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4/28/2010 Chapter 4 Aqueous Reactions and Solution Stoichiometry Today’s Goals Water as a unique solvent Properties of aqueous solutions Dissolution/Disassociation Electrolytes/Non-electrolytes Acids, bases and ionization Some reaction types: precipitation, metathesis, neutralization Ionic Equations Water: where the action is • Water is found in all three states on Earth. • Most common liquid solvent found in nature. • Unique properties of water make it a perfect environment for chemical and biological reactions • Without water, life as we know it could not exist. 1 4/28/2010 What makes water so special? • Polar nature of covalent bond – – – – Ability to form hydrogen bonds Relatively high boiling point Strong solvent for ionic compounds Low density of ice Dissolution of Ionic compounds • Water molecules surround the NaCl crystal. • The – side of the H2O dipole is attracted to the Na+ ions while the + side of the H2O dipole heads for the Cl- ions. • The H2O molecules surround and carry off each ion, until the crystal is completely dissolved. 2 4/28/2010 Dissolution/Dissociation • When ionic compounds dissolve in water, the anions and cations are separated from each other. This is called dissociation. – However, not all ionic compounds are soluble in water! • When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion. • When molecular compounds dissolve in water, the only ones that can form ions in solution are acids and bases. The rest will just dissolve as the complete molecule (if that molecule is soluble in water) Solutions There are hundreds of aromatic compounds in coffee • Solutions are defined as homogeneous mixtures of two or more pure substances. • The solvent is present in greatest abundance. • All other substances are solutes. • Aqueous solutions such as coffee, seawater and wine have many solutes, both liquid and solid 3 4/28/2010 Properties of aqueous Solutions: Electrical conductivity eCl- Na Na++ • For electric current to be conducted, charged, mobile carriers such as free e- or free ions must be present. – Metals have mobile electrons to carry current. – Aqueous solutions which contains ions can conduct electricity. • Strong electrolytes produce a high concentration of ions and are good conductors • Weak electrolytes produce fewer ions and are poor conductors. • Non-electrolytes don’t form ions or conduct electricity Electrolytes & Non-electrolytes Ionic compound → Complete dissociation (after dissolving) CH3COOH ↔CH3COO- + H3O+ ↔ + Non-acid molecular → No dissociation (methanol in water) - 4 4/28/2010 Strong ElEctrolytES ArE… • Soluble ionic compounds • Strong acids • Strong bases Weak Electrolytes: • Weak acids – incomplete ionization – Memorize the 7 strong acids; the rest are weak • Weak Bases – incomplete ionization – Usually contain N-H in the compound (like NH4OH) 5 4/28/2010 Reaction Types • Precipitation reactions – Two soluble reactants combine to form an insoluble product • Metathesis (transpose) or displacement/ replacement – Exchange ions in reactants to form new products • Acid-base reactions – Neutralization reaction forming ionic salt and water (usually) • Acid reactions with gas formation – Carbonic acid decomposition – Formation of H2S Solution Chemistry • We need to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). • If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction. • Sometimes there is no visible change in the solution, but the reaction still occurred 2Al(s) + 3Br2(l) → 2AlBr3(s) Precipitation reactions 2 AgNO3 + Cu Cu(NO3)2 + 2 Ag 6 4/28/2010 Precipitation Reactions • Some reactions involve the combining of ions in solution resulting in formation of a material that is insoluble in water. These are called precipitation reactions. AgNO3 (aq) + NaCl (aq) g AgCl (s) + NaNO3 Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq) • Precipitation reactions are generally done with the reactants dissolved in water to allow the ions to move more freely. 2+ 2- Mg 2+ + CO32- g MgCO3 i Precipitation Reaction with Free Ions Combining Precipitation Reactions 2 KI(aq) + Pb(NO3)2(aq) Dissociated ions in solution (both beakers) 2 KNO3(aq) + PbI2(s) PbI2 precipitate with K+ and NO3- ions in solution 7 4/28/2010 No Precipitate Formation g No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq) All ions still present in solution no reaction. Predicting Precipitation • Need to know solubility of reaction products – Low solubility in reaction media (usually water) means product will precipitate • Solubilities for many ions can be predicted based on historical data • Solubility (a physical property) can be used to predict whether a reaction will occur and can be used to separate the reaction products 8 4/28/2010 Solubility Rules: Compounds that Are Generally Soluble in Water Compounds containing the following ions are generally soluble Exceptions (when combined with ions on the left the compound is insoluble) Li+, Na+, K+, NH4+ none NO3–, C2H3O2– none Cl–, Br–, I– Ag+, Hg22+, Pb2+ SO42– Ca2+, Sr2+, Ba2+, Pb2+ Solubility Rules: Compounds that Are Generally Insoluble Compounds containing the following ions are generally insoluble OH– S2– CO32–, PO43– Exceptions (when combined with ions on the left the compound is soluble or slightly soluble) Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ Li+, Na+, K+, NH4+, Ca2+, Sr2+, Ba2+ Li+, Na+, K+, NH4+ 9 4/28/2010 Metathesis (Exchange) Reactions • Metathesis comes from a Greek word that means “to transpose.” • It appears the ions in the reactant compounds exchange, or transpose, ions. • Also known as double displacement or replacement reactions AgNO3 (aq)+KCl (aq) → AgCl(s)+KNO3 (aq) acetic acid Properties of Acids • Formula starts with H (hydrogen), acids are H+ donors. • Sour taste. • Change color of vegetable dyes. • React with bases to form ionic salts and often H2O. • React with “active” metals, not noble metals. – i.e., Al, Zn, Fe, but not Cu, Ag or Au. – Zn + 2 HCl ZnCl2 + H2 – Corrosive. phosphoric acid Tartaric acid Citric acid 10 4/28/2010 Acids There are only seven strong acids: Cl: icks! • • • • • • • Properties of Bases • • • • Hydrochloric (HCl) Chloric (HClO3) Perchloric (HClO4) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO3) Sulfuric (H2SO4) Ammonium hydroxide Bases are H+ acceptors Not so tasty - bitter Feel slippery. sodium hydroxide Change color of vegetable dyes. – Different color than acid. • Strong bases contain hydroxide ion (OH-) • React with acids to form ionic salts and water (usually) – Neutralization. quinine 11 4/28/2010 Strong Bases The strong bases are the soluble metal salts of hydroxide ion (OH-) • Alkali metals • Calcium • Strontium • Barium Acid–Base Reactions • Also called neutralization reactions because the acid and base neutralize each other’s properties. • In the reaction , the H+1 from the acid combines with the OH-1 from the base to make water. • The cation from the base combines with the anion from the acid to make the salt. acid + base salt + water 2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l) HC2H3O2(aq) + NH4OH (aq) NH4C2H3O2 (aq)+ H2O(l) Think of water as H-OH 12 4/28/2010 Predicting Acid–Base Reaction Products 1. Determine what ions each aqueous reactant has. 2. Exchange ions. – (+) ion from one reactant with (-) ion from the other. – H+ combines with OH− to make water. 3. Balance charges of combined ions to get formula of the salt. 4. Balance the equation. 5. Determine solubility of the salt. – Use the solubility rules. – If the salt is insoluble or slightly soluble, it will precipitate. Acid reactions w/ Gas Evolution • Some reactions form a gas directly: K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g) • Other reactions form a gas by the decomposition of one of the products into a gas and water. NaHCO3 + HC2H3O2 H2CO3 NaC2H3O2 + H2CO3 exothermic CO2(g) + H2O endothermic 13 4/28/2010 Gas-Forming Reactions When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water. CaCO3 (s) + HCl (aq) NaHCO3 (aq) + HBr (aq) CaCl2 (aq) + CO2 (g) + H2O (l) NaBr (aq) + CO2 (g) + H2O (l) Similarly, when a sulfite reacts with an acid, the products are a salt, sulfur dioxide, and water. SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l) WhAt’S rEAlly going on? • Molecular equations lists the reactants and products in “molecular” or formula unit form: 2 KI(aq) + Pb(NO3)2(aq) 2 KNO3(aq) + PbI2(s) • In ionic equations, all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are shown dissociated into their ions: 2K+(aq) + 2I-(aq)+ Pb+(aq)+2NO3-(aq) 2K+(aq)+2NO3-(aq) + PbI2(s) • This shows what’s really going on in that water 14 4/28/2010 Net Ionic Equation • The net ionic equation shows us what reaction actually happened – if any. • To make the net ionic equation, start with the ionic equation and cross out anything that does not change from the left side of the equation to the right. 2K+(aq) + 2I-(aq)+ Pb+(aq)+2NO3-(aq) 2I-(aq)+ Pb+(aq) 2K+(aq)+2NO3-(aq) + PbI2(s) PbI2(s) • Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Writing Net Ionic Equations 1. Write a balanced molecular equation. AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) 2. Dissociate all strong electrolytes – write as individual ions (keep the correct coefficients). Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3-(aq) 3. Cross out anything that is unchanged on the left side and right sides of the equation (the spectator ions ) 4. Write the net ionic equation with the species that remain. Ag+(aq) + Cl-(aq) AgCl (s) 15 4/28/2010 Neutralization Reactions When a strong acid reacts with a strong base, the reaction is… Acid + Base → Salt + Water HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) The ionic equation is: H+ (aq) + Cl x- (aq) + Na x + (aq) + OH-(aq) Na x + (aq) + Cl x - (aq) + H2O (l) The net ionic equation is: H+ (aq) + OH- (aq) H2O (l) Some practice Write the Molecular Equation, Total Ionic Equation and Net Ionic Equation for each reaction: #1 CaCO3(s) + HCl (aq) → ? #2 Fe(s) + AgNO3(aq) →? #3 K3PO4(aq) + 3HNO3(aq) →? #4 KNO3(aq) + HCl(aq) →? 16 4/28/2010 Some Practice Example 1: CaCO3(s) + HCl → ? Molecular: CaCO3(s) + 2HCl(aq) gCaCl2(aq) + H2O(l) + CO2(g) Total Ionic : CaCO3(s) + 2H+(aq) + 2Cl-(aq) gCa2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g) Net Ionic : CaCO3(s) + 2H+(aq) gCa2+(aq) + H2O(l) + CO2(g) Example 2: Fe(s) + AgNO3(aq) g? Molecular : Fe(s) + 2AgNO3(aq) g2Ag(s) + Fe(NO3)2(aq) Total Ionic : Fe(s) + 2Ag+(aq) + 2NO3-(aq) g2Ag(s) + Fe2+(aq) + 2NO3-(aq) Net Ionic : Fe(s) + 2Ag+(aq) g2Ag(s) + Fe2+(aq) Example 3: K3PO4(aq) + 3HNO3(aq) g? Molecular : K3PO4(aq) + 3HNO3(aq) ® H3PO4(aq) + 3KNO3(aq) Total Ionic : 3K+(aq) + PO43-(aq) + 3H+(aq) + 3NO3-(aq) ® H3PO4(aq) + 3K+(aq) + 3NO3-(aq) Net Ionic : PO43-(aq) + 3H+(aq) ® H3PO4(aq) Example 4: KNO3(aq) + HCl(aq) ® ? Molecular : KNO3(aq) + HCl(aq) ® KCl(aq) + HNO3(aq) Total Ionic : K+(aq) + NO3-(aq) + H+(aq)+ Cl-(aq) ® K+(aq) + Cl-(aq) + H+(aq) + NO3-(aq) Net Ionic : There is no net because all species remained as aqueous ions For thursday • • • • Keep reading in Chapter 4 Lab Quiz 1 Purity of Hydrate Lab due Work on Chapter 4 Mastering Chemistry 17