Unit 2 – Chemical Reactions Chemical Equations

What is a Chemical Reaction? A chemical reaction is a process that involves the creation of new substances.

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. Writing Chemical Equations: REACTANTS PRODUCTS Ex: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) reacts with yields and

oxygen gas + hydrogen gas  water vapour Word Equation This equation is used to identify substances by name. Ex: When ignited by a flame, oxygen gas and hydrogen gas react to form water vapour. oxygen gas + hydrogen gas  water vapour An aqueous solution of sodium hydroxide reacts with hydrochloric acid to yield an aqueous solution of sodium chloride and water.

Skeleton Chemical Equation Skeleton equation lists the chemical formulas of the reactants and products along with their states. Ex: When ignited by a flame, oxygen gas and hydrogen gas react to form water vapour. O2 (g) + H2(g)  H2O(g) States: (g) – gas (l) -liquid (s) - solid (aq) - aqueous

All atoms IN = all atoms OUT mass of reactants = mass of products Law of Conservation of Mass In any chemical reaction The mass of the reactants is conserved in the reaction. All atoms IN = all atoms OUT Balancing Chemical Equations: -all atoms in the reactants must equal the # of atoms in the products. mass of reactants = mass of products **Use an ATOM TABLE

Relative # of Entities: Balanced Chemical Equation use coefficients to balance -place coefficients in front of THE WHOLE COMPOUND to change the # of atoms. 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

But you cannot change any of the chemical formulas 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   Ex: Fe(s) + O2(g)  Fe2O3(s) Ex: 4 Fe(s) + 3 O2(g)  2 Fe2O3(s)

Steps for Balancing Chemical Equations Write out the skeletal equations. Double check that you have written all the chemical formulas correctly. Balance only with coefficients. Fractions can be used at first but need to be changed to whole numbers later. Using an atom table, balance the atoms that occur in the largest numbers on either side of the equation first. Leave hydrogen, oxygen and other atoms until later. Balance any polyatomic ions that occur on both sides of the chemical equation as one unit (do not split the atoms apart) Balance the hydrogen and/or oxygen atoms last Check your answer. Make sure all the coefficients are whole numbers in the lowest terms.

Ex 1: A magnesium metal reacts with a solution of iron (II) chloride to produce iron metal and magnesium chloride. Word Equation: Balanced Chemical Equation:

Ex. 2 Hydrogen gas and sodium sulfite are produced from a reaction between sulfurous acid and sodium metal Word Equation: Balanced Chemical Equation:

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

Homework p121 textbook: #10, 12, 13, 14 Finish the Word Equation questions #1-10