2Question… What happens to the ice cream you (or I!) eat?? What happens to the gasoline you put into your car?Do they just disappear?Ok, so where do their atoms go?They are rearranged into new substances!!So when we say we’ve “used something up”…have we?
3Conservation Atoms are FOREVER!! #'s & types of atoms in Products Law of Conservation of Matter—in a chemical reaction, matter can neither be created nor destroyed – just rearranged.The numbers & kinds of atoms present in the products are the same as those in the reactants.Balanced equations illustrate this law.#'s & types of atomsin Products=in Reactants
4Reactants --> Products Why?Chemical reactions cannot create or destroy atoms (they can only be rearranged), chemical equations must be balanced.The atoms involved as reactants (Left of arrow) must still have the same number as on the product side (right of arrow), but they can be a part of new molecules.Reactants --> Products
5Atom InventoryList atoms in reactants and products under the arrowDetermine the number of each atom.Subscripts—show a ratio of atoms to each other in a particular compound.H2O =‘s 2 hydrogens bonded to 1 oxygenHOSubscripts are NEVER changed when balancing!Coefficients—(#’s in front of compound) show how many of each compound is present H2O =‘s 2 watersHO
6Example of Atom Inventory: CH4 + 2 O2 CO2 + 2 H2OReactants Products____ C ________ H ________ O ____114444What does it mean if there is no coefficient in front of the compound?Is this equation balanced?Yes!!!It is an implied “1”!
7Close, but not yet Balanced! Answers to BS 62 Methane gas (CH4) reacts with oxygen gas (O2) to produce carbon dioxide gas (CO2) & water vapor (H2O)CHO++1 C 1H 4O 4Balanced!3. HBr + Mg H2 + MgBr2Not Balanced!Close, but not yet Balanced!4Ag + 4H2S +O2 2Ag2S + 4H2O
8Cellulose reacts with oxygen gas to form carbon dioxide gas & liquid water. C6H10O5 + 6O2 6CO2 + 5H2O6 C 6H 10O 17Balanced!!!Nitroglycerin decomposes to form nitrogen gas, oxygen gas, carbon dioxide gas & water vapor2 C3H5(NO3)3 3N2 + O2 + 6CO2 + 5H2O6 C 6H 106 N 6O 19Not Balanced!
9Steps to Balance Do an atom inventory. Balance by using coefficients to show multiple number of elements or compounds.(NEVER change subscripts to balance…is H2O the same as H2O2? If so, would you like a big cold glass of hydrogen peroxide?!)(Coefficients only inserted in front of compound, never split compound or the ratio would D)Recount both sides.
10Example: H2 (g) + O2 (g) H2O (l) Atom Inventory:Reactants Products2 H 22 O 1Insert coefficients to balance3. Double check to make sure it is all BALANCED!442
11Is this equation balanced? Al O2 Al2O3432Reactants Products____ Al ________ O ____1-- 4-- 422-- 6-- 63Do an atom inventoryNoIs this equation balanced?Use coefficient to multiply COMPOUND with element in it by # that balances it.
12Additional RulesPolyatomic ions, (like NO3- and CO32-) that appear on both sides of equation should be balanced as units rather than balancing their atoms individuallyAgNO3 + Cu ---> Cu(NO3) AgBalance atoms in pure elements w/ no subscripts lastRecount all atoms one last time to be sure you are correct!22
13Balancing EquationsFor labs, demos, and whenever possible, indicate the states of each element or compound.(s) solid (g) gas(l) liquid (aq) aqueousDiatomic Elements?H O F Br I N ClRemember these elements need a subscript of 2 if in elemental state. If in compound then criss- cross charges to get subscripts
15Balanced Equations show Proportions Coefficients show how many of each compound reacts with, or creates another substance.Does that mean ONLY 2 hydrogen molecules will react with only 1 oxygen molecule?NO!! Could be 250 H2 react with 125 O2
17Example: 2 H2 (g) + O2 (g) 2 H2O (l) Does that mean ONLY 2 hydrogen molecules will react with only 1 oxygen molecule?NO!! Could be 250 H2 react with 125 O2
18Types of Chemical Reactions SynthesisDecompositionSingle replacement (displacement)Double replacement (displacement)CombustionExamplesA + B ABN2 + 3H2 2NH3H2O2 H2 + O2AB A + BA + BC AC + BZn + 2HCl ZnCl2 + H2A + BD BA + DAB + CD AD + CBNaCl + AgNO3 NaNO3 + AgClCxHy + O2 CO2 + H2OCH4 + 2O2 CO2 + 2H2O
196. C8H18 + O2 CO2 + H2O What could we do? 16 18 2 25 8 9 16 +9 = 25 O’s – to be put in pairs on the other side???What could we do?Double everything…???What you do to 1 thing, you have to do to everything…