Chemical Reactions

1
Unit 7

• Chemical Reactions

2
Writing Chemical Equations

• All chemical reactions have two parts
• Reactants - the substances you start with (on
  left side of arrow)
• Products - the substances you end up with (on
  right side of arrow)
• The reactants turn into the products.
• Reactants ? Products

3
In a chemical reaction

• The way atoms are joined is changed.
• Atoms arent 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)

4

• 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 names
• Copper 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)

5
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.

6
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?

7
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.

8
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.

9
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
Remember HON17 !!!
10
Write a skeleton equation for

• Solid iron (III) sulfide reacts with gaseous
  hydrogen chloride to form solid iron (III)
  chloride and hydrogen sulfide gas.

11
Types of Reactions

• There are probably millions of reactions.
• We cant remember them all, but luckily they will
  fall into several categories.
• We will learn a) the 5 major types.
• We will be able to b) predict the products.

12
Types of Equations

• Synthesis (or Combination)
• Equation in Symbols A B ? AB
• Sample Equation
• 2Cu (s) O2 (g) ? 2 CuO (s)
• 2 or more elements combine to form only 1 product

13
Types of Equations

• Decomposition
• Equation in Symbols AB ? A B
• Sample Equation
• 2 NH3 (g) ? N2 (g) 3 H2 (g)
• 1 reactant breaks down into more than 1 product

14
Types of Equations

• Single Replacement
• Equation in Symbols A BC ? AB C
• Metal replacing metal
• Nonmetal replacing nonmetal
• Sample Equation
• Mg (s) CuCl2 (aq) ? Cu (s) MgCl2 (aq)

15
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

16
Types of Equations

• Combustion
• Equation in Symbols
• CxHy O2 ?CO2 H2O
• Sample Equation
• CH4(g) O2 (g) ? CO2 (g) H2O (l)
• Hydrocarbons react to form CO2 and H2O

17
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

18
Balancing Chemical Equations

• Atoms cant 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.

19
Balancing Equations
Chemical bookkeeping of atoms involved in the
reaction
H2(g) O2(g) ? H2O(g)
H 2 O 2 Reactants
H 2 O 1 Products
20
Rules for Balancing Chemical Equations

• Count the number of atoms of each type of element
  appearing on both sides
• 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.

21
Never

• 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.

22
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

23
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
Remember HON17 !!!
24
Balancing Equations Practice

• Balance the following chemical equations using
  the appropriate coefficients
• ____ Al(s) _____ Br2 (l) ? _____ Al2Br6 (s)
• ____ Na3PO4 ____ Fe2O3 ? ____ Na2O ____ FePO4

25
Reaction Stoichiometry

• CS2 3O2 ? CO2 2 SO2
• Interpretation in terms of moles
• 1 mole of CS2 3 moles of O2 form
• 1 mole of CO2 2 moles of SO2

26
Predicting Products - Single Replacement Rxns.

• One element replaces another
• Reactants must be an element and a compound.
• Products will be a different element and a
  different compound.
• Na KCl K NaCl
• F2 LiCl LiF Cl2

(Cations switched)
(Anions switched)
27
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

28
Single Replacement Rxns. Practice

• Fe CuSO4
• Pb KCl
• Al HCl

29
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 Na1 Cl-1 Fe3 OH-1
• NaOH FeCl3 Fe(OH)3 NaCl

30
Complete and balance

• CaCl2 NaOH
• CuCl2 K2S
• KOH Fe(NO3)3
• (NH4)2SO4 BaF2

31
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 be
  CO2 and H2O.
• C3H8 O2 ? CO2 H2O

32
Mixed Practice Examples

• H2 O2
• H2O
• Zn H2SO4
• HgO
• KBr Cl2
• AgNO3 NaCl
• Mg(OH)2 H2SO3