Day 18 Balancing Chemical Equations Sci 10 Chemistry

5H2 Review What number represents the Coefficient? _____ What number represents the Subscript? _____ What element is represented by the letter "H"? _____ How many "H's" do you have? _____

Counting Atoms How many of each type of atom are present in the compounds below? H2SO4 CaCl2 4NaF 2KNO3 3K2SO4 2Mg3(PO4)2 Type of Atom # of Atoms

Law of Conservation of Mass Law of conservation of mass: mass of products = mass of reactants Video: http://www.youtube.com/watch?v=dExpJAECSL8 the # of atoms of each type in the reactants = the # of atoms of each type in the products Matter can’t be created or destroyed: atoms can’t be gained or lost during a chemical reaction A + B → C + D “reacts with” Reactants Products “produces/ gives”

Balancing Chemical Equations Skeleton equation: CH4(g) + O2(g)  CO2(g) + H2O(l) The skeleton equation needs coefficients to balance both sides. Balanced equation: CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) Subscripts show the # of atoms *YOU CANNOT CHANGE THESE TO BALANCE AN EQUATION State of matter (aq) = aqueous/dissolved in water (s) = solid ( l ) = liquid (g) = gas Coefficients indicate the ratio of compounds in the reaction here, there is twice as much NO and NO2 than there is O2 5

Types of Chemical Equations Word Equation (words only, no formulas): Methane + oxygen  carbon dioxide + water Skeleton Equation (formulas, but not balanced): CH4(g) + O2(g)  CO2(g) + H2O(l) Balanced Equation:

Remember: You can only change the coefficients (the number in front) to balance You can not have coefficients that are fractions Double check to make sure coefficients can’t be reduced

Examples Hg + O2  HgO Aluminum reacts with oxygen to form aluminum oxide. CaO + H2O  Ca(OH)2

Strategies for Balancing Equations Balance chemical equations by following these steps: Trial and error will work, but can be very inefficient Balance compounds first, elements last Balance one compound at a time Only add coefficients; NEVER change subscripts! If H and O appear, attempt to balance them LAST Polyatomic ions (such as SO42–) can be balanced as a whole group. They don’t break apart! Always double-check after you think you are finished! 9

More examples Balance the following: __ N2 (g) + ___ H2 (g) → ___ NH3 (g) __ N2 (g) + _3_ H2 (g) → _2_ NH3 (g) __ Fe (s) + ___ H2SO4 (aq) → ___ H2 (g) + Fe2(SO4)3 (aq) 2_ Fe (s) + _3_ H2SO4 (aq) → _3_ H2 (g) + Fe2(SO4)3 (aq)

Balancing Word Equations Write the skeleton equation. Remember to ensure ionic compounds have a neutral charge (+ and – charges must add to zero) Then balance Check coefficients for common factors. If yes, reduce them.

Try It! sodium + chlorine  sodium chloride iron (II) oxide  iron + oxygen methane + oxygen  carbon dioxide + water copper (I) oxide  copper + oxygen

Try balancing these: Stop & Think! Fe + Br2  FeBr3 CaCl2 + Na  NaCl + Ca Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4 C2H6 + O2  CO2 + H2O

To Do: Day 18 Practice 1 Text: p.133 #13, 18 p.137 #8, 9, 11-18