Principles of Chemistry and Physics

Chemical Reactions  When one or more substances (reactants) are changed into one or more new substances (products), a CHEMICAL REACTION has occurred and can be represented as a chemical equation Reactants  products

Chemical Equations  Replaces words with chemical formulas skeleton equations- does not indicate the relative amounts of reactants and products  Here is the skeleton equation for rusting:  Fe + O 2  Fe 2 O 3

Evidence a reaction has occurred:  Color change  Gas released  Precipitate formed  Temperature change (endo or exothermic)  Odor produced  Smoke  Light  Flames  pH change  Flammable to nonflammable or vice versa

Types of Reactions There are five types of chemical reactions: 1. Synthesis reactions 2. Decomposition reactions 3. Single displacement reactions 4. Double displacement reactions 5. Combustion reactions Unit objective: identify the type of reaction and predict the product(s).

1. Synthesis reactions Synthesis reactions occur when two substances (generally elements) combine to form a compound. reactant + reactant  1 product Basically: A + B  AB Example: 2H 2 + O 2  2H 2 O Example: C + O 2  CO 2

Synthesis Reactions Another example of synthesis:

2. Decomposition Reactions Decomposition reactions occur when a compound breaks up into the elements or into a few simpler compounds 1 Reactant  Product + Product In general: AB  A + B Example: 2 H 2 O  2H 2 + O 2 Example: 2 HgO  2Hg + O 2

Decomposition Reactions Another view of a decomposition reaction:

Decomposition Reaction AB  A + B

3. Single Replacement Reactions Single Replacement Reactions occur when one element replaces another in a compound. A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). element + compound  element + compound A + BC  AC + B (if A is a metal) OR A + BC  BA + C (if A is a nonmetal) (remember the cation always goes first!) When H 2 O splits into ions, it splits into H + and OH - (not H+ and O -2 !!)

Single Replacement Reactions Another view:

Single Replacement Example Example: Zn + CuCl 2  Zn Cl Cu + General: AB + C  AC + B Cl Zn Cu + LIKE replaces LIKE

Activity Series lists metals in order of decreasing reactivity. As a general rule, more reactive metals replace less reactive metals in a compound Li K Ba Ca Na Mg Al Zn Fe Cd Ni Sn Pb H Cu Hg Ag Au Li K Ba Ca Na Mg Al Zn Fe Cd Ni Sn Pb H Cu Hg Ag Au

4. Double Replacement Reactions Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound Compound + compound  compound+ compound AB + CD  AD + CB

Decomposition reactions cont..  Solubility rules-  The formation of a precipitate is a driving force of a double replacement reaction

5. Combustion Reactions Combustion reactions occur when a fuel reacts with oxygen gas, which produces heat! Fuel + O 2 (+ Heat)  Product

Hydrocarbon Combustion Reactions Hydrocarbon Combustion: C x H y + O 2  CO 2 + H 2 O Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by- products like carbon monoxide) Combustion is used to heat homes (CH 4 )and run automobiles (octane: C 8 H 18 )

Carbon Monoxide Effects Edgar Allen Poe’s drooping eyes and mouth are potential signs of CO poisoning.

 Exothermic process – a process that results in the evolution of heat- energy flows out of the system  Endothermic process- a process that absorbs energy from the surroundings- energy flows into the system

Exothermic or endothermic?  1. Your hand gets cold when you touch ice  2. ice melts when you touch it  3. Ice cream melts  4. Propane is burning in a propane torch.  5. Water drops on your skin evaporate after swimming  6. Two chemicals mixing in a beaker give off heat

 Exothermic  Endothermic  Exothermic  Endthermic  exothermic

Writing Chemical Equations  Word Equations  Names of reactants on the left of an arrow separated by plus signs  Names of products to the right of the arrow separated by plus signs  Ex: flour + water + yeast + salt  bread - Ex: carbon + oxygen  carbon dioxide

Diatomic Elements  Some elements exist naturally in pairs, as diatomic molecules. You will be expected to memorize these: Br 2, I 2, N 2, Cl 2, H 2, O 2, F 2.

Law of Conservation of Mass  Mass is never created or destroyed-ALL must be conserved and accounted for during a chemical reaction  The same number of atoms of reactant elements must equal the atoms of product elements

Rules for balancing equations:  Write correct skeleton formula  Determine number of atoms of each element of reactants and products. COUNT POLYATOMIC ION AS A SINGLE UNIT if it appears unchanged on both sides of the equation  Balance elements one at a time by using coefficients-never change subscripts  Begin with the easiest elements first  Check both sides to see if they match  Make sure coefficients are in the lowest possible ratio

Counting with Moles  Chemists use the unit mole to measure the amounts of small particles  1 mole = 6.02 x particles (atoms, molecules) 6.02 x is known as Avogadro’s number

 The molar mass of any two elements contain the same number of atoms  Ex: a dozen apples – 12 apples  a dozen oranges – 12 oranges

Molar mass  Molar mass is the amount of one mole of that element or compound (use the periodic table)  Once you know the molar mass of the compound, you can convert moles of that substance into moles

Molar Mass  Ex: the molar mass of one mole of CO2 is 44 g this means that 44 g CO21 mole 1 mole44 g CO2 So, if I have 55 g of CO2, how many moles do I have?