Chemical Reactions

Describing Chemical Change• Chemical reactions take place

around you everyday

• Chemical reactions are expressed on paper as chemical equations

Describing Chemical Change• Word Equations

• Written in the form of reactants products (words only; arrow means “yields” or “produces”)

• Example:Iron + oxygen iron (III) oxide

Chemical Equations• Word equations adequately

describe reactions, but they are inconvenient

• In chemical equations, the formulas of the reactants are on the left, the formulas of the products are on the right

Chemical Equations

• Fe + O2 Fe2O3

• Equations such as the one above, which only show the formulas of the reactants and products are called skeleton equations

Chemical Equations• You can indicate states of

substances in parentheses• (s) – solid• (g) – gas• (l) – liquid• (aq) - aqueous

Balancing Equations• Remember the Law of Conservation of

Matter?

• All chemical reactions must obey that law, which means what you begin with must be the same as what you end with (although maybe in a different form)

Balancing Equations• Let’s go back to our example:

Iron + oxygen iron (III) oxideFe + O2 Fe2O3

Are there the same amounts of iron and oxygen on the reactants side and the products side?

Balancing Equations• An equation that does not indicate

the quantity of reactants needed to make the products is unbalanced

Balancing Equations• To balance an equation, numbers

called coefficients are placed in front of the symbols (when there is no coefficient, it is assumed to be one)

Balancing Equations

• Fe + O2 Fe2O3

How many Fe? Left = ? Right = ? How many O? Left = ? Right = ?

What coeffcients will we add to balance this equation?

Balancing Equations• Once you have added coefficients,

you check to make sure there are equal numbers of each element on both sides of the equation.

• Why?

Practice• Try balancing these equations on

your own at your seat.

• Sodium + chlorine sodium chloride

• CH4 + O2 CO2 + H2O

Practice - Answers• Sodium + chlorine sodium

chloride• 2Na + Cl2 2NaCl

• CH4 + 2O2 CO2 + 2H2O

Types of Reactions• Combination/Synthesis

• Two or more reactants make ONE product

2K + Cl2 2KCl2S + 3O2 2SO3

Types of Reactions• Decomposition

• ONE reactant is broken down into two or more products

(most require energy)

CaCO3 CaO + CO2

Types of Reactions• Single-Replacement

• A single element replaces a second element in a compound

(whether a reaction will occur depends on the activity series of metals – pg 217)

Mg + Zn(NO3)2 Mg(NO3)2 + ZnMg + LiNO3 no reaction

Types of Reactions• Double-Replacement

• Involve an exchange of positive ions between two reacting solutions (usually in solution and form percipitate)

3CuCl2 + 2Na3PO4 Cu3(PO4)2 + 6NaCl

Types of Reactions• Combustion

• Element or compound (usually hydrocarbon) reacts with oxygen. Products for a hydrocarbon are CO2 and H2O

2Mg + O2 2MgOCH4 + 2O2 CO2 + 2H2O

Practice • Predict the products of the

following reactions and balance:Na + Cl2 Mg + CuSO4 NaI + Pb(NO3)2

Practice - Answers2Na + Cl2 2NaClMg + CuSO4 MgSO4 + Cu2NaI + Pb(NO3)2 2NaNO3 + PbI2