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Unit 6Chemical Reactions
Writing Chemical Equations• All chemical reactions have two parts:• ReactantsReactants - the substances you start with (on
left side of arrow)• Products Products - the substances you end up with (on
right side of arrow)• The reactants turn into the products.
Reactants Products
In a chemical reaction…
• The way atoms are joined is changed.• Atoms aren’t created or destroyed; they just
combine together in new ways.– Law of Conservation of Mass
• Can be described using sentences, word equations, or skeleton equations (symbols):
#1. In a sentence every item is a word Copper reacts with chlorine to form
copper (II) chloride.#2. In a word equation symbols used w/
element or compound namesCopper + chlorine copper (II) chloride
#3. In a skeleton equation formulas and symbols are used to describe a reaction
Cu + Cl2 CuCl2– These are NOT balanced (# of atoms on each side is
not necessarily equal)
Symbols used in Equations• The arrow () separates the reactants from the
products; means “reacts” or “yields”
• The plus sign (+) = between products or between reactants; means “and”
• Subscripts are used to describe the number of atoms in a FORMULA.
• Coefficients are used to describe the number of molecules in the REACTION. They are the only things changed when balancing a reaction.
Subscripts vs. Coefficients
3 CuCl2 + 2 Al 2 AlCl3 + 3 Cu
On the reactant side of the equation:How many copper (II) chloride molecules react?
How many total chlorine atoms are there?
How many aluminum atoms are there?
States of Matter• Solid--(s) after the formula
– Precipitate -- a solid formed in a reaction
• Gas--(g) after the formula
• Liquid—(l) after the formula
• Aqueous-- (aq) after the formula - dissolved in water.
Other Symbols used in Equations
• indicates a reversible reaction (More later)
• show that heat is supplied to the reaction
• is used to indicate a catalyst used or supplied, in this case, platinum.
heat ,
Pt
Remember HON17 !!!
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
Write a skeleton equation for:Solid iron (III) sulfide reacts with gaseous hydrogen
chloride to form solid iron (III) chloride and
hydrogen sulfide gas.
Types of Reactions• There are probably millions of
reactions.• We can’t remember them all, but luckily
they will fall into several categories.• We will learn the 5 major types.
– We will then be able to predict the products of a reaction when given just the reactants!
Types of Equations
Synthesis (or Combination)Equation in Symbols: A + B ABSample Equation:
2Cu (s) + O2 (g) 2 CuO (s)
2 or more elements combine to form only 1 product
Types of Equations
DecompositionEquation in Symbols: AB A + BSample Equation:
2 NH3 (g) N2 (g) + 3 H2 (g)
1 reactant breaks down into more than 1 product
Types of EquationsSingle ReplacementEquation in Symbols: A + BC AB + CSample Equation:
Mg (s) + CuCl2 (aq) Cu (s) + MgCl2 (aq)
• Metal replaces metal; Nonmetal replaces nonmetal– Positive ion replaces positive ion; negative ion
replaces negative ion!
Types of Equations
Double Replacement – Equation in Symbols: AX + BY BX + AY
Sample Equation: 2AgNO3(aq) + CuCl2 (aq) Cu(NO3)2 (aq) + 2AgCl (s)
2 ionic compounds switching “partners”
Types of Equations
CombustionEquation in Symbols:
CxHy + O2 CO2 + H2O
Sample Equation: CH4(g) + O2 (g) CO2 (g) + H2O (l)
Hydrocarbons react to form CO2 and H2O
Examples: Identify the Type of Rxn.1. 2 AgNO3 + Ni Ni(NO3)2 + 2 Ag
2. BaCO3 BaO + CO2
3. 4 Cr + 3 O2 2 Cr2O3
4. Ca + 2 HCl CaCl2 + H2
5. Cu(OH)2 + 2 HC2H3O2 Cu(C2H3O2)2 + 2 H2O
6. C8H18 + O2 CO2 + H2O
Balancing Chemical Equations• Atoms can’t be created or destroyed in
an ordinary reaction:– All the atoms we start with we must end
up with (meaning: balanced!)• A balanced equation has the same
number of atoms of each element on both sides of the equation.
Balancing EquationsChemical “bookkeeping” of atoms
involved in the reaction:
H2(g) + O2(g) H2O(g)
H – 2 O – 2Reactants
H – 2 O – 1Products
COEFFICIENTS must be added so reactant atoms EQUAL product atoms!
Note the imbalance in oxygen atoms
Rules for Balancing Chemical Equations1) Count the number of atoms of each type
of element appearing on both sides
2) Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST!
(HINT: save O until the very last)
3) Double-Check to make sure it is balanced.
• Never change a subscript to balance an equation (You can only change coefficients)– If you change the subscript (formula) you are
describing a different chemical.– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula; they must go only in the front
2NaCl is okay, but Na2Cl is not.
Hints & Tips for Balancing Equations• Take one element at a time, working from left to right
• Save H for next to last and O for last.
• (Shortcut) polyatomic ions that appear on both sides of the equation should be balanced as independent units
• IF EVERYTHING BALANCES EXCEPT FOR O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is a diatomic element)
– Same goes for HON17
Remember HON17 !!!
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
Balancing Equations Practice
Balance the following chemical equations using the appropriate coefficients:
____ Al(s) + _____ Br2 (l) _____ Al2Br6 (s)
Balancing Equations Practice
Balance the following chemical equations using the appropriate coefficients:
____ Na3PO4 + ____ Fe2O3 ____ Na2O + ____ FePO4
Predicting Products - Single Replacement Rxns.
• One element replaces another• Reactants must be 1 element and 1 ionic
compound.• Products will be a different element and a
different compound.
• Na + KCl K + NaCl
• F2 + LiCl LiF + Cl2
(Cations switched)
(Anions switched)
Predicting Products - Single Replacement Rxns.
• Metals will replace other metals (and they can also replace hydrogen)
• K + AlN • Zn + HCl • Think of water as: HOH
– Metals replace the first H, and then combines with the hydroxide (OH).
• Na + HOH
Single Replacement Rxns. Practice:
• Fe + CuSO4
• Pb + KCl
• Al + HCl
Predicting Products - Double Replacement Rxns.
• Two things replace each other.– Reactants must be two ionic compounds, in
aqueous solution
• NaOH + FeCl3 – The positive ions change place.
NaOH + FeCl3 Na+1 Cl-1 + Fe+3 OH-1
NaOH + FeCl3 Fe(OH)3 + NaCl
Complete and balance:__CaCl2 + __NaOH
__CuCl2 + __K2S
__KOH + __Fe(NO3)3
__(NH4)2SO4 + __BaF2
Predicting Products – Combustion Rxns.• Combustion means “add oxygen”• Normally, a compound composed of
only C, H, (and maybe O) is reacted with oxygen (O2) – usually called “burning”
• In a combustion reaction, the products will ALWAYS be CO2 and H2O.
C3H8 + O2 CO2 + H2O
Mixed Practice Examples:• H2 + O2
• H2O
• Zn + H2SO4
• HgO
• KBr + Cl2
• AgNO3 + NaCl
• Mg(OH)2 + H2SO3