Combining and Breaking Down Substances

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

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. e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- 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-) Valence e- C F Element symbol

8. 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)

9. 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

10. 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 .. .

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

12. 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”.)

13. *The bonds between C and O in carbon dioxide are covalent. CO2 exists as independent molecules. Substances with covalent bonds exist as molecules (combinations of at least 2 nonmetal atoms) O-C-O

14. Each H has 1 e- The 2 e- are shared, and are likely to be found anywhere between the atoms. *The simplest molecule: H2 The 1st energy levels of the H atoms overlap.

15. Partial positive charge Partial negative charge + - H Cl Cl has a greater attraction than H for the 2 shared e- Shared e- are closer to Cl, making Cl slightly negative *Hydrogen chloride molecule, HCl • If the elements are not the same, the bond is polar covalent; the electrons are shared unequally. :

16. 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. 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

17. 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

18. 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

19. 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.

20. 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.