Unit 4.1: Describing and Balancing Chemical Equations

Vocabulary: Reactant: a substance present at the start of a reaction; to the left of the arrow Product: substance produced as part of a reaction; to the right of the arrow Skeleton equation: a chemical equation that does not indicate the relative amounts of reactants and products Coefficients: small whole numbers placed in front of formulas in an equation to balance it; (you can change the coefficients, but if you try to change the subscripts to balance an equation, I will shake my head in disappointment!) Spectator ions: an ion that appears on both sides of an equation and is not directly involved in the reaction Net Ionic Equation: an equation that shows only those particles that are directly involved in the chemical change and is balanced in mass and charge 2

Combination reaction: two or more substances react to form a single new substance Decomposition reaction: a single compound breaks down into two or more simpler products Single-replacement reaction: one element replaces another element in a compound Double-replacement reaction: involves an exchange of positive ions between two compounds Combustion reaction: an element or compound reacts with oxygen and produces energy as heat or light. Precipitate: a solid that forms and settles out of a mixture 3

I. Writing Chemical Equations (10.1) A. Word equations: describe chemical reactions with reactants written to the left of arrow and products written to the right: reactant + reactant  product + product 1.Ex: methane + oxygen  carbon dioxide + water 4

B.Chemical equations: describe reactions using formulas in place of words 1.Skeleton equation shows just formulas, but are not balanced: a.Ex: CH 4 + O 2  CO 2 + H 2 O 2.Balanced equation shows conservation of mass – each side of the equation has the same number of atoms of each element a.Ex: CH 4 + 2O 2  CO 2 + 2H 2 O b.Use coefficients in front of substances but do not change the subscripts 1)Subscripts = # of each atom in a compound 2)Coefficients = # of each reactant or product in a reaction 5

C.Other information included in equations: 1.State of matter: If substance is gas: (g) Liquid: ( l ) Solids: (s) Dissolved in water, aqueous: (aq) 2.Reversible arrow () indicates that reaction can occur in both forward and reverse directions. 3.If heat is needed as part of the reaction, then “heat” or “Δ” is written over the arrow. 4.If a catalyst is used, then written over the arrow. Ex: 2H 2 O 2 2H 2 O + O 2 6 Pt

D.Rules for writing balanced equations: 1.Write a skeleton equation with correct formulas for reactants, products a.Draw boxes around these compounds. Do not write inside the boxes! 2.Balance atoms that appear only once on each side of the equation: a.Use coefficients to change the number of compounds/elements… b.DO NOT CHANGE THE SUBSCRIPTS! EVER! c.It (sometimes) helps to “double” any elements that have an “odd” number on one side. d.Balance hydrogen and oxygen last. 3.If you add coefficients, re-check all other atoms affected by that coefficient. 4.All coefficients must be in lowest possible ratio. 7

E.Balance the following equation: ___AlBr 3 + ___K 2 SO 4  ___Al 2 (SO 4 ) 3 + ___KBr Step 1: draw boxes around all substances: ___ AlBr 3 + ___K 2 SO 4  ___ Al 2 (SO 4 ) 3 + ___KBr Step 2: Inventory all atoms before and after. Step 3: balance an atom that is not oxygen or hydrogen: 2 AlBr 3 + ___K 2 SO 4  ___ Al 2 (SO 4 ) 3 + ___KBr Recheck all atoms affected by coefficient: Step 4: balance atoms affected by new coefficient: 2 AlBr 3 + ___K 2 SO 4  ___ Al 2 (SO 4 ) KBr Then recheck all new atoms affected by coefficient: Step 5: balance atoms affected by new coefficient: 2 AlBr K 2 SO 4  ___ Al 2 (SO 4 ) KBr Finally: double-check that all atoms balance on both sides of the equation. 8 BA Al12 Br31 K21 SO 4 13 BA Al22 Br61 K21 SO 4 13 BA Al22 Br66 K26 SO 4 13 BA Al22 Br66 K66 SO 4 33

II. Types of Chemical Reactions (10.2) A.5 types of reactions: 1.Combination reaction: two or more reactants combine into a single product. a.Also called “synthesis” reactions b/c they “join together” b.Ex: 2Mg (s) + O2 (g)  2MgO (s) 9

2.Decomposition reaction: a single reactant breaks down (“decomposes”) into two simpler products. a.The opposite of a combination reaction b.Ex: 2HgO (s)  2Hg ( l ) + O 2(g) 10

3.Single-replacement reaction: one element replaces a second element in a compound. Also called single-displacement reactions a.Ex: 2K (s) + 2H 2 O ( l )  2KOH (aq) + H 2(g) 11

b.Whether one metal replaces another depends on reactivity, called the activity series of metals: A metal that is higher on the list is more active, will replace any metal below it in the list c.Halogens can also replace other halogens in compounds according to hierarchy: Smaller halogens can replace larger halogens, but larger halogens cannot replace smaller halogens. 12

4.Double-replacement reaction: an exchange of positive ions between two compounds Also called double-displacement reactions a.Ex: Na 2 S (aq) + Cd(NO 3 ) 2(aq)  CdS (s) + 2NaNO 3 (aq) 13

b.In a double-replacement reaction, one of the following is generally true: 1)One of the products is insoluble and will form a solid “precipitate” that settles out of the solution. 2)One of the products is a gas. 3)One product is a molecular compound like water. 14

5.Combustion reaction: an element or compound reacts with oxygen, often producing heat and light. a.Other reactant is generally a hydrocarbon b.Ex: 2C 8 H 18( l ) + 25O 2(g)  16CO 2(g) + 18H 2 O ( l ) octane + oxygen  carbon dioxide + water c.But not always complete, so often leaves elemental carbon (C (s) )and carbon monoxide (CO) as products 15

III. Reactions in Water (10.3) Most reactions occur in water. A.When ionic compounds dissolve in water, they dissociate (break apart) into cations and anions 1.NaCl (aq)  Na+ + Cl- B.A complete ionic equation will show ionic compounds as dissociated ions: 1.AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq) becomes Ag + + NO Na + + Cl -  AgCl (s) + Na + + NO

C.But NO 3 - and Na + appear unchanged on both sides of the equation so they can be eliminated from the overall equation (just like Algebra): Ag + + NO Na + + Cl -  AgCl (s) + Na + + NO Called “spectator ions” b/c they contribute to but don’t participate in the actual reaction D.So net ionic equation is written without spectator ions (and charge is balanced): Ag + + Cl -  AgCl (s) 17

E.You can predict whether a precipitate will form based on general solubility rules: 1.The following are insoluble, will form a precipitate: a.Most carbonates (CO 3 2- ), phosphates (PO 4 3- ), chromates (CrO 4 2- ), sulfides (S 2- ), and hydroxides (OH - ) b.Sulfates (SO 4 2- ) of Pb 2+, Ag +, Hg 2 2+, Ba 2+, Sr 2+, and Ca 2+ c.Chlorides (Cl-) of Pb 2+, Ag +, and Hg Predict whether the following reaction will form a precipitate: 2Na + + CO Ba NO 3 -  ? According to rules, all nitrates are soluble, and all sodium compounds are soluble, but most carbonates are insoluble, so BaCO 3 will form a precipitate. 18

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