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 2H 2 + O 2  2H 2 O Total mass of Reactants must equal the total mass of Products

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 Must be reduced 2H 2 + O 2  2H 2 O

Balancing Reactions 4Na + O 2  2Na 2 O 2Al + 3Br 2  2AlBr 3 4Ni + 3O 2  2Ni 2 O 3 2HNO 3 + Ca(OH) 2  Ca(NO 3 ) 2 + 2H 2 O

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 A + B  AB 2Mg + O 2  2MgO

Decomposition Chemical change in which a single compound breaks down into two or more simpler products 1 reactant AB  A + B 2NaCl  2Na + Cl 2

Single Replacement Chemical change in which one element replaces a second element in a compound Metal replaces metal (hydrogen included) Nonmetal replaces nonmetal A + BC  AC + B A + BC  BA + C Zn + 2HCl  ZnCl 2 + H 2

Double Replacement Chemical change involving an exchange of positive ions between compounds AB + CD  AD + CB AgNO 3 + NaCl  NaNO 3 + 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 + O 2  2MgO

Combustion Complete combustion of a hydrocarbon produces carbon dioxide and water 2C 2 H 6 + 7O 2  4CO 2 + 6H 2 O

Chemical Changes Indications of a chemical change Transfer of Energy Change in temperature Production of Light Formation of a gas Formation of a precipitate Change in color

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? ZnCO 3 + Mg  MgCO 3 + Zn PbSO 4 + Mn  MnSO 4 + Pb 3KNO 3 + Al  Al(NO 3 ) 3 + 3K 2HCl + Zn  ZnCl 2 + H 2 2NaBr + I 2  2NaI + Br 2 YES NO YES NO

Spontaneous Reactions A double replacement reaction will only occur if: A precipitate (solid) is produced A liquid is produced H 2 O(l) A gas is produced

Table F

Spontaneous or not? NaCl + AgNO 3  NaNO 3 + AgCl K 2 CO 3 + MgSO 4  K 2 SO 4 + MgCO 3 NH 4 OH + NaNO 3  NaOH + NH 4 NO 3 Yes No

Stoichiometry Calculations of quantities in chemical reactions

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

Process for Solving Determine Reaction Products 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? ratio X amount X = 5 eggs

Chemical Example How many moles of NH 3 can be made with 6 moles H 2 and excess N 2 ? ratio 132 6X amount X = 4 mol NH 3 =

Chemical Example How many moles of N 2 are needed to produce 7 moles of NH 3 ? ratio 132 7X amount X = 3.5 mol N 2

Chemical Example How many moles of O 2 are needed to produce 9 moles of H 2 O? ratio X amount X = mol O 2 4

Conservation of Energy Reactions must maintain conservation of energy Energy term written in reaction

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 How much energy is produced when 6 moles H 2 reacts with excess N 2 ? ratio 132 6X amount 91.8 X =183.6 kJ