Chemical Reactions

A chemical reaction is a process by which one or more substances are changed into one or more different substances. Evidence of a chemical reaction might include: A permanent color change Formation of a gas Formation of a precipitate Absorption or release of energy. This may include heat, sound, light and electrical energy.

Chemical Equations Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The numbers in the front are called (stoichiometric) coefficients The letters (s), (g), (aq) and (l) indicate the physical states of compounds.

Chemical Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---give---> 2 molecules of Al 2 O 3 OR… 2 molecules of Al 2 O 3 OR… 4 moles of Al + 3 moles of O 2 ---give---> 2 moles of Al 2 O 3 2 moles of Al 2 O 3

Chemical Equations Because the same atoms are present in a reaction at the beginning and at the end, the amount of matter in a system does not change.Because the same atoms are present in a reaction at the beginning and at the end, the amount of matter in a system does not change. The Law of the Conservation of MatterThe Law of the Conservation of Matter

Chemical Reactions In a chemical equation, reactants (the items on the left side) change into products (the items on the right side). Rusting: Iron + oxygen --> iron (III) oxide This equation is a chemical sentence.

Chemical Reactions Symbols Used in Chemical Equations +Used to separate reactants or products Yield sign: Separates reactants and products.

Chemical Reactions Pt A formula above a yield sign tells you a catalyst was used - Pt in this case. A catalyst is a substance that speeds up a reaction without being used up in the reaction. Ex. MnO 2 with decomposition of H 2 O 2. heat Indicates heat is supplied to reaction. Often a triangle may be used rather than the word.

Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Chemical Equations Lavoisier, 1788

Chemical Reactions Rules for balancing equations: The law of conservation of mass must be followed. In order to do this, then each side of the equation must have the same number of atoms. We do this by adding coefficients to the reactants and products. (Always in WHOLE NUMBERS!) ___BaCl 2(aq) + ___ H 3 PO 4(aq) --> ___Ba 3 (PO 4 ) 2(s) + ___HCl (aq)

Rules for Balancing Equations ___BaCl 2(aq) + ___H 3 PO 4(aq) --> ___Ba 3 (PO 4 ) 2(s) + ___ HCl (aq) Rules for Balancing Equations: 1.Be sure you have correct formulas for reactants and products. 2.If the same complex ion appears on both sides, then treat it as one group.

Rules for Balancing Equations 3. Check for 2 to 3 ratios – do them first! Balance highest (noncomplex ion) #’s next 4.Save single elements for last. 5. Check all numbers once you’re done… 3BaCl 2(aq) +2 H 3 PO 4(aq) > 1 Ba 3 (PO 4 ) 2(s) + 6 HCl (aq)

Balancing Equations ___ Al(s) + ___ Br 2 (l) ---> ___ AlBr 3 (s)

Balancing Equations 2 Al(s) + 3 Br 2 (l) ---> 2 AlBr 3 (s)

Balancing Equations __Ca 3 P 2 + __ Na --  __Ca + __ Na 3 P __LiOH(aq) + __H 2 (g)  __Li (s) + _HOH(l)

1 Ca 3 P Na ---> 3 Ca + 2 Na 3 P 2 LiOH(aq) + 1 H 2 (g) -----> 2 Li (s) + 2 HOH(l)