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 + O 2  Fe 2 O 3 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) oxide Fe + O 2  Fe 2 O 3 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 + O 2  Fe 2 O 3 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 CH 4 + O 2  CO 2 + H 2 O

Practice - Answers Sodium + chlorine  sodium chloride 2Na + Cl 2  2NaCl CH 4 + 2O 2  CO 2 + 2H 2 O

Types of Reactions Combination/Synthesis Two or more reactants make ONE product 2K + Cl 2  2KCl 2S + 3O 2  2SO 3

Types of Reactions Decomposition ONE reactant is broken down into two or more products (most require energy) CaCO 3  CaO + CO 2

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(NO 3 ) 2  Mg(NO 3 ) 2 + Zn Mg + LiNO 3  no reaction

Types of Reactions Double-Replacement Involve an exchange of positive ions between two reacting solutions (usually in solution and form percipitate) 3CuCl 2 + 2Na 3 PO 4  Cu 3 (PO 4 ) 2 + 6NaCl

Types of Reactions Combustion Element or compound (usually hydrocarbon) reacts with oxygen. Products for a hydrocarbon are CO 2 and H 2 O 2Mg + O 2  2MgO CH 4 + 2O 2  CO 2 + 2H 2 O

Practice Predict the products of the following reactions and balance: Na + Cl 2  Mg + CuSO 4  NaI + Pb(NO 3 ) 2 

Practice - Answers 2Na + Cl 2  2NaCl Mg + CuSO 4  MgSO 4 + Cu 2NaI + Pb(NO 3 ) 2  2NaNO 3 + PbI 2