Honors text: Ch 10 & 12 Unit 06 Notes: Balancing Chemical Equations

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1 Notes: Balancing Chemical Equations Effects of chemical reactions: Chemical reactions rearrange atoms in the reactants to form new products. The identities and properties of the products are completely different from that of the reactants. Production of gases and color changes are signs of chemical reactions. Energy and Reactions Energy must be to bonds. Energy is when bonds are. Chemical energy is in chemical reactions. EXOTHERMIC REACTIONS: release energy (More energy is released as the products form bonds than is absorbed to break the bonds in the reactants.) ENDOTHERMIC REACTIONS: Chemical equations are used to represent or describe chemical reactions. An equation shows: The + means reacts with The! means yields or reacts to produce To show physical states of each substance: Consider the reaction of iron with oxygen to form iron (III) oxide, or rust. COEFFICIENTS: numbers in front of compound that represents the number of molecules of that compound SUBSCRIPTS: small numbers that help define the compound. Ex: H 2 O: 2H 2 O: H 2 O 2 : During a chem. rxn.; atoms are rearranged (NOT created or destroyed!) Chemical equations must be balanced to show the relative amounts of all substances. Balanced means: each side of the equations has the same # of atoms of each element.

2 RULES to follow in balancing: 1. Correct formulas for all reactants & products. Fe + O 2! Fe 2 O 3 2. Reactants! Products 3. Count the # of atoms of each element in reactants & products. 4. Balance one at a time using coefficients. 5. Check for balance. 6. Are the coefficients in the lowest possible ratio? Examples: a) CuCl 2(aq) + Al (s)! Cu (s) + AlCl 3(aq) b) Propane, C 3 H 8, burns in oxygen, O 2, to form carbon dioxide and water. c) Pentane, C 5 H 12, burns in oxygen, O 2, to form carbon dioxide and water. d) Silver nitrate reacts with copper to produce silver and copper nitrate. e) reacts with oxygen, O 2, to produce disphosphorus pentoxide. f) C 7 H 14 + O 2! CO 2 + H 2 O Net Ionic Equations Reactions in Aqueous Solutions (solutions where water is the solvent) When a soluble substance is dissolved in water, the substance often breaks into ions. This solution is said to be an aqueous solution. An aqueous solution is. OVERALL CHEMICAL REACTION: Consider the reaction TOTAL IONIC EQUATION: What is really going on is Note that the Na + ion and the NO 3 - ion are not reacting. They are said to be spectator ions. NET IONIC EQUATION: It is often useful to write an equation showing only the species that are actually reacting. This is called a net ionic equation. It does not show the spectator ions.

3 Precipitates: To predict precipitates, use the Precipitation Diagram (see Table 1 below) Table 1: Precipitation Diagram - Generalized Solubility of Salts in Water Type of cation NO - 3 Cl - Br - I - Group 1 cations (Li+, Na +, K +, Cs+, Rb+) and NH 4 + (labeled X below) SO 2-4 OH - S 2 - CO 2-3 CrO 2-4 PO 3-4 Group 2 cations (Mg 2 +, Ca 2 +, Ba 2 +) BaSO 4 CaSO 4 Mg(OH) 2 Transition metal cations AgX PbX 2 Hg 2 Cl 2 Ag 2 SO 4 PbSO 4 Hg 2 SO 4 Choose the cation row and read across to the anion column. If the block is colored, a precipitate will form from dilute solution. Where a formula is given, that is the only cation-anion combination in that block that will precipitate. For a more complete list of solubility rules, visit Examples: For each, write the overall reaction, the total ionic equation and the net ionic equation. Circle spectator ions. NaOH(aq) + Cu(NO 3 ) 2 (aq) " Na 3 PO 4 (aq) + Fe(NO 3 ) 3 (aq) " MgSO 4 (aq) + AgNO 3 (aq) " KOH(aq) + NaNO 3 (aq) "

4 Types of Chemical Reactions In chemistry, there are 5 general types of reactions: 1) Synthesis or Combination: 2 or more reactants combine to form 1 product. 2) Decomposition: 1 reactant decomposes to form 2 or more products. 3) Single Replacement: One metal replaces another metal in an ionic compound, producing a new ionic compound and a metal. 4) Double Replacement: Two positive ions switch places forming 2 new ionic compounds: 5) Combustion: a hydrocarbon (containing C and H) or other substance burns in the presence of oxygen gas (O 2 ) to produce CO 2 and H 2 O.

5 Chemical Reaction Reference Tables Table 2: Activity Series (more reactive elements are capable of replacing less reactive elements in a compound) DECREASING ACTIVITY ACTIVITY SERIES OF METALS Name Symbol Lithium Li Potassium K Calcium Ca Sodium Na Magnesium Mg Aluminum Al Zinc Zn Iron Fe Lead Pb (Hydrogen) (H)* Copper Cu Mercury Hg Silver Ag *Metals from Li to Na will replace H from acids and water; from Mg to Pb they will replace H from acids only. Table 3: Keys to identifying general types of chemical reactions General Equation Reactants Probable Rxn Type Probable Products R + S " RS Two elements Combination A single compound Two compounds (or at least one is a molecular compound) RS " R + S A single binary compound A single tertiary compound Combination Decomposition Decomposition A single compound Two elements 2 or more elements and/or compounds T + RS " TS + R An element and a compound Single replacement A different element and a new compound R + S - + T + U - " R + U - + T + S - Two ionic compounds Double Two new compounds Replacement C xh y + #O 2 " CO 2 + H 2O Oxygen and a compound of C, H,(O) Combustion CO 2 and H 2O (with incomplete combustion, C and/or CO may be additional products) STOICHIOMETRY Calculations Based on Balanced Chemical Equations Iron (III) oxide reacts with carbon monoxide to form iron and carbon dioxide. How many CO molecules are required to react with 25 particles of Fe 2 O 3? How many iron atoms can be produced by the reaction of 2.5 x 10 5 particles of Fe 2 O 3 with excess CO?

6 *Formulas can also represent MOLES of substances involved in chem. rxns. *Equations define reaction ratios, i.e. the mole ratios of reactants and products. What mass of CO is required to react with 146 grams of iron (III) oxide? What mass of iron (III) oxide is required to produce 8.65 g of carbon dioxide? Limiting Reactants (Reagents) Calculations need to be based on the limiting reactant, or the reactant that is present in lesser amount. Example: Suppose a box contains 87 bolts, 110 washers, and 99 nails. How many sets consisting of one bolt, two washers and one nail, can you construct from the contents of the box? Example: What is the maximum mass of sulfur dioxide that can be produced by the reaction of 95.6 g carbon disulfide with 110 g oxygen? CS 2 + O 2! CO 2 + SO 2 Example: What mass of CO 2 could be formed by the reaction of 8.0 g CH 4 with 48 g O 2? CH 4 + O 2! CO 2 + H 2 O

7 Percent Yield Many chem. rxns do not go to completion (reactants are not completely converted to products) Percent yield indicates how much of a desired product is obtained. Percent yield may be represented as: % Yield = actual yield x 100 theoretical yield So far the masses we have calculated from chemical equations were based on the assumption that each reaction occurred 100%. The theoretical yield is the yield calculated assuming 100% reaction and isolation of 100% of the desired product. The actual yield is the yield actually obtained form a particular run of a rxn. Example: A 10.0 g sample of ethanol, C 2 H 5 OH, was boiled with excess acetic acid, CH 3 COOH, to produce 14.8 g of ethyl acetate, CH 3 COOC 2 H 5. What percent yield of ethyl acetate is this? CH 3 COOH + C 2 H 5 OH! CH 3 COOC 2 H 5 + H 2 O