Unit 2 – Chemical Reactions Chemical Equations

Before we begin…. Do you remember the signs that a chemical change has taken place? Chemical Change = any change in which a new substance is formed Evidence of Chemical Change: Change in colour Change in odour Formation of gas/solid Release/absorption of heat

Collision-Reaction Theory A theory stating that chemical reactions involve collisions and rearrangements of atoms or groups of atoms and that the outcome of collisions depends on the energy and orientation of the collisions No reaction occurs if: Molecules don’t have enough energy Molecules don’t collide in the right orientation

Chemical Equations A short form method used to represent chemical reactions, using symbols and formulas. Indicates the: chemical formulas, relative number of entities and states of matter of the reactants and products. In general: REACTANTS PRODUCTS Ex: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) reacts with yields and

Word Equation This equation is used to identify substances by name. Ex: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) An aqueous solution of sodium hydroxide reacts with hydrochloric acid to yield an aqueous solution of sodium chloride and water.

Chemical Equations Skeleton equation lists the chemical formulas of the substances. Ex: Fe(s) + O2(g)  Fe2O3(s) Is this equation balanced? Law of Conservation of Mass In any chemical reaction mass of reactants = mass of products

Relative # of Entities: Balanced Chemical Equation use coefficients to balance Coefficient = a whole number indicating the ratio of molecules of each substance involved in a chemical reaction The large number on the left side of a molecule’s formula Example: Mg + 2 Cl  MgCl2 Example: 6 K + N2  2 K3N

Ex: Fe(s) + O2(g)  Fe2O3(s) Example: Mg + 2 Cl  MgCl2 Example: 6 K + N2  2 K3N Notice…there are equal numbers of each type of atom on both sides of the equation But you cannot change any of the chemical formulas   Follow the “trial and error” method. (see handout provided) Ex: Fe(s) + O2(g)  Fe2O3(s) Ex: 4 Fe(s) + 3 O2(g)  2 Fe2O3(s)

State of Matter Example: 6 K(s) + N2(g)  2 K3N(s) Solid = (s) Liquid = (l) Gas = (g) Solution = (aq) Example: 6 K(s) + N2(g)  2 K3N(s)

Now Try This Worksheet “Balancing Chemical Equations” # 1,3,5 ... Worksheet “Word Equations” page 1 all

Homework p121 textbook: #1, 2, 4, 5, 6, & 10