Word Equations. Symbols used in equations. Unit 7: Chemical Reactions. hydrogen peroxide water + oxygen

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1 Unit 7: Chemical Reactions Chemical reactions involve a reorganization of the atoms in one or more substances. Can be represented in different ways 1 Word Equations hydrogen peroxide reacts to produce water and oxygen reactants products hydrogen peroxide water + oxygen 2 Symbols used in equations Symbol what it means separates two reactants or products yields: separates reactants and products = yields: for reversible reactions solid liquid gas aqueous energy in the form of heat catalyst above arrow: KI 3

2 Skeleton Equations Using symbols to represent reactions H 2 O 2 H 2 O + O 2 Only bare bones information given. 4 Complete Balanced Equation catalyst: increases rate of reaction but does not get consumed 2H 2 O 2 l 2H 2 O ( l) + O 2( g) + Δ ( ) KI heat produced: exothermic reaction If heat was absorbed we would place the delta (Δ) on the reactant side: endothermic reaction. 5 Complete Balanced Chemical Equation A total representation of a chemical reaction: C2H5OH(g) + 3O2(g) 2CO2(g) + 3H2O(g) +Δ Coefficient: multiplier Represents the number of molecules needed in reaction. 6

3 Steps for balancing 1) Determine the correct formulas for all reactants and products. 2) Write formulas for reactants on left and formulas for products on right of arrow 3) Count the number of atoms of each element in the reactants and products. (If polyatomic ion is present on both sides of equation count it as a single unit.) 4) Balance the elements one at a time using coefficients. 5) DO NOT attempt to balance an equation by changing the subscripts in the chemical formula of a substance. 6) Make sure that all the coefficients are in the lowest possible ratio. Hints: a) Do the most complex one first b) Save any element that appears alone (not in a compound) for last. c) Keep polyatomic ions together if possible. d) Leave O2 and/or H2 towards the end to balance. 7 Lets work some out! 1. H 2 + O 2 H 2O 2. Ca(OH)2 + H3PO4 Ca3(PO4)2 + H2O 3. Cu + AgNO 3 Cu(NO 3) 2 + Ag 4. KNO 3 KNO 2 + O 2 5. Pb(NO 3) 2 + AlCl 3 PbCl 2 + Al(NO 3) 3 8 Combination General form: A + B AB Example: 2Al + 3Cl2 2AlCl3 1. Identified by having two single reactants. 3. Diatomics may be one of single reactants Special cases Metal oxide + water metal hydroxide (base) CaO(s) + H2O(l) Ca(OH)2(aq) nonmetal oxide + water acid SO3(g) + H2O(l) H2SO4(aq) 9

4 Decomposition General form: AB A + B Example: 2H2O 2H2 + O2 1. Identified by having one compound as a lone reactant 3. Binary compounds will split into individual elements Special Case carbonates 1. metal carbonates metal oxide + carbon dioxide 2. example: Li2CO3 Li2O + CO2 chlorates 1. metal chlorate metal chloride + oxygen gas 2. example: Mg(ClO3)2 MgCl2 + 3O2 hydroxides (bases) 1. metal hydroxide metal oxide + water 2. example: 2NaOH Na2O + H2O 10 (Single) Displacement General form: A + BC AC + B OR A + BC BA + C (A = metal) (A = nonmetal) Ex: 2Li + FeCl2 2LiCl + Fe OR 3F2 + 2GaBr3 2GaF3 + 3Br2 1. Identified by an element or diatomic plus an ionic compound 2. Compound is usually in solution Assume will always will happen if single element is nonmetal If single element is a metal you must check activity series. 1. If metal is higher than cation in compound it will be able to replace it and the reaction will happen 2. If metal is lower than cation in compound it will not be able to replace it and we have a No Reaction. 11 Combustion General form: hydrocarbon + O2 CO2 + H2O + heat Example: CH4 + 2O2 CO2 + 2H2O + heat 1. Identified by having a hydrocarbon combining with oxygen 3. Needs a mechanism to light hydrocarbon (spark etc.) 4. Always exothermic Make sure you don t mistake them of single replacement! 12

5 Double Displacement General form: Example: AB + CD AD + CB AlCl3(aq) + Na3PO4(aq) AlPO4(s) + 3NaCl(aq) 1. Identified by having two compounds as reactants 2. To happen one of the following must occur 1. produce a gas (g) 2. produce water (l) (neutralization of an acid and base) 3. produce a precipitate (s) This is most common situation! You can determine if a compound will precipitate out by checking the solubility table 1. If it is soluble it will stay in aqueous solution and we get a No Reaction. 2. If is insoluble it will precipitate out as a solid and reaction will happen. 13 Net Ionic Equations Chemistry in solutions Ionic compounds separate into their ions in solution. To simplify the reaction we cancel out the ions that stay in solution. Rewrite equations showing only the net ionic reaction. 14 Lets work one out 2Na 3 PO 4(aq) + 3Cu(NO 3 ) 2(aq) Cu 3 (PO 4 ) 2(s) + 6NaNO 3(aq) Now try this one: Write the net ionic equation for the reaction between aqueous iron (II) chlorate and aqueous ammonium sulfide. 15