3.1 Recognizing and Understanding Chemical Changes Unit 2 – Chemical Reactions

Before we begin…. Do you remember the signs that a chemical change has taken place? Chemical Change or Reaction = any change in which a new substance is formed Evidence of Chemical Change: Change in colour Change in odour Formation of gas Formation of precipitate Heat or light is emitted

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

Equations A short form method used to represent chemical reactions. Two different types: word and chemical equations In general: REACTANTS PRODUCTS ex: NaOH (aq) + HCl (aq) NaCl (aq) + H 2 O (l) reacts withyields or produces and

State of Matter Solid =(s) Liquid=(l) Gas=(g) Solution (dissolved in water) =(aq) ex: Mg (s) + 2HCl (aq)  MgCl 2(s) + H 2(g)

Word Equation This equation identifies substances by name. sodium hydroxide (aq) + hydrochloric acid (aq) sodium chloride (aq) + water (l) An aqueous solution of sodium hydroxide reacts with hydrochloric acid to yield an aqueous solution of sodium chloride and water.

Chemical Equation a skeleton equation lists the chemical formulas of the substances. Ex: Fe (s) + O 2(g)  Fe 2 O 3(s) Is this equation balanced? Law of Conservation of Mass In any chemical reaction, mass of reactants = mass of products Therefore, we must balance all chemical equations

Balanced Chemical Equations 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: 2 Na (s) + Cl 2(g)  2 NaCl (s) example: 6 K (s) + N 2(g)  2 K 3 N (s)

example: 2 Na (s) + Cl 2(g)  2 NaCl (s) example: 6 K (s) + N 2(g)  2 K 3 N (s)  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 (DO NOT CHANGE THE SUBSCRIPTS !!!) Skeleton equation: Fe (s) + O 2(g)  Fe 2 O 3(s) Balanced equation: 4Fe (s) + 3O 2(g)  2Fe 2 O 3(s)

Hints for Balancing: First, balance the atoms which only occur in one molecule on each side (eg. Pb in equation a) Balance whole groups (polyatomic ions) whenever possible, rather than considering the atoms of the groups separately (eg. SO 4, NO 3 in equation b) Be systematic; do not jump all over an equation balancing a bit here and a bit there. Instead, balance atoms and groups one at a time. Balance atoms which occur in elemental form last. By elemental form we mean that the atoms are not combined with any atoms of a DIFFERENT kind (eg. O 2, K, Cu etc.) a)PbO 2 + HBr → H 2 O + PbBr 2 + Br 2 b)Al 2 (SO 4 ) 2 + Ba(NO 3 ) 2 → Al(NO 3 ) 3 + BaSO4 c)KOH + Cr 2 O 3 + O 2 → K 2 CrO4 + H 2 O

Homework pg. 111 # 1, 2, 4 pg. 113 # 5 – 7 (Practice) pg.113 – 114 #1 (Section 3.1 questions) Classwork Worksheet “Balancing Chemical Equations / Writing Chemical Equations”