Chapter 5 Chemical reactions
Chemical equations CaO(s) + CO 2 (g) CaCO 3 (s) + CO(g) Chemical equation - representation of a chemical reaction; uses the symbols of the elements and formulae of the compounds The reacting species (starting materials or reactants) are listed on the left, and the products of the reaction are listed on the right, an arrow separating them.
Chemical equations Symbols commonly used in chemical equations are listed in table 5-1 (know these) We balance chemical equations because of the Law of the Conservation of Mass Use coefficients in front of the element or compound formula - whole numbers that adjust the amounts of the species Note that we cannot balance an equation by changing the subscripts of the compound Example 1
5 Types of chemical reactions COMBUSTION REACTIONS Combustion reactions - reaction of an element or a compound with elemental oxygen (O 2 ) These reactions are often accompanied by the release of large amounts of heat (and flame) Consider: 2C 2 H 6 (g) + 7O 2 (g) 4CO 2(g) + 6H 2 O (g)
Combination reactions Often called synthesis reactions Involves two or more substances combining to form a single substance, or two compounds Consider: 2Mg(s) + O 2 (s) 2MgO(s) CaO(s) + CO 2 (g) CaCO 3 (s) + CO(g)
Decomposition reactions The reverse of a combination reaction One substance breaking apart into two or more substances Consider: H 2 CO 3 (aq) H 2 O (l) + CO 2 (g) CH 3 COOOH(g) CH 3 (g) + CO 2 (g) + OH(g)
Formation of ions in solution (side road ) When a substance dissolves in another substance, the solute is the substance in lesser amount, the solvent is the substance in greater amount Solubility is the relative degree to which a substance dissolves in a solvent Soluble substances dissolve to a considerable degree (NaCl table salt) Insoluble substances do not dissolve at all or only to a small degree (gravel)
Dissolution of ionic compounds When ionic compounds dissolve in water, they dissociate into separate anions and cations: NaCl(s) Na + (aq) + Cl - (aq) Acids are so named because when they dissolve, the ionize to yield H + and the associated anion HCl(aq) H + (aq) + Cl - (aq)
Dissolution of ionic compounds
Acids Strong acids completely dissociate into ions upon dissolution in water Examples are HCl, H 2 SO 4 Weak acids dissolve, but do not completely dissociate into H + and anions Examples are CH 3 CO 2 H, HCN
Take a break Balancing coefficients The subscripts of the compound are fixed: they cannot be changed in an equation e.g. CH 4 + 2 O 2 CO 2 + H 2 O The coefficients used should be the smallest whole numbers possible The coefficient multiplies every number in the formula (e.g. 2 MgCl 2 means that there are 2 Mg atoms and 4 Cl atoms) Balance easy equations by inspection
OB JEC TIV ES So Far Section You Should be able to 5-1 Write a chemical equation from a word description of the reaction. Examples Exercises 5-1 1a, 1b, 1e, 2c Chapter Problems 10, 11, 12, 13 Balance a simple chemical equation by inspection. 5-2 Classify certain chemical reactions as being combustion, combination, or decomposition reactions. 5-3 Write the ions formed when ionic compounds or acids dissolve in water. 5-1, 5-2, 5-3 1c, 1d 2, 3, 4, 5 5-3 2a, 2b, 2c 14, 15, 16, 18, 19, 20, 21, 22, 23 5-4 3a, 3b, 3c 24, 25, 26, 27 5-4 Given the activity series, complete several single-replacement reactions as balanced molecular, total ionic, and net ionic equations. 5-5 4a, 4b, 4c, 4d 28, 29, 30, 33 5-5 Given a table of solubility rules, determine whether a specific ionic compound is soluble or insoluble in water. Write balanced molecular, total ionic, and net ionic equations for precipitation reactions. 5-6 Write balanced molecular, total ionic, and net ionic equations for neutralization reactions. 5-6 5a, 5b, 5c 34, 35, 40, 42 5-7, 5-8, 5-9 5d, 5e 44, 45, 46, 47, 48, 49 5-10 6a, 6b 55, 56, 57, 58
Procedure for balancing eqns Examples
Single replacement reactions 4 th type of chemical reaction An element replaces another in a compound Zn(s) + CuCl 2 (aq) ZnCl 2 (aq) + Cu(s) This particular form of the equation is termed the molecular equation, where all reactants and products are shown as neutral compounds
Double replacement reactions Where the anions and cations in an ionic equation swap AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq)
Compare single replacement vs double replacement single replacement Zn(s) + CuCl 2 (aq) ZnCl 2 (aq) + Cu(s) double replacement AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq)
Single replacement reactions 4 th type of chemical reaction An element replaces another in a compound Zn(s) + CuCl 2 (aq) ZnCl 2 (aq) + Cu(s) This particular form of the equation is termed the molecular equation, where all reactants and products are shown as neutral compounds
OB JEC TIV ES So Far Section You Should be able to 5-1 Write a chemical equation from a word description of the reaction. Examples Exercises 5-1 1a, 1b, 1e, 2c Chapter Problems 10, 11, 12, 13 Balance a simple chemical equation by inspection. 5-2 Classify certain chemical reactions as being combustion, combination, or decomposition reactions. 5-3 Write the ions formed when ionic compounds or acids dissolve in water. 5-1, 5-2, 5-3 1c, 1d 2, 3, 4, 5 5-3 2a, 2b, 2c 14, 15, 16, 18, 19, 20, 21, 22, 23 5-4 3a, 3b, 3c 24, 25, 26, 27 5-4 Given the activity series, complete several single-replacement reactions as balanced molecular, total ionic, and net ionic equations. 5-5 4a, 4b, 4c, 4d 28, 29, 30, 33 5-5 Given a table of solubility rules, determine whether a specific ionic compound is soluble or insoluble in water. Write balanced molecular, total ionic, and net ionic equations for precipitation reactions. 5-6 Write balanced molecular, total ionic, and net ionic equations for neutralization reactions. 5-6 5a, 5b, 5c 34, 35, 40, 42 5-7, 5-8, 5-9 5d, 5e 44, 45, 46, 47, 48, 49 5-10 6a, 6b 55, 56, 57, 58
What is happening in single replacement reactions 2 e -
Activity series Table 5-2 indicates the common metals in decreasing order of their ability to replace metal ions in aqueous solution When comparing two metals, the one higher in the table replaces the one lower in the table (e.g. Zn can replace Cu ion but not Al)
Activity Series Metal Metal Ion Reactivity Lithium Potassium Calcium Sodium Magnesium Aluminum Manganese Zinc Chromium Iron Lead Copper Mercury Silver Platinum Gold Li+ K + Ca 2+ Na + Mg 2+ Al 3+ Mn 2+ Zn 2+ Cr 2+, Cr 3+ Fe 2+, Fe 3+ Pb 2+ Cu 2+ Hg 2+ Ag + Pt 2+ Au +,Au3+ Most Reactive - - - - - - - - - - - - - - Least Reactive Zn 0 + Cu 2+ Cl 2 (aq) Cu 0 + Zn 2+ Cl 2 (aq)
Ionic equations The total ionic equation writes all of the anions and cations separately Zn (s) + Cu 2+ (aq) + 2Cl - Zn 2+ (aq) + 2Cl - +Cu (s) Spectator ions (such as the Cl - ions) are those that are in the same state on both sides of the equation
Net ionic equation The net ionic equation leaves out the spectator ions The complete ionic equation Zn (s) + Cu 2+ (aq) + 2Cl - Zn 2+ (aq) + 2Cl - + Cu (s) The net ionic equation Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s)
Compare single replacement vs double replacement single replacement Zn(s) + CuCl 2 (aq) ZnCl 2 (aq) + Cu(s) double replacement AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq)
Double replacement reactions Where the anions and cations in an ionic equation swap AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq)
What causes double replacements? There has to be some process to cause the reaction to take place Precipitation of a solid Evolution of a gas Neutralization reaction between an acid and a base If one of these processes does not occur, no reaction takes place For precipitation rxns, use solubility table, Table 5-3
Double replacement reactions Precipitation AgNO 3 (aq) + NaCl (aq) AgCl (s)+ NaNO3 (aq) Gas evolution 2HCl (aq) + Na 2 CO 3 (aq) 2NaCl (aq) + CO 2 (g)
Double replacement reactions Neutralization an acid is a proton donor a base is a hydroxide donor proton plus hydroxide yields water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l)