativity of

nearby atoms which we can see with this

carbon in the methyl lithium

you

and we'll see more examples of inductive

effects later on alright so for our

practice problem use the partial

positive partial negative to indicate

the direction of expected polarity for

each of these so we have ch3 ch2 O H if

you were to draw those out I have ch3

ch2 and then Oh H oxygen has two lone

pairs each carbon has four bonds oxygen

has two like it should if I look at the

electron density of this one the carbon

is partial positive because it is less

electronegative than oxygen which is one

of the electronegative atoms the dipole

arrow was going to move in this

direction so this is the structure of

ethanol so that is a polar compound

because there's an uneven distribution

of electron density if we look at CCL 4

we have C CL CL CL CL chlorine is very

electronegative because it's right under

fluorine so I can draw partial positive

here partial negative to each of these

chlorines

and dipole arrows to each of these as

well

so there are polar bonds here

but because each of these are pulled in

different and equal directions the

overall molecule is nonpolar

if we look at the definition of a

bronsted-lowry acid base which is often

shortened to bronsted a bronsted acid is

a substance that donates a hydrogen

cation

and the bronsted base

one that accepts the H+

H+ can also be referred to as a proton

a proton is a synonym for H+ if we look

at this reaction here we have HCl plus

HOH which gives CL minus and h3o plus if

we try to figure out what is acid and

what its base we can look at the other

half of the reaction so if we notice

here the h and the CL on the other side

this is losing the h which means that

this HCl is the one that donated it the

one that donates is the acid

which by default makes this the base to

figure out the conjugate acid and

conjugate base we can look at the

reaction going backwards in this case

the CL will CL minus and it becomes HCl

so it is accepting a proton which means

this is the conjugate base

and by default this is the conjugate

acid and the conjugate acid is the

is going to be donating a proton h3o

plus donates its proton to CL - which

makes it h2o we can use PKS

which is the negative log of ka to

measure the acid strength PKA is the

negative log of ka this measures acid

strength

so if you have a smaller PKA this

indicates a stronger acid

the pKa of water is fifteen point seven

four this is a values you should

definitely become familiar with so what

a ka or PKA tells us is how far the

reaction proceeds in an acid-base

reaction so a KA is a concentration of

acid times base over H a which is also

the same as h3o plus times o h- over

water and you can see these values here

so for water it is 1.8 times 10 to the

negative 16 the pKa equals fifteen point

seven for the KA is a measure of how

much of the acid will dissociate into

hydronium ion and conjugate base we can

look at their relative strengths of

different common acids ethanol is a

weaker acid water as we know is a weak

acid and we forget down to HCl that is a

very strong acid if you notice the pKa

is of ethanol 16 negative seven is a

very strong acid for HCl and the

conjugate base of each of these you can

observe here as well so a weaker acid

stronger acid weaker base over here

stronger base so one thing that you

might observe is a stronger acid

means that it will have a weaker

conjugate base

so this is HCl which is a strong acid

but cl- is a weak base you will not need

to memorize these PK's except for water

so if we want to rank these substances

in order of creasing acidity we want to

remember that high PKA

hi P ka equals weak acid so the value

with the highest will be the least

acidic

and the one with the lowest value will

be the most acidic so if I put this in

order acetone is a pKa of 19 the pentane

Dino diode has a pKa of 9 this is nine

point nine four point seven six the

weakest one will be acetone the

strongest is four point seven six

then 9.9 the phenol is next

and then the pentane died on

so we can just move these from highest

pKa to lowest PKA

this is least acidic

this is most acidic

we can use PKS to predict how the

reaction is going to proceed if we have

acetic acid with hydroxide ion acetate

ion plus water we can use the pKa values

that are related to their logarithms to

determine the equilibrium constant so we

can use this to determine whether or not

a reaction will happen which side the

equilibrium lies on so if you have a

higher PKA that means it's harder to

remove a proton

proton being H+ so we just saw in the

last slide the pKa of acetic acid was

around 4.8 water is fifteen point seven

four so when you're trying to predict

the acid-base reactions compare acids

because we have the PKA values to

support that the value with the higher

PKA is water which has fifteen point

seven four which means it's going to be

harder to remove this proton if it's

hard to remove this proton that means

that most of the product is going to be

on this side because the acetate ion

will not be able to take that proton off

to go to this side so this side of the

equilibrium is favored

so the equilibrium favors the high PKA

because the high PKA means that we can't

remove this proton very well if we can't

take the proton off then it can't go in

the opposite direction

organic acids come in different shapes

and sizes they're generally

characterized by the presence of

polarized hydrogen atoms they can lose a

proton from an O H

so these would be organic acids methanol

can lose this H so can acetic acid they

can also lose a proton from a CH

and this is usually next to a co bond

so this one here would be an H next to a

C double bond o so these qualify as

organic acids the pKa of each of these

methanol is fifteen point five four four

point seven six and this is 19.3 an

organic base has an atom with a lone

pair of electrons that can bond to H+

nitrogen-containing compounds are the

most common organic basis

you

oxygen containing compounds can react as

bases when with a strong acid or as

acids with strong pieces so methylamine

here has this lone pair it's a nitrogen

that's going to act as a base the oxygen

here can act as a base as well and so

can this the oxygen lone pair of acetone

you

so Lewis acids are different from

bronsted acids loose acids are electron

pair acceptor x' so we're dealing with

electrons here and not protons and the

Lewis base is an electron pair donor

bronsted acids are not lewis acids

because they can't accept an electron

directly only a proton would be a Lewis

acid and there's no scale of strengths

as an operon state definition of a PKA

Lewis bases can donate their electrons

Lewis acids can accept electrons if we

look at boron trifluoride which is a

Lewis acid this is the electron space

fill of this here the boron can accept

electron lone pairs from the oxygen here

so this would be the Lewis base this is

the Lewis acid this is the acid base

complex so Lewis bases can accept

protons as well as Lewis acids

most oxygen and nitrogen compounds are

Lewis spaces because they have lone

pairs of electrons

some asses can act as both depending on

the reaction so we can see here some

examples of lewis bases alcohols ethers

aldehydes these all have the electron

lone pairs the sulfur as well nitrogen's

and then we can see phosphorus or this

phosphate compound because of the oxygen

lone pairs so if we look at this which

of these are likely to act as lewis

bases we saw on the last slide oxygen

nitrogen are very good at acting as

lewis bases because of the lone pairs so

this would be and this would be these

two are going to be considered the risk

acids so if we look at an application of

organic chemistry all foods are organic

organic in the dietary industry or in

the food industry is commonly thought to

mean it has no synthetic chemicals or

pesticides or is less dangerous or risky

this is not always a good definition of

that because organic food farmers do use

chemicals and pesticides they might be

natural ones but they do use some to

some extent and the other thing is that

the FDA does not really regulate what is

considered organic or not if there's a

compound that is organic in the whole

food item that can be considered organic

so they don't really police that if

you're making cereal and one ingredient

on there was organic they could slap the

organic label on there so pesticides are

used for weeds they use herbicides for

that for insects

they use insecticides and they use

fungicides for molds and fungi if you

use no pesticides whatsoever and you may

have noticed this if you've ever had a

garden over the summer there is a

significant drop in crop production

which would increase your food costs if

you were a farmer so it's not really

ideal for an organic food farmer to not

use any pesticides at all because they

have to deal with weeds insects or molds

so one of the things that is used is

atrazine which is used on crops there's

100 million pounds used each year to

kill weeds in sorghum corn and sugarcane

and there's trace amounts of this that

persists in the environment exposure can

pose health risks to humans and animals

but there's an alternative but to use

this the EPA will not ban this pesticide

due to the resulting dramatic increase

in food costs currently the benefits

outweigh the risk so what I mean by that

they use atrazine on food crops and if

you use them your crops of sorghum will

increase by 13 bushels increases seven

bushels of corn per acre by not using

these things your food costs and your

farming costs parts a lot more if we

look at the risk versus the benefit

analysis strychnine is a nasty poison

that has a lethal dose of point zero

zero five grams per kilogram which means

that in a group of a hundred people 50

people would die at this dosage level

for arsenic trioxide it would be point

zero one five and you can see the

different things here ethyl alcohol

which is in our alcoholic beverages is

ten point six so they tested the

atrazine in animals at a much higher

level than you would see at normally

exposure the results were converted to

an ld50 value which is at one hundred

people fifty died oh so the results were

converted to an ld50 of value which came

out to be one to four grams per kilogram

that was lethal for 50 percent of the

testing done in animals this has no

information on long-term exposure risk

so there was a small amount of the ld50

is low especially considering that

aspirin also has a similar ld50 so

because of the benefits of the atrazine

outweigh the risk of health the

FDA will continue to allow the use of

atrazine on crops

you