1. the chemistry of life 2 © zanichelli editore 2015

The chemistryof life

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© Zanichelli editore 2015

Biomoleculesand intermolecular bonds

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The chemical bases of life: biomolecules

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Living organisms contain and produce biomolecules.

Each biomolecule has a specific function and is produced in a controlled way. Biomolecules are similar in all organisms.


The chemical bases of life:water and carbon

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The six most important elements for life are hydrogen, oxygen, nitrogen, phosphorous, sulphur and carbon.

Hydrogen and oxygen are mainly present as molecules of water, which represents over 70% of an organism’s weight.

Carbon is an essential component of biomolecules.


Each molecule has a specific shape

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The shape of a molecule mainly depends on bond angles.

The water molecule has a V shape.Carbon can form molecules with different geometries.Bond angle: 104.5°


Dipole molecules are polar

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All molecules are neutral, but in some cases electrical charges are distributed asymmetrically: these molecules are called polar or dipole molecules.

Water is a very polar molecule.


Carbon form many compounds

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Carbon is an essential element for life and it forms a large number of compounds. Its atoms can make strong covalent bonds with other carbon atoms or atoms of different elements.

The compounds formed by chains of carbon atoms are called organic compounds.

A complex molecule like eme group contains carbon, oxygen, hydrogen, iron and nitrogen atoms.


Hydrocarbons

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Hydrocarbons are made up of carbon and hydrogen.

Aliphatic hydrocarbons are molecules made up of linean chains which can be open, closed or branched.

Aromatic hydrocarbons have a cyclic structure with specific properties.


Intermolecular bonds

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Intermolecular bonds are weak electrical attractions that do not involve the division or transfer of electrons. They can form when molecules are close together.

There are three types of intermolecular bonds:•dipole-dipole bonds;•hydrogen bonds;•London forces.

Their force depends on the polarity of the molecules involved.


Dipole-dipole bonds

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The dipole-dipole bond is the attraction between the opposite poles of polar molecules.

δ–δ+ δ–δ+

+ –

Attraction

+ –


The hydrogen bond

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The hydrogen bond is a particular case of dipole-dipole bond.

It is formed between molecules that are strongly polarized, like water, where a hydrogen atom has a covalent bond with small, very electronegative atoms (F, O, N).

H OOδ–δ+


London forces

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London forces are the weakest intermolecular bonds. They can form between apolar molecules and they are due to the presence of temporal dipoles between molecules which are close together.

δ–δ+ δ–δ+

Attraction

+ – + –


Intermolecular bonds in solutions

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Intermolecular bonds are very important in liquid solutions.

Ionic and polar substances, like ammonia, dissolve well in polar solvents.

Apolar substances, like hydrocarbons, dissolve better in apolar solvents.


The propertiesof water

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Water is a special liquid

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Water has properties that make it indispensible for life due to hydrogen bonds between its molecules.


Cohesion determines surface tension /1

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Water has a very strong cohesion - the force that keeps molecules together.

Cohesion determines surface tension: molecules inside the liquid are attracted by all the surrounding molecules, while the molecules on the surface are only attracted towards the inner part of the liquid.


Cohesion determines surface tension /2

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Due to surface tension, the surface of water behaves like an elastic membrane, which allows some insects to walk on it.

Cohesion between molecules of water is crucial for plants, which collect water through their roots and transport it to their leaves.


Water has a high thermal capacity

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Water has a high thermal capacity: in order to change its temperature it is necessary to apply a large quantity of energy.

This is why water is usually cooler than sand even on very hot days.


Ice is less dense than liquid water

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When water passes from the liquid to the solid state, its density decreases: this is why ice floats on water.

In the solid state, water molecules are disposed in a reticular crystalline structure. The distance between molecules increases and then the density decreases.


Water and self-ionization

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In liquid water, there are always small quantities of hydronium and hydroxyl ions, produced by a reaction of self-ionization:

2H2O H⇄ 3O+ + OH–

The reaction is reversible.


Ionic solutes in water: dissociation

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Ionic substances dissociate in water. Sodium chloride (NaCl), for example, dissociates into Na+ and Cl– ions.

Ions are called electrolytes when their presence makes water a conductor of electricty.


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Sometimes water molecules modify the structure of polar molecules, and transform them in ions (ionization). Hydrochloric acid molecules (HCl), for example, are transformed in ions H+ and Cl–. Other substances, like sodium hydroxide (NaOH), release ions OH–.

Molecular solutes in water: ionization


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The relative quantities of H+ and OH– ions released by a substance is indicated by the pH scale.

Pure water releases an equal number of H+ and OH– ions by self-ionization: it is neutral, with a pH of 7.

Acids are substances that release an excess of H+ ions: they have a pH lower than 7.

Bases are substances that release an excess of OH– ions: they have a pH greater than 7.

The pH scale: acids and bases /1


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The pH scale: acids and bases /2


1. the chemistry of life 2 © zanichelli editore 2015

Documents

dipole molecules

molecules of water

attraction zanichelli

apolar molecules

water molecule

chains of carbon atoms

weakest intermolecular

types of intermolecular