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Reactions and Stoichiometry
Chapters 11-12
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General Reaction Format
Reactants Products Starting Material Ending Material Science equivalent of Math’s =
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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 Total mass of Reactants must equal the total mass of Products
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Balancing Reactions Reactions must maintain conservation of mass, charge, and energy Reactants must have the same total charge as Products Cu Fe+3 Cu Fe+2
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Balancing Reactions To balance a reaction:
Do Chemistry (determine products) Do NOT change chemistry (compounds, subscripts) Only change coefficients (whole numbers in front of chemicals) Must be reduced 2H2 + O2 2H2O
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Balancing Reactions 4Na + O2 2Na2O 2Al + 3Br2 2AlBr3
4Ni + 3O2 2Ni2O3 2HNO3 + Ca(OH)2 Ca(NO3) H2O
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Reaction Types Synthesis Decomposition Single Replacement
Double Replacement Combustion
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Synthesis Chemical change in which two or more substances react to form a single new substance (1 product) Also called Combination (old) A + B AB 2Mg + O2 2MgO
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Decomposition Chemical change in which a single compound breaks down into two or more simpler products 1 reactant AB A + B 2NaCl 2Na + Cl2
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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 ZnCl2 + H2
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Double Replacement Chemical change involving an exchange of positive ions between compounds AB + CD AD + CB AgNO3 + NaCl NaNO3 + AgCl
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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
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Combustion Complete combustion of a hydrocarbon produces carbon dioxide and water 2C2H O2 4CO H2O
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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
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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
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Table J The more active element: Does not want to be alone
Wants to be combined with someone else
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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
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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
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Table F
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Spontaneous or not? NaCl + AgNO3 NaNO3 + AgCl
K2CO3 + MgSO4 K2SO4 + MgCO3 NH4OH + NaNO3 NaOH + NH4NO3 Yes Yes No
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Stoichiometry Calculations of quantities in chemical reactions
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Food Example 8 X How many pancakes can be made with 8 cups flour, excess eggs and milk? X = 20 pancakes
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Process for Solving Determine Reaction Products Balance reaction
Place numbers from question above coefficients in reaction (Setting up proportion) Solve for X using a proportion
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Food Example How many eggs are needed to make 25 pancakes? X 25
X = 5 eggs
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Chemical Example 6 X How many moles of NH3 can be made with 6 moles H2 and excess N2? X = 4 mol NH3
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Chemical Example X 7 How many moles of N2 are needed to produce 7 moles of NH3? X = 3.5 mol N2
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Chemical Example X 9 How many moles of O2 are needed to produce 9 moles of H2O? X = mol O2
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Conservation of Energy
Reactions must maintain conservation of energy Energy term written in reaction
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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
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Example 6 X How much energy is produced when 6 moles H2 reacts with excess N2? X =183.6 kJ
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