17-2 Describing Chemical Reactions

Symbol Represents one kind of an element Ex: C = carbon Ex: Na = sodium

Formula Represents a compound and shows ratio of elements in the compound. Ex: H 2 O 2 hydrogen peroxide Ex: H 2 O water Subscript shows number of atoms of an element.

Equation Shows reactants and products of a chemical reaction. Reactant + Reactant Product + Product H 2 + O 2 H 2 O 2

Law of Conservation of Mass Never end up with more atoms than start with. All atoms present at start of a reaction are present at the end. * Total mass of the reactants must equal the total mass of the products. * Matter is not created or destroyed in a chemical reaction.

Balancing Chemical Equations Use coefficients to determine how many molecules are needed to follow the Law of Conservation of Mass. Ex: hydrogen + oxygen water H 2 + O 2 H 2 O 2H 2 + O 2 2H 2 O

Classifying Chemical Reactions 3 Categories: 1.Synthesis = 2 or more substances combine Ex: 2H 2 + O 2 2H 2 O Ex: 2SO 2 (sulfur dioxide) + O 2 + 2H 2 O 2H 2 SO (sulfuric acid) This is formula for acid rain

2. Decomposition = breaks down compounds into simpler forms Ex: 2H 2 O 2 2H 2 O + O 2

3.Replacement = one element replaces another in a compound – or, elements in different compounds trade places. Ex: 2CuO (copper oxide) + C (charcoal) 2Cu (copper) + CO 2 (carbon dioxide) This is the method for obtaining copper metal from rock.