Reactions and Stoichiometry Chapters 11-12

Reactions Reactants Products

Balancing Reactions Reactions must maintain conservation of mass, charge, and energy Reactants and Products must have the same number of atoms of each element 2H2 + O2  2H2O

Balancing Reactions Reactions must maintain conservation of mass, charge, and energy Reactants must have the same total charge as Products Cu+1 + Fe+3  Cu+2 + Fe+2

Balancing Reactions To balance a reaction: Do NOT change chemistry (compounds, subscripts) Only change coefficients (big numbers in front of chemicals) Coefficients can only be whole numbers 2H2 + O2  2H2O

Balancing Reactions 4Na + O2  2Na2O 2Al + 3Br2  2AlBr3 4Ni + 3O2  2Ni2O3 2HNO3 + Ca(OH)2  Ca(NO3)2 + 2H2O

Reaction Types Synthesis Decomposition Single Replacement Double Replacement Combustion

Synthesis Chemical change in which two or more substances react to form a single new substance (1 product) Also called Combination 2Mg + O2  2MgO

Decomposition Chemical change in which a single compound breaks down into two or more simpler products 1 reactant NaHCO3  NaOH + CO2

Single Replacement Chemical change in which one element replaces a second element in a compound Metal replaces metal (hydrogen included) Nonmetal replaces nonmetal Zn + 2HCl  ZnCl2 + H2

Double Replacement Chemical change involving an exchange of positive ions between compounds AgNO3 + NaCl  NaNO3 + AgCl

Combustion Chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of heat and light 2Mg + O2  2MgO

Combustion Complete combustion of a hydrocarbon produces carbon dioxide and water 2C2H6 + 7O2  4CO2 + 6H2O

Spontaneous Reactions A single replacement reaction will only occur if: The single element in the reactants is more active than the element it replaces in the compound

Table J The more active element: Does not want to be alone Wants to be combined with someone else

Spontaneous or not? ZnCO3 + Mg  MgCO3 + Zn PbSO4 + Mn  MnSO4 + Pb 3KNO3 + Al  Al(NO3)3 + 3K 2HCl + Zn  ZnCl2 + H2 2NaBr + I2  2NaI + Br2 YES YES NO YES NO

Spontaneous Reactions A double replacement reaction will only occur if: A precipitate (solid) is produced A liquid is produced H2O(l) A gas is produced

Table F

Spontaneous or not? NaCl + AgNO3  NaNO3 + AgCl K2CO3 + MgSO4  K2SO4 + MgCO3 NH4OH + NaNO3  NaOH + NH4NO3 Yes Yes No

Stoichiometry Goal The role of coefficients in balanced formula equations Mole ratios in a balanced formula Mole-mole stoichiometry problems

Stoichiometry Calculations of quantities in chemical reactions

Food Example 8 amount X 4 2 1 10 ratio X = 20 pancakes How many pancakes can be made with 8 cups flour, excess eggs and milk?

Process for Solving Balance reaction Copy coefficients from reaction into ratio row Place numbers from question in row above ratio row (Setting up proportion) Solve for X using a proportion

Food Example How many eggs are needed to make 25 pancakes? 25 X 4 2 1 amount X 4 2 1 10 ratio X = 5 eggs How many eggs are needed to make 25 pancakes?

Chemical Example 6 X amount = 1 3 2 ratio X = 4 mol NH3 How many moles of NH3 can be made with 6 moles H2 and excess N2?

Chemical Example X 7 amount 1 3 2 ratio X = 3.5 mol N2 How many moles of N2 are needed to produce 7 moles of NH3?

Chemical Example X 9 amount 1 5 3 4 ratio X = 11.25 mol O2 How many moles of O2 are needed to produce 9 moles of H2O?

Endo/Exothermic Endothermic – Energy is absorbed Energy term is on the left side Exothermic – Energy is released Energy term is on the right side Treat just like a coefficient

Example 6 X amount ratio 1 3 2 91.8 X =183.6 kJ How much energy is produced when 6 moles H2 reacts with excess N2?