1 Combining and Breaking Down Substances

2 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.

3 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 gravel 2. 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.

4 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.

5 e- 1 st Period: Hydrogen (#1)Helium (#2) nucleuse- 2 nd Period: Lithium (#3)Neon (#10) e- 1 st energy level 2 nd energy level Electron Structures of Atoms

6 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.

7

8 Lewis structures of atoms Lewis structure: shows only valence e - of an atom or ion.  Uses dots, representing e -, at top, bottom, right, and left sides Ex: Carbon (6 e-)Fluorine (9 e-) C Valence e- Element symbol F e-

9 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)

10 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 H 2 0

11 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....

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

13 Covalent Compounds  Covalent bonds occur in most “natural” compounds like methane (CH 4 ), ammonia (NH 3 ), 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”.)

14 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. CO 2 exists as independent molecules.

15 *The simplest molecule: H 2 The 1 st 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-

16 HCl *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 :

17 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  NaCl ex. 2H 2 + O 2  2H 2 O 2. Decomposition Reaction- Opposite of a synthesis reaction. This a reaction in which a compound breaks down into two or more simpler substances. ex. 2H 2 O  2H 2 + O 2

18 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 + 2AgNO 3  2Ag + Cu(NO 3 ) 2

19

20 Chemical Reactions Cont. 4. Combustion Reactions – is a reaction in which a substance reacts rapidly with oxygen, which often produces heat and light. ex. CH 4 + 2O 2  CO 2 + 2H 2 O + heat & light

21 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.

22 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.