Atom Inventory & Balancing Equations Reactants --> Products

Question… 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?

Conservation 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 atoms in Products = in Reactants

Reactants --> 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

Atom Inventory List atoms in reactants and products under the arrow Determine 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 oxygen H O Subscripts are NEVER changed when balancing! Coefficients—(#’s in front of compound) show how many of each compound is present. 2H2O =‘s 2 waters H O

Example of Atom Inventory: CH4 + 2 O2  CO2 + 2 H2O Reactants Products ____ C ____ ____ H ____ ____ O ____ 1 1 4 4 4 4 What does it mean if there is no coefficient in front of the compound? Is this equation balanced? Yes!!! It is an implied “1”!

Close, but not yet Balanced! Answers to BS 6 2 Methane gas (CH4) reacts with oxygen gas (O2) to produce carbon dioxide gas (CO2) & water vapor (H2O) C H O + + 1 C 1 4 H 4 4 O 4 Balanced! 3. HBr + Mg  H2 + MgBr2 Not Balanced! Close, but not yet Balanced! 4Ag + 4H2S +O2  2Ag2S + 4H2O

Cellulose reacts with oxygen gas to form carbon dioxide gas & liquid water. C6H10O5 + 6O2  6CO2 + 5H2O 6 C 6 10 H 10 17 O 17 Balanced!!! Nitroglycerin decomposes to form nitrogen gas, oxygen gas, carbon dioxide gas & water vapor 2 C3H5(NO3)3  3N2 + O2 + 6CO2 + 5H2O 6 C 6 10 H 10 6 N 6 18 O 19 Not Balanced!

Steps 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.

Example: H2 (g) + O2 (g)  H2O (l) Atom Inventory: Reactants Products 2 H 2 2 O 1 Insert coefficients to balance 3. Double check to make sure it is all BALANCED! 4 4 2

Is this equation balanced? Al + O2  Al2O3 4 3 2 Reactants Products ____ Al ____ ____ O ____ 1 -- 4 -- 4 2 2 -- 6 -- 6 3 Do an atom inventory No Is this equation balanced? Use coefficient to multiply COMPOUND with element in it by # that balances it.

Additional Rules Polyatomic ions, (like NO3- and CO32-) that appear on both sides of equation should be balanced as units rather than balancing their atoms individually AgNO3 + Cu ---> Cu(NO3)2 + Ag Balance atoms in pure elements w/ no subscripts last Recount all atoms one last time to be sure you are correct! 2 2

Balancing Equations For labs, demos, and whenever possible, indicate the states of each element or compound. (s) solid (g) gas (l) liquid (aq) aqueous Diatomic Elements? H O F Br I N Cl Remember these elements need a subscript of 2 if in elemental state. If in compound then criss- cross charges to get subscripts

Practice NaCl + H2SO4  Na2SO4 + HCl 2 NaCl + H2SO4  Na2SO4 + 2 HCl BaF2 + Al(OH)3  Ba(OH)2 + AlF3 3 BaF2 + 2 Al(OH)3  3 Ba(OH)2 + 2 AlF3 C2H6 + O2  CO2 + H2O 2C2H6 + 7O2  4CO2 + 6H2O (had to double everything to balance oxygen)

Balanced 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

Example: 2 H2 (g) + O2 (g)  2 H2O (l) 2 H2 molecules produce 2 H2O molecules 1 O2 molecule produces 2 H2O molecules 2 H2 molecules react with 1 O2 molecules

Example: 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

Types of Chemical Reactions Synthesis Decomposition Single replacement (displacement) Double replacement (displacement) Combustion Examples A + B  AB N2 + 3H2  2NH3 H2O2  H2 + O2 AB  A + B A + BC  AC + B Zn + 2HCl  ZnCl2 + H2 A + BD  BA + D AB + CD  AD + CB NaCl + AgNO3  NaNO3 + AgCl CxHy + O2  CO2 + H2O CH4 + 2O2  CO2 + 2H2O

6. 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…