water and carbon: the chemical basis of life2

7/18/2019 Water and Carbon: The Chemical Basis of Life2

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2011 Pearson Education, Inc.

Water and Carbon: The Chemical

Basis of Life


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Basic Atomic Structure

Atoms are composed of:

Protonspositively charged particles Neutronsneutral particles

Electronsnegatively charged particles

Protons and neutrons are located in the nucleus.

Electrons are found in orbitalssurrounding the nucleus.


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ElementsThe Building Blocks of Chemical Evolution

Every different atom has a characteristic number of protons inthe nucleus, called the atomic number.

Atoms with the same atomic number have the same chemicalproperties and belong to the same element.

Forms of an element with different numbers of neutrons areisotopes.

The mass numberis the number of protons + neutrons

Eg: Carbon -6 neutrons +6 protons=12 mass number

- 7 neutrons + 6 protons =13 mass number (isotope)


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Electron Arrangement around the Nucleus

Electrons move around atomic nuclei in specific regions called

orbitals.

Each orbital can hold up to two electrons.

Orbitals are grouped into levels called electron shells.

Electron shells are numbered, with smaller numbers closer tothe nucleus.

The electrons in the outermost shell are called valence

electrons.

Elements commonly found in organisms have at least one unpaired

valence electron. The number of unpaired electrons in an atom is its

valence.


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Chemical Bonding

Unfilled electron orbitals allow formation of chemical bonds, and

atoms are most stable when each electron orbital is filled.

Covalent bond: Each atoms unpaired valence electrons are

shared by both nuclei to fill their orbitals.

Substances held together by covalent bonds are calledmolecules.

Ionic bond: Electrons are transferred from one atom to

another.



Covalent Bonds

Electrons are not always shared equally. An atom in a molecule

with a high electronegativitywill hold the electrons more tightly

and have a partial negative charge (), whereas the other atom will

have a partial positive charge (+).

O> N> C=~ H

Differences in electronegativity dictate how electrons are

distributed in covalent bonds.

Nonpolar covalent bond: Electrons are evenly shared between

two atoms and the bond is symmetrical.

Polar covalent bond: Electrons are asymmetrically shared.



Nonpolar Covalent Bond


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Ions and Ionic Bonds

An atom or molecule that carries a charge is called an ion.

Cation: An atom that loses an electron and becomes positivelycharged.

Anion: An atom that gains an electron and becomes negatively

charged.

The resulting attraction between oppositely charged ions is an ionic

bond.


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The Electron-Sharing Continuum

The degree to which electrons are shared in chemical bonds forms

a continuum, from equal sharing in nonpolar covalent bonds, to

unequal sharing in polar covalent bonds, to the transfer of

electrons in ionic bonds.


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How Many Bonds Can an Atom Have?

The number of unpaired electrons determines the number of bonds

an atom can make.

Atoms with more than one unpaired electron can form multiple

single bonds or double or triple bonds.


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Representing Molecules

The shape of a simple molecule is governed by the geometry of its

bonds.

Molecular formulasindicate the numbers and types of atoms in a

molecule (e.g., H2O, CH4).

Structural formulasindicate which atoms are bonded together and

whether the bonds are single, double, or triple bonds.

Ball-and-stick models andspace-filling models show 3Dgeometry.


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Chemical Reactions

Chemical reactions occur when:

1. One substance is combinedwith another.

Atoms are rearranged in molecules, or small molecules

combine to form larger molecules.

2. One substance is broken down into another substance.

Molecules are split into atoms or smaller molecules.

In most cases, chemical bonds are broken and new bonds form.


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Quantifying Molecules

The molecular weightof a molecule is the sum of the mass

numbers of all the atoms in the molecule.

Eg: water =1+1+16=18g

One mole, or 6.022 1023molecules, has a mass equal to the

molecular weight expressed in grams.

The concentration of a substance in a solutionis typically

expressed as molarity(M), which is the number of moles per liter.

Eg: NaCl=23+35=58g =1 mole

58g in 1l of water = 1M(molarity)


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Why Is Water Such an Efficient Solvent?

Life is based on water because water is a great solvent.

The covalent bonds in water are polar because oxygen has a greater

electronegativity than hydrogen.

Oxygen has a partial negative charge.

Hydrogen has a partial positive charge.

Hydrogen bonds are the weak electrical attractions between the

partially negative oxygen of one water molecule and the partially

positive hydrogen of a different water molecule. Can also form between a water molecule and another polar

molecule.


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Water and Hydrogen Bonds

Ions and polar molecules stay in solution because of their

interactions with waters partial charges. These atoms and

molecules are said to be hydrophilic.

Eg?

Uncharged and nonpolar compounds do not dissolve in water and

are said to be hydrophobic.

Eg?

Hydrogen bonding makes it possible for almost any charged orpolar molecule to dissolve in water.


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C i f S i


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Correlation of Waters Structure and Properties

Water is unique due to its small size, bent shape, highly polar

covalent bonds, and overall polarity.

Water also has several remarkable properties, largely due to its

ability to form hydrogen bonds. Water is:

1. Cohesive

2. Adhesive

3. Denser liquid is denser than solid

4. Able to absorb large amounts of energy-

5. Surface tension-water surface elastic property

A Cl L k h P i f W


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A Closer Look at the Properties of Water

Cohesionbinding between like molecules

Results in high surface tension

Adhesionbinding between unlike molecules

Water expands as it changes from a liquid to a solid. This is why ice floats

Water has an extraordinarily large capacity for absorbing heat.

High specific heat (amount of energy required to raise the

temperature of 1 g of substrate by 10C)

High heat of vaporization (the energy required to change 1g

of it from liquid to gas)


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A id B R ti d H


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AcidBase Reactions and pH

Proton [hydrogen ion (H+)] concentration is the basis of the pH

scale.

pH expresses proton concentration in a solution.

The pH of pure water is 7.

Acids have a pH of less than 7. Bases have a pH of greater than 7.

In acidbase reactions, a proton donor (acid) transfers a proton to

a proton acceptor (base).

Th H S l d B ff


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The pH Scale and Buffers

The pH scale is logarithmic:

pH = log [H+]

Greater H+concentrationlower pHmore acidic

Lower H+concentrationhigher pHmore basic/alkaline

Buffersare compounds that minimize changes in pH.


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Ch i l E l ti Th


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Chemical Evolution Theory

Simple molecules present on ancient Earth reacted to create larger,

more complex molecules.

Eg: protein, carbohydrates

This may have happened in:

The atmosphere -volcanic gasses (carbon dioxide, nitrogen,water vapor, methane, carbon monoxide, hydrogen)

Deep-sea vents-rich in metals (iron ,nickel)

Ho Do Chemical Reactions Happen?


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How Do Chemical Reactions Happen?

Chemical reactions have reactantsand products. For example:

CO2(g) + H2O(l)H2CO3(aq)

Chemical equilibrium occurs when the forward and reverse

reactions proceed at the same rate and the quantities of reactantsand products remain constant.

Endothermic reactions must absorb heat to proceed, but

exothermic reactions release heat.

What Is Energy?


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What Is Energy?

Energyis the capacity to do work or supply heat. This capacity

exists in one of two waysas a stored (potential) or

-as an active motion (kinetic).

Potential Energy and Kinetic Energy


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Potential Energy and Kinetic Energy

Stored energy is called potentialenergy. An objects position

determines its ability to store energy. For example:

Electrons in an outer shell (farther from the positively

charged nucleus) have more potential energy than do

electrons in an inner shell.

The energy of movement is called kineticenergyor thermal

energy, which is measured as temperature.

Low-temperature objects have slower molecules than high-

temperature objects.


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Heat and the First Law of Thermodynamics


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Heat and the First Law of Thermodynamics

Heat is the thermalenergy transferred between objects of different

temperatures.

The first law of thermodynamicsstates that energy is conserved

it cannot be created or destroyed, but it can be transferred or

transformed.

What Makes a Chemical Reaction Spontaneous?


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What Makes a Chemical Reaction Spontaneous?

Chemical reactions are spontaneous if they proceed on their own,without any continuous external influence such as added energy.

The spontaneity of a reaction is determined by two factors:

1. The amount of potential energy

Products of spontaneous reactions have less potential energy

than the reactants.Eg: methane react with oxygen to produce carbon dioxide and

water

2. The degree of order

Products of spontaneous reactions are less ordered than thereactants.

Eg:TNT explodes and give carbon dioxyde,carbon monoxide,various nitrogen oxide gasses


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The Second Law of Thermodynamics


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The Second Law of Thermodynamics

Entropy(S) is the amount of disorder in a group of molecules.

Thesecond law of thermodynamics states that entropy always

increases in an isolate system.

In other words, chemical reactions result in products with less

ordered (usable) energy.

In general, physical and chemical processes proceed in the direction

that results in lower potential energy and increased disorder.

Gibbs Free-Energy Change


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Gibbs Free-Energy Change

The Gibbs free-energy change(G) determines whether a

reaction is spontaneous or requires energy.

G< 0 is an exergonicspontaneous reaction.

G> 0 is an endergonicreaction that requires energy input.

G= 0 is a reaction that is at equilibrium.

Temperature and Concentration Affect Reactions


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Temperature and Concentration Affect Reactions

Breaking and forming bonds depends on collisions between

substances.

This allows electrons to interact.

The rate of a reaction depends upon the number of collisions.

The number of collisions is dependent on the temperature and

concentration of the reactants:

Higher temperaturemore collisionsfaster reaction

Higher concentrationmore collisionsfaster reaction

Energy Inputs and the Start of Chemical Evolution


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Energy Inputs and the Start of Chemical Evolution

Formation of formaldehyde (H2CO) and hydrogen cyanide (HCN)

is the first step in chemical evolution and requires energy input.

Photonsare packets of light energy emitted by the Sun.

High-energy photonscan break molecules apart by knockingelectrons away from valence shells. The resulting free radicals

have unpaired electrons and are extremely unstable and highly

reactive.


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Chemical Energy Is a Form of Potential Energy


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Chemical Energy Is a Form of Potential Energy

Significant amounts of H2CO and HCNcould form under the

temperature and concentration conditions that were likely on

ancient Earth.

These products have more potential energy than the reactants.

Potential energy stored in chemical bonds is calledchemical

energy.

Thus: solar energy (energy of the Sun) was converted into chemical

energy (in H2

CO and HCN).

The Importance of Carbon


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The Importance of Carbon

Carbon is the most versatile atom on Earth. Because of its four

valence electrons, carbon can form many covalent bonds.

Carbon-containing molecules can form an almost limitless arrayof molecular shapes with different combinations of single and

double bonds.

The formation of carboncarbon bonds was an important event inchemical evolution.


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Functional Groups: Determinants of Chemical Behavior


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Functional Groups: Determinants of Chemical Behavior

The carbon atoms in an organic molecule furnish the skeleton that

gives the molecule its overall shape.

Amino and carboxyl groups:Attract or drop a proton,

respectively

Carbonyl groups:Sites of reactions that link molecules intolarger, more-complex compounds

Hydroxyl groups:Act as weak acids

Phosphate groups:Have two negative charges

Sulfhydryl groups:Link together via disulfide bonds


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Key Concepts


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Key Concepts

Molecules form when atoms bond to each other. Chemical bonds

are based on electron sharing. The degree of electron sharing

varies depending on the type of bond formed.

Of all small molecules, water is the most important for life. Water

is highly polar and readily forms hydrogen bonds, making it an

extremely efficient solvent.

Key Concepts


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Key Concepts

Energy is the capacity to do work or supply heat, and can be (1) a

stored potential or (2) an active motion. Chemical energy is a

form of potential energy, stored in chemical bonds.

Chemical reactions tend to be spontaneous if they lead to lower

potential energy and higher entropy, and nonspontaneous if they

require an input of energy.

Most of the important compounds in organisms contain carbon.

water and carbon: the chemical basis of life2

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