Chapter 8 Notes Chemical Reactions

I. Describing Chemical Change Arrow separates the reactants (on left) from the products (on right); plus signs separate multiple reactants and products Chemical Equation: Ex. Fe + O 2 → Fe 2 O 3 Be sure you have these filled in from page 206 in your textbook Symbols Used in Chemical Equations: A substance that speeds up a reaction without being used up in the process; often shown above the reaction arrow Catalyst: Ex. H 2 + O 2 H 2 O Pt

Whole numbers placed to the LEFT of reactants and products; used to balance equations; COEFFICIENTS AND SUBSCRIPTS MULTIPLY Coefficient: Each side of the equation (left & right sides of the arrow) has the same number of atoms of each element Balanced Chemical Equation: Coefficients represent _____ ________ of products and reactants, NOT ____________! the ratios masses

(A Few) Rules for Balancing Chemical Equations: 1. Never change the subscripts (small numbers) in formulas 2. Only use coefficients to balance 3. Remember that coefficients and subscripts MULTIPLY to count atoms 4. Balance one element at a time 5. If polyatomic ions appear on both sides of the equation, treat them as units 6. Make sure all coefficients are in lowest possible ratio (Ex. 1 : 2 : 2 NOT 2 : 4 : 4) 7. PRACTICE, PRACTICE, PRACTICE! *Tip for the very difficult equations: If you have an odd number of atoms of one element on one side of the equation, put a 2 in front of it, then attempt to balance the other side See board for examples

Pg. 209: #3 a.) AgNO 3 + H 2 S → Ag 2 S + HNO 3 b.) MnO 2 + HCl → MnCl 2 + H 2 O + Cl 2 c.) Zn(OH) 2 + H 3 PO 4 → Zn 3 (PO 4 ) 2 + H 2 O (HOH) 3 2 6

Pg. 210: #7 a.) FeCl 3 + NaOH → Fe(OH) 3 + NaCl b.) CS 2 + Cl 2 → CCl 4 + S 2 Cl 2 c.) CH 4 + Br 2 → CH 3 Br + HBr Balanced!

II. Types of Chemical Reactions 1. Combination Reaction: Multiple reactants form ONE PRODUCT Ex. A: A + B → AB Ex. B: 2 S + 3 O 2 → 2 SO 3 2. Decomposition Reaction: ONE REACTANT forms multiple products Ex. A: AB → A + B Ex. B: PbO 2 → Pb + O 2

3. Single Replacement Reaction: An element replaces another element in a compound; ONE ELEMENT and ONE COMPOUND ON BOTH SIDES Ex. A: A + BX → AB + X Ex. B: 2 Na + ZnO → Zn + Na 2 O 4. Double Replacement Reaction: an exchange of positive ions between two compounds; TWO COMPOUNDS ON BOTH SIDES Ex. A: AB + XY → AX + BY Ex. B: 3 KOH + AlPO 4 → Al(OH) 3 + K 3 PO 4

An element or compound reacts with O 2 ; O 2 IS A REACTANT (on left) 5. Combustion Reaction: Ex. A: CH O 2 → CO H 2 O Ex. B: S + O 2 → SO 2

Predicting Products in a Double Replacement Reaction: If you remember that in a double replacement reaction, the cations and anions switch places, you can predict the products for the reaction. Just be careful to write the formulas correctly on the product side. Ex. 1: MgCl 2 + NaBr → Ex. 2: Na 3 PO 4 + Al 2 (SO 4 ) 3 → Don’t carry the subscripts (little numbers) from the left to the right side of the arrow UNLESS they are part of a polyatomic ion formula (ex. PO 4 3- ). The subscripts are determined the same way here that they were in Ch 6: criss-cross the ionic charges. MgBr 2 + NaCl Na 2 SO 4 + AlPO 4 (Mg 2+ Br 1- )(Na 1+ Cl 1- ) (Na 1+ SO 4 2- )(Al 3+ PO 4 3- )