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Combining and Breaking Down Substances

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Compounds & Mixtures:

What happens when you combine two or more substances?

1. Compounds – is a substance that is made from two or more simple substances that can be broken down by chemical means.

* a compound always contains two or more elements joined in fixed proportions.

2. Mixture – is a physical combination of two or more substances.

* a mixture does not have a set number of elements joined in fixed proportions.

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Types of mixtures: There are two types of mixtures:

1. Heterogeneous Mixtures – the parts of the mixture are noticeably different from one another.

ex. sand or gravel2. Homogeneous Mixtures – the substances are so evenly distributed that

is difficult to distinguish one from another.

* Solution – a mixture that forms when a substance dissolves (solute) in a liquid (solvent) and forms a homogeneous mixture.

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A little review: Compounds are much more complex

than mixtures. We will have to review the structure of

an atom to understand how compounds work.

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e-1st Period: Hydrogen (#1) Helium (#2)

nucleus

e-

e-

2nd Period: Lithium (#3) Neon (#10)

e- e-e- e-e- e- e-

e-

e-

e-e-

e-e-

1st energy level

2nd energy level

Electron Structures of Atoms

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Elements in the same group have similar properties because they have same number of valence electrons (e- in outermost shell).

The number of valence electrons increases as you go from left to right across a period; there is no change going down a group.

*Ex: Alkali metals all have one valence electron: they all will form a white powder with cl.

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Lewis structures of atomsLewis structure: shows only valence e- of an atom or

ion. Uses dots, representing e-, at top, bottom, right, and

left sidesEx: Carbon (6 e-) Fluorine (9 e-)

CValence

e-

Element symbol

F

e-

e-e-

e-

e-e-

e-

e-e-

e-

e-

e-e-

e-

e-

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Lewis structures of first 20 elements

Most elements “want” a set of valence electrons like that of the chemically-stable noble gases, which have 8 valence electrons- (except He)

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Chemical Bonds Chemical bond – force that holds atoms or

ions together Interaction occurs between valence electrons Examples: ionic, covalent

Chemical formula – shows the elements in a compound and the ratio of the atoms in the compound Example: formula for water is H20

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Ionic Compounds When one or more e- are transferred from a

metal atom to a nonmetal atom, ions are formed.

Ionic bond: attractive force between oppositely charged ions

Na atom + :Cl: atom → Na +:Cl:sodium chloride

..:Cl:

....

. ...

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Mg: atom + 2 :Cl: atom → Mg + 2 :Cl:magnesium chloride

.

.

.....

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Covalent Compounds Covalent bonds occur in most “natural”

compounds like methane (CH4), ammonia (NH3), and large biological molecules (proteins, DNA, etc.)

Nonmetal atoms still “want” to have filled e- levels, but instead of transferring e- and forming ions, they share e-, forming a covalent bond. (“Co-valent” means “sharing valence”.)

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Substances with covalent bonds exist as molecules (combinations of at least 2 nonmetal atoms)O

-C-O

*The bonds between C and O in carbon dioxide are covalent. CO2 exists as independent molecules.

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*The simplest molecule: H2

The 1st energy levels of the H atoms

overlap.

The 2 e- are shared, and are

likely to be found anywhere

between the atoms.

Each H has 1 e-

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H Cl

*Hydrogen chloride molecule, HCl If the elements are not the same, the bond is polar

covalent; the electrons are shared unequally.

Cl has a greater attraction than H for the 2 shared

e-

+Partial

positive charge -

Partial negative charge

Shared e- are closer to Cl,

making Cl slightly negative

:

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Chemical Reactions Just as we can classify matter, we can classify

chemical reactions. Some of the general types of reactions follow:

1. Synthesis Reaction- is a reaction in which two or more substances react to form a single substance.

ex. Na + Cl NaClex. 2H2 + O2 2H2O

2. Decomposition Reaction- Opposite of a synthesis reaction. This a reaction in which a compound breaks down into two or more simpler substances.

ex. 2H2O 2H2 + O2

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Chemical Reactions Cont.1. Replacement Reaction- is a reaction in which one or more elements take

the place of one or more elements in a compound, resulting in a new compound.

ex. Cu + 2AgNO3 2Ag + Cu(NO3)2

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Chemical Reactions Cont.4. Combustion Reactions – is a reaction in which a substance reacts

rapidly with oxygen, which often produces heat and light.

ex. CH4 + 2O2 CO2 + 2H2O + heat & light

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Factors that effect Reactions: Reaction rates depend on how often the particles collide. If the collisions occur more often the rate will increase

and vice versa. Factors that affect reaction rates include the following:

1. Temperature - Generally, an increase in temperature will increase a reaction rate.

ex. milk stored in a refrigerator -vs- on the counter

2. Surface Area – The more area exposed the faster the reaction will be.

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Factors that effect Reactions Cont.

3. Stirring – Also increases the exposure of reactants to each other.

ex. washing machine 4. Concentration – The more reactants, the faster the particles will react.

ex. dye solution concentration 5. Catalysts – is a substance that affects the reaction rate without being

used up in the reaction.

* Used to speed up reactions or have a reaction occur at a lower temperature.

* Weakens the bonds holding substance together.