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