Unit 8: Reactions And Stoichiometry

What is a chemical reaction? Chemical Reaction: one or more substances are made into one or more NEW substances (bonds are broken and/or formed) Reactant: a substance that undergoes a change in a chemical rxn(LEFT side) Product(s): what is formed during chemical rxn (RIGHT side) Arrow means “yield”

Balancing Chemical Equations **LAW OF CONSERVATION Mass (matter), charge and energy are always conserved in a chemical rxn Mass at the beginning = mass at the end Same kind and number of atoms on each side of the reaction

Word equation  formula equation Solid potassium chlorate is heated in the presence of a manganese (IV) oxide catalyst to form solid potassium chloride and oxygen gas.

Balancing Rules Adjust coefficients ONLY!!! NEVER touch the subscripts. Report the final answer using the smallest, whole # coefficients. Tip: Keep PAIs together as a unit and balance these first!

Balancing Practice (ex) ____Ag + ____AuCl3  ____AgCl + ____Au (ex) ____Pb(NO3)2 +____NaCl ____PbCl2 + ____NaNO3 **(ex) ____CH4 + ____O2  ____CO2 + ____H2O (ex) ____Fe + ____O2  ___Fe2O3

Types of Reactions Combustion Decomposition Synthesis Double Replacement Single Replacement

1. Synthesis (Combination) Reaction Two or more reactants combine to form a single product A + B  AB

2. Decomposition One reactant breaks apart into two or more simpler substances AB  A + B

3. Single Replacement Both the reactant and product side consist of 1 element and 1 compound

4. Double Replacement Both the reactant and product side have 2 compounds

5. Combustion CH4 + 2O2 CO2 + 2H2O Hydrocarbon + O2  CO2 + H2O Chemical change in which an element or a compound reacts with oxygen, producing energy Often, the reactant is a hydrocarbon (contains hydrogen and carbon) Hydrocarbon + O2  CO2 + H2O CH4 + 2O2 CO2 + 2H2O

Writing Synthesis Reactions Magnesium and oxygen  Aluminum and chlorine 

Table J: SR Reactions More reactive metals replace less reactive metals K + NaCl  KCl + Na Na + KCl  NR More reactive non-metals replace less reactive non-metals Cl2 + 2 NaI  2 NaCl + I2 I2 + 2NaCl  NR

Practice 1) Zn (s) + CaF2 (aq)  2) Mg (s) + Al(NO3)3 (aq) 3) Br2 (l)+ NaI (aq)  4) Na (s) + Al2(SO4)3 (aq)  Exit Slip: Ca + HBr  F2 + KCl 

Writing Double Replacement Rxns Two ionic compounds exchange ions to form 2 new compounds CuSO4 (aq) + Na2S (aq)  _______ + ________ K2CO3 (aq) + Pb(NO3)2 (aq)  _______ + _______ NaCl (aq) + NH4NO3 (aq)  _______ + _______

Double Replacement Demo

Practice Using Table F: Determine if the following are soluble (aq) or insoluble (s): PbCl2 Mg3(PO4)2 Ba(OH)2 Sr(ClO3)2 Na2CO3

Warm-Up Predict the products of the following single replacement rxn. Write both a word equation and a complete, balanced chemical equation. aluminum + copper (II) chlorate 

Stoichiometry The calculation of quantities in chemical reactions is called stoichiometry

Using Mole Ratios A mole ratio is a conversion factor made from the coefficients of a balanced equation

Stoichiometry Problems: Summary Mole  Mole Grams  Moles or Moles  Grams Grams  Grams

Mole  Mole 4Fe (s) +O2 (g)  2Fe2O3 (s) How many moles of iron are required to produce 1 mol of iron (II) oxide? How many moles of oxygen gas are required to produce 4 mol of iron (II) oxide? How many grams of iron (II) oxide are produced when 1.29 mol oxygen gas react completely?

Gram  Gram 4Fe (s) +O2 (g)  2Fe2O3 (s) How many grams of iron are required to produce 66.7 grams of Fe2O3?

Table I Stoichiometry Shows the ΔH for various different reactions (combustion, synthesis, dissolving) ΔH (heat of reaction) Negative sign indicates heat is released (exo) Positive sign indicates heat is absorbed (endo) To find ΔH for the reverse of ANY reaction, switch the sign of ΔH

Table I Problems 1) How much heat is released during the combustion of 0.450 mol C3H8 (g)? 2) How much heat is absorbed when forming 1.20 mol of HI from its elements? 3) How much heat is absorbed when forming 62.3 g NO2 (g) from its elements?

Limiting and Excess Reactants (Reagents) Limiting- the substance used up first Excess means that substance is NOT used up completely in the rxn

Example: Limiting Reactant Problem If 28 g of Fe react with 24 g of S to produce FeS What is the limiting reactant? How many grams of FeS are produced? How many grams of excess reactant?

Percent Yield Theoretical yield: the maximum amount of product that could be formed from the given amounts of reactants (mass:mass calc.) Actual yield: the amount of product that actually forms when the rxn is carried out in the lab (measured result)

Percent Yield Percent Yield= experimental yield x 100% theoretical yield