Unit 8 Chemical Reactions

What is a Chemical Reaction? When a substance is changed into another substance by chemical means When a substance is changed into another substance by chemical means Reactants = what you start with Reactants = what you start with Products = what you end up with Products = what you end up with

During a Chemical Reaction… Bonds are formed, broken, or both Bonds are formed, broken, or both Atoms are rearranged, NOT created or destroyed Atoms are rearranged, NOT created or destroyed Conservation of Atoms Conservation of Atoms Law of Conservation of Mass Law of Conservation of Mass

Evidence of Chemical Reactions Evolution/production of a gas Evolution/production of a gas Bubbling or odor change Bubbling or odor change Forming a precipitate Forming a precipitate Two clear solutions  cloudy/particles Two clear solutions  cloudy/particles Release or absorb energy Release or absorb energy Change temperature or give off light Change temperature or give off light Color change in reaction system Color change in reaction system

Writing Chemical Equations Reactants  Products Reactants  Products HC 2 H 3 O 2 + NaHCO 3  NaC 2 H 3 O 2 + H 2 O + CO 2 HC 2 H 3 O 2 + NaHCO 3  NaC 2 H 3 O 2 + H 2 O + CO 2 Can also include info about state of matter Can also include info about state of matter s = solid s = solid l= liquid l= liquid g = gas g = gas aq = aqueous solution (water solution) aq = aqueous solution (water solution) HC 2 H 3 O 2 (aq) + NaHCO 3 (s)  NaC 2 H 3 O 2 (aq)+ H 2 O (l) + CO 2 (g) HC 2 H 3 O 2 (aq) + NaHCO 3 (s)  NaC 2 H 3 O 2 (aq)+ H 2 O (l) + CO 2 (g)

Writing Chemical Reactions is used to indicate a catalyst used in the reaction (in this case, it’s platinum) is used to indicate a catalyst used in the reaction (in this case, it’s platinum) What is a catalyst? What is a catalyst? A substance that speeds up a reaction without being changed by the reaction. A substance that speeds up a reaction without being changed by the reaction. Enzymes are biological catalysts. Enzymes are biological catalysts.

Writing Chemical Reactions Word Equations – replaces symbols and formulas with names Word Equations – replaces symbols and formulas with names Cu + Cl  CuCl 2 Cu + Cl  CuCl 2 Copper + chlorine  copper (II) chloride Copper + chlorine  copper (II) chloride Fe + O 2  Fe 2 O 3 Fe + O 2  Fe 2 O 3 Iron + Oxygen → Iron (III) oxide Iron + Oxygen → Iron (III) oxide

Balancing Chemical Equations

Balancing Equations Law of Conservation of Mass Law of Conservation of Mass Mass of products = Mass of reactants Mass of products = Mass of reactants Atoms can’t be created or destroyed Atoms can’t be created or destroyed A balanced equation has the same number of atoms of each element on both sides of the equation. A balanced equation has the same number of atoms of each element on both sides of the equation.

C + O 2  CO 2 C + O 2  CO 2 Start with 1 carbon, 2 oxygens Start with 1 carbon, 2 oxygens End with 1 carbon, 2 oxygens End with 1 carbon, 2 oxygens This equation is already balanced This equation is already balanced C + O O  C O O

C + O 2  CO C + O 2  CO We need one more oxygen in the products. We need one more oxygen in the products. We can’t change the formula, because it describes what actually happens We can’t change the formula, because it describes what actually happens +  O C O O C O CC

Must have started with two C Must have started with two C Balanced equation = 2 C + O 2  2 CO Balanced equation = 2 C + O 2  2 CO +  O CC O CC The O must be used to make another CO The O must be used to make another CO But where did the other C come from? But where did the other C come from? O O

Rules for balancing  Write the correct formulas for all the reactants and products  Count the number of atoms of each type appearing on both sides  Balance the elements one at a time by adding coefficients (the numbers in front)  Check to make sure it is balanced.

Never… Change a subscript to balance an equation. Change a subscript to balance an equation. If you change the formula you are describing a different reaction. If you change the formula you are describing a different reaction. H 2 O is a different compound than H 2 O 2 H 2 O is a different compound than H 2 O 2 Put a coefficient in the middle of a formula Put a coefficient in the middle of a formula 2 NaCl is okay, Na2Cl is not. 2 NaCl is okay, Na2Cl is not.

How do I balance an equation? Start with elements that only appear once on each side. Start with elements that only appear once on each side. CH 4 + O 2  CO 2 + H 2 O CH 4 + O 2  CO 2 + H 2 O If an atom appears more than once on a side, balance it last. If an atom appears more than once on a side, balance it last. CH 4 + O 2  CO 2 + H 2 O CH 4 + O 2  CO 2 + H 2 O

Example H 2 +H2OH2OO2O2  Make a table to keep track of where you are at RP H O Need twice as much O in the product H 2 +H2OH2OO2O2  2 2 Changes the OAlso changes the H 4 Need twice as much H in the reactant 2 Recount 4 The equation is balanced, has the same number of each kind of atom on both sides

Example H 2 +H2OH2OO2O2  RP H O This is the answer Not this

Example N 2 + H 2  NH 3 N 2 + H 2  NH 3

Examples P + O 2  P 4 O 10 P + O 2  P 4 O 10

Examples Al + N 2  Al 2 N 3 Al + N 2  Al 2 N 3

Examples CH 4 + O 2  CO 2 + H 2 O CH 4 + O 2  CO 2 + H 2 O

Examples AgNO 3 + Cu  Cu(NO 3 ) 2 + Ag

Examples Na + H 2 O  H 2 + NaOH Na + H 2 O  H 2 + NaOH

Examples AgNO 3 + H 2 S → Ag 2 S + HNO 3 AgNO 3 + H 2 S → Ag 2 S + HNO 3

Word Equations Iron (III) chloride + calcium hydroxide → Iron (III) hydroxide + calcium chloride FeCl 3 + Ca(OH) 2 → Fe(OH) 3 + CaCl 2

Types of Chemical Rxns Synthesis Synthesis Decomposition Decomposition Single Replacement Single Replacement Double Replacement Double Replacement Combustion Combustion Neutralization Neutralization

Synthesis Reactions 2 elements or compounds combine to make 1 compound 2 elements or compounds combine to make 1 compound 2Ca +O 2  CaO 2Ca +O 2  CaO SO 3 + H 2 O  H 2 SO 4 SO 3 + H 2 O  H 2 SO 4 We can predict the products if there are two elements as reactants We can predict the products if there are two elements as reactants Mg + N 2  MgN Mg + N 2  MgN Mg + N 2  Mg + N 2  Mg 3 N 2

Write and balance Al + O 2  Al + O 2  First step - write the formula First step - write the formula Next – balance the equation Next – balance the equation

Decomposition Reactions One reactant breaks apart into two or more elements or compounds One reactant breaks apart into two or more elements or compounds NaCl Na + Cl 2 NaCl Na + Cl 2 CaCO 3 CaO + CO 2 CaCO 3 CaO + CO 2

Decomposition Reactions We can predict the products if the reactant is a binary compound We can predict the products if the reactant is a binary compound Binary compound = Made up of only two elements Binary compound = Made up of only two elements H 2 O H 2 O H 2 + O 2

Decomposition Reactions HgO HgO

Decomposition Reactions If the reactant compound has more than two elements, you must be given one of the products If the reactant compound has more than two elements, you must be given one of the products The other product will be from the missing pieces The other product will be from the missing pieces NiCO 3 NiO + NiCO 3 NiO + C

Single Replacement One element replaces another One element replaces another Reactants must be an element and a compound. Reactants must be an element and a compound. Products will be a different element and a different compound. Products will be a different element and a different compound. Na + KCl  K + NaCl Na + KCl  K + NaCl F 2 + LiCl  LiF + Cl 2 F 2 + LiCl  LiF + Cl 2

Na + KCl  K + NaCl Na K Cl

F LiCl  2 LiF + Cl 2 F Li Cl F Li Cl Li

Single Replacement Metals replace metals (and hydrogen) Metals replace metals (and hydrogen) Al + CuSO 4  Al + CuSO 4  Zn + H 2 SO 4  Zn + H 2 SO 4  Water - Think of water as HOH Water - Think of water as HOH Metals replace one of the H to combine with hydroxide Metals replace one of the H to combine with hydroxide Na + H 2 O  Na + H 2 O  Na + HOH  Na + HOH 

Single Replacement Single Replacement When it comes to metals, we can actually predict whether a reaction is possible When it comes to metals, we can actually predict whether a reaction is possible Some metals are more active than others Some metals are more active than others More active replaces less active but not vice versa More active replaces less active but not vice versa Can replace an element below it, but not above it on this list. Can replace an element below it, but not above it on this list.

Single Replacement Al + HCl  Al + HCl 

Single Replacement Fe + CuSO 4  Fe + CuSO 4 

Single Replacement Pb + KCl  Pb + KCl 

Single Replacement Nonmetals can replace other nonmetals Nonmetals can replace other nonmetals F 2 + HCl  F 2 + HCl  Br 2 + KCl  Br 2 + KCl 

Double Replacement Two things replace each other Two things replace each other Reactants must be two ionic compounds or acids Reactants must be two ionic compounds or acids NaOH + FeCl 3  NaOH + FeCl 3  The positive ions change place. The positive ions change place. NaOH + FeCl 3  Fe 3+ OH - + Na + Cl - NaOH + FeCl 3  Fe 3+ OH - + Na + Cl - NaOH + FeCl 3  Fe(OH) 3 + NaCl NaOH + FeCl 3  Fe(OH) 3 + NaCl

3NaOH + FeCl 3  Fe(OH) 3 + 3NaCl Na + O-O- H+H+ O-O- H+H+ O-O- H+H+ Fe 3+ Cl -

Complete and balance Assume all of the reactions are possible Assume all of the reactions are possible CaCl 2 + NaOH  CaCl 2 + NaOH  CuCl 2 + K 2 S  CuCl 2 + K 2 S 

Combustion Reactions A compound composed of only C, H, and maybe O is reacted with oxygen A compound composed of only C, H, and maybe O is reacted with oxygen Composed of C & H = hydrocarbon Composed of C & H = hydrocarbon O 2 will always be the second reactant O 2 will always be the second reactant If the combustion is complete, the products will be CO 2 and H 2 O If the combustion is complete, the products will be CO 2 and H 2 O If the combustion is incomplete, the products will be C, CO, & H 2 O If the combustion is incomplete, the products will be C, CO, & H 2 O

Examples Complete combustion of C 4 H 10 Complete combustion of C 4 H 10 Incomplete combustion of C 4 H 10 Incomplete combustion of C 4 H 10

Types of Chemical Reactions Neutralization – Chemical reaction where an acid and base combine to form a salt Neutralization – Chemical reaction where an acid and base combine to form a salt HR + SOH  RS + H 2 O (HOH) HR + SOH  RS + H 2 O (HOH) HCl + NaOH  NaCl + HOH HCl + NaOH  NaCl + HOH