Post on 21-Jan-2016
Chapter 8
Chemical Reactions
Reactants Products
• + - separates reactants and products
• - “yields”
• - reversible reaction
• (s) - solid
• (l) - liquid
• (g) - gas
• (aq) – aqueous solution, dissolves in H2O
• - heat
• Pt - catalyst
• Fe (s) + O2 (g) Fe2O3 (s)
Examples
4Fe (s) + 3O2 (g) 2Fe2O3 (s)
Balancing Chemical Equations
• Balanced equations obey the law of conservation of matter.
• Balanced Equation – must have the same number of atoms of each element in both sides of the equation.
Examples
• C(s) + O2(g) CO2(g)
1 C + 2O 1C + 2O
More Examples
• 2C(s) + O2(g) 2CO
2C 2O 2C 2O
Rules for Balancing Equations
• 1) Determine the correct formulas for reactants and products
• 2) Write the reaction• 3) Count the number of atoms of each
element in the reactants and products (a poly-atomic ion appearing unchanged on both sides of the equation is counted as a single unit)
Rules Cont’d
• 4) Balance elements 1 at a time using coefficients–Coefficient: small whole # that
appears in front of a formula in an equation•Balance O & H last•You cannot change subscripts
Rules Cont’d
• 5) Check to see if each atom is balanced
• 6) Make sure Coefficients are lowest possible numbers
Example
• H2 + O2 H2O
2 1 2
4H2 + 2O2 4H2O
Combination Reactions
• Two or more substances react to form a single substance
Formation of Binary Salts
• M + X MX
–Example
•2Na + Cl2 2NaCl
Formation of Binary Oxides
• M + O2 MO
–Example
•2Zn + O 2 2ZnO
Formation of Binary Acids
• Examples
– H2 + X HX
– H2 + Cl2 2HCl
Formation of Ternary Acids
• Examples
– H2O + nonmetallic oxide oxyacid
– P2O5 + 3H2O 2H3PO4
Formation of Bases
• Examples
– MO + H2O MOH
– CaO + H2O Ca(OH)2
Formation of Ternary Salts
• Examples
– MO + XOn MXOn+1
– CaO + CO2 CaCO3
Decomposition Reactions
• A single compound is broken down into 2 or more simpler products.
– A B + C
– CaCO3 CaO + CO2
• They can be a combination of elements and compounds
• Binary Compounds break down into constituent elements
• Decomposition reactions require energy - (heat, light or electricity)
– Example
• 2H2O(l) electricity 2H2(g) + O2(g)
Single Replacement Reactions
• Atoms of an element replace the atoms of a second element in a compound
– Gen Rxn:
A + BC AC + B
– Example• Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)
How do you know this will happen?
• Whether one metal will replace another is determined by the relative activity of the 2 metals.
Activity Series
• Lists metals in order of decreasing reactivity
• A positive metal will replace any metal found below it in the activity series
• Metals from Li to Na replace H from acids and H20; Mg to Pb replace H from acids only.
Double Replacement Rxns
• Involve an exchange of positive ions between two compounds.– Generally take place in aqueous
solutions
For a double displacement rxn to occur one of the following must be true:
• A precipitate forms
• A gas forms and bubbles out of mixture
• A molecular compound forms (ie H2O)
Examples
• AB + CD AD + CB
AgNO3(aq) + NaCl(aq) AgCl(s) +
NaNO3(aq)
Combustion Rxns
• Oxygen reacts with another substance, producing energy in the form of heat or light.
– Example
• CxHy + O 2 CO2 + H2O