Section 2 - Balancing Chemical Equations Reactions Conserve Mass This law states that in ordinary chemical or physical changes, mass is neither created nor destroyed. This law is based on the fact that the products and the reactants of a reaction are made up of the same number and kinds of atoms. The atoms are just rearranged and connected differently. Balancing Equations To balance an equation, you need to make the number of atoms for each element the same on the reactants’ side and on the products’ side. But there is a catch. You cannot change the formulas of any of the substances. You can only place numbers called coefficients in front of the formulas. A coefficient multiplies the number of atoms of each element in the formula that follows.

Balancing Chemical Equations Step 1 – Balance polyatomic ions if any are present Step 2 – Balance the metals if any are present Step 3 – Balance the oxygen's last Step 4 – Check the law of conservation of mass **Remember Never Change Subscripts to Balance an Equation

Balance the following equations. a. ZnS + O 2 → ZnO + SO 2 b. Fe 2 O 3 + CO → Fe + CO 2 c. AgNO 3 + AlCl 3 → AgCl + Al(NO 3 ) 3 d. Ni(ClO 3 ) 2 → NiCl 2 + O 2

Section 3 - Classifying Chemical Reactions Reaction Types We have learned about a lot of chemical reactions. But they are just a few of the many that take place. To make learning about relations simpler, it is helpful to classify them and to start with a few basic types. Combustion Reactions A combustion reaction is the reaction of a hydrocarbon compound with oxygen. The products are carbon dioxide and water C 3 H 8 + 5O 2 → 3CO 2 + 4H 2 O

Synthesis Reactions In synthesis reactions a single compound forms from two or more reactants. CaO + H 2 O → Ca(OH) 2 Decomposition Reactions Decomposition reactions are the opposite of synthesis reactions—they have only one reactant. In a decomposition reaction a single compound breaks down, often with the input of energy, into two or more elements or simpler compounds. 2H 2 O → 2H 2 + O 2

Single-Displacement Reactions In this single displacement reaction, a single element reacts with a compound and displaces another element from the compound Mg + Pb(NO 3 ) 2 → Pb + Mg(NO 3 ) 2 Double-Displacement Reactions Occurs when two compounds in aqueous solution appear to exchange ions and form two new compounds. HCl + NaOH → HOH + NaCl