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Predicting Chemical Reactions Exchange Reactions (Metathesis Reaction) 1 Precipitation Reactions Predicting Products Consider the reaction between AgNO3 and NaCl in aqueous solution: AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq) Notice that the anions and cations appear to have exchanged partners. 2 Precipitation Reactions • Metathesis Reactions – reactions in which the positive ions and negative ions present in the reactants appear to exchange partners – also called exchange reactions AX + BY AY + BX AgNO3 + NaCl AgCl + NaNO3 3 Precipitation Reactions • How can you predict the products of a reaction between ionic compounds? – Identify the ions (including charge) present in the reactants. – Exchange the anions between the two cations and write the correct formulas (electrically neutral) for the possible products. 4 Precipitation Reactions • How can you predict the products of a reaction between ionic compounds? -Determine if any of the products are insoluble in water. • If any are insoluble, the reaction will procede! • Also, if possible products are in gas form (CO2) or are in the liquid (not aq) form, the reaction will take place. • If none of the products are a solid, a gas, or a liquid (i.e. different physical state, physical state other than aq,) there is no reaction….nothing happens. -Write a balanced equation including the physical states of reactants and products. 5 Predicting Reactions Example: Write the chemical equation for the aqueous reaction between Ba(NO3)2 and Na2SO4. Predict which product(s) will be insoluble. Ions: Ba2+ , NO3 -, Na+, At this point, you don’t care how many of each ion you started with. SO426 Predicting Reactions Possible Products: Ba2+ and SO42- BaSO4 Na+ and NO3 - NaNO3 Make sure that you combine the ions so that total positive charge = total negative charge 7 Predicting Reactions Determine solubility of possible products: BaSO4 (s) Insoluble = solid NaNO3 (aq) Soluble = aqueous 8 Predicting Reactions Write a balanced equation: Ba(NO3)2(aq) + Na2SO4(aq) BaSO4 (s) + NaNO3 (aq) unbalanced You must now make sure that you balance the equation at this point. Ba(NO3)2(aq) + Na2SO4(aq) BaSO4 (s) + 2 NaNO3 (aq) balanced 9 Practice • Predict the identity of the precipitate that forms when solutions of Fe2(SO4)3 and LiOH are mixed. • Ions present in solution: Fe3+ SO42Li+ OH- 10 Practice (contd) • Write a balanced equation for the reaction: Fe2(SO4)3(aq)+ LiOH(aq) Fe(OH)3 (s) + Li2SO4(aq) Unbalanced!! Fe2(SO4)3(aq)+ 6 LiOH(aq) 2 Fe(OH)3 (s) + 3 Li2SO4(aq) Balanced 11 Molecular Equation The molecular equation lists the reactants and products in their molecular form. AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) 12 Ionic Equation • In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. • This more accurately reflects the species that are found in the reaction mixture. AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq) Ag+ (aq) + NO3- (aq)+ K+(aq) + Cl-(aq) AgCl (s) + K+ (aq) + NO3- (aq) 13 Net Ionic Equation • To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3-(aq) Ag+(aq) + Cl-(aq) AgCl (s) The only things left in the equation are those things that change (i.e., react) during the course of the reaction. 14 Net Ionic Equation • To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. • The only things left in the equation are those things that change (i.e., react) during the course of the reaction. • Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3-(aq) 15 Writing Net Ionic Equations 1. Write a balanced molecular equation. 2. Dissociate all strong electrolytes. 3. Cross out anything that remains unchanged from the left side to the right side of the equation. 4. Write the net ionic equation with the species that remain. 16 Practice • Write a net ionic equation for the precipitation reaction that occurs when aqueous solutions of silver nitrate and potassium phosphate are mixed. 17 More practice • Lab 8: • Write molecular, as well as ionic and net ionic reactions (13 of them) if there is a reaction taking place. • Reaction #1: Sodium carbonate plus hydrochloric acid 18 Acids and Bases and Their Reactions 19 Acids • Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water. • Brønsted and Lowry defined them as proton donors. 20 Acids Increase the H+ concentration in water: Example: nitric acid HNO3 (aq) H+ (aq) + NO3- (aq) 21 Acids • Examples of Acids: – HCl – HNO3 – CH3COOH – H2SO4 – H3PO4 hydrochloric acid nitric acid acetic acid sulfuric acid phosphoric acid Note: Acids can form different numbers of H+ ions! 22 Acids • Monoprotic acids – have one H in the formula – form a single H+ ion when they ionize HNO3 (aq) H+ (aq) + NO3- (aq) 23 Acids • Diprotic acids – have two H’s in the formula – can form two H+ ion when they ionize completely H2SO4 (aq) 2H+ (aq) + SO42- (aq) • Polyprotic acids: – Have two or more H’s in the formula – Form two or more H+ ions when they ionize completely 24 Bases • Arrhenius defined bases as substances that increase the concentration of OH− when dissolved in water. • Brønsted and Lowry defined them as proton acceptors. 25 Bases • Bases: – substances that accept (react with) H+ ions. – any substance that increases the OHconcentration when added to water Examples: – Hydroxide ion (OH-) OH- (aq) + H+ (aq) H2O (l) 26 Bases • Examples (cont) – Common hydroxide containing bases: • NaOH, KOH, Ca(OH)2 • Note: These are strong electrolytes! • NaOH (aq) Na+ (aq) + OH- (aq) 27 Bases • Examples (cont): – Ammonia (NH3) • Does not contain OH• Accepts H+ ion from water and increases the OHconcentration in the water NH3(aq) + H2O (l) NH4+ (aq) + OH- (aq) NH3 is a weak electrolyte!! (double arrow!) 28 Acids & Bases • Strong Acid: – an acid that is a strong electrolyte • ionizes completely in solution • Weak Acid: – an acid that is a weak electrolyte • an acid that does not ionize completely 29 Acids & Bases • Strong acids: – Know the names and formulas of the 7 common strong acids: • • • • • • • HCl (aq) HBr (aq) HI (aq) HClO3 HClO4 HNO3 H2SO4 hydrochloric acid hydrobromic acid hydroiodic acid chloric acid perchloric acid nitric acid sulfuric acid 30 Acids & Bases • Examples of Weak Acids HF (aq) H3PO4 CH3COOH hydrofluoric acid phosphoric acid acetic acid 31 Acids & Bases • Strong Base: – a base that is a strong electrolyte – ionizes completely in solution • Weak Base: – a base that is a weak electrolyte – does not ionize completely in solution 32 Acids & Bases • Strong Bases: Know the names and formulas of the strong bases – Alkali metal (1A) hydroxides • • • • • LiOH NaOH KOH RbOH CsOH lithium hydroxide sodium hydroxide potassium hydroxide rubidium hydroxide cesium hydroxide 33 Acids & Bases • Strong bases to know (con’t): – Heavy alkaline earth metal (2A) hydroxides • Ca(OH)2 • Sr(OH)2 • Ba(OH)2 calcium hydroxide strontium hydroxide barium hydroxide 34 Acids & Bases • Examples of Weak Bases: ammonia (NH3) sodium bicarbonate (NaHCO3) • baking soda • a component of Alka-Seltzer 35 Acids + Bases Generally, when solutions of an acid and a base are combined, the products are a salt and water. CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l) Neutralization Reaction 36 Neutralization Reactions When a strong acid reacts with a strong base, the molecular equation is… HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) 37 Neutralization Reactions When a strong acid reacts with a strong base, the ionic equation is… HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) Na+ (aq) + Cl- (aq) + H2O (l) 38 Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq) Na+ (aq) + Cl- (aq) + H2O (l) H+ (aq) + OH-(aq) H2O (l) 39 Neutralization Reactions The products of these reactions have very different properties than the reactants. HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq) Sharp sour bitter slippery salt 40 Acid-Base Reactions • Salt: – any ionic compound whose cation comes from a base and whose anion comes from an acid – an ionic compound that is neither an acid nor a base • In general, acid + metal hydroxide a salt + water 41 Acid-Base Reactions • Neutralization reactions are a type of metathesis reaction. • To predict the products: – identify the ions present – exchange anions – write the correct formulas for the products including physical states – write a balanced equation 42 Acid-Base Reactions Example: Write the balanced equation for the reaction between HBr (aq) and Ca(OH)2 (aq). Ions: H+ Br- Possible Products: H-OH = H2O Ca2+ OH- CaBr2 2HBr (aq) + Ca(OH)2 (aq) CaBr2 (aq) + 2H2O (l) 43 Gas-Forming Reactions • Some metathesis reactions do not give the product expected. • In this reaction, the expected product (H2CO3) decomposes to give a gaseous product (CO2) and H2O. CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l) 44 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) CaCl2 (aq) + CO2 (g) + H2O (l) NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l) “H2CO3” 45 Gas-Forming Reactions 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) “H2SO3” 46 Gas-Forming Reactions • This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! • But just as in the previous examples, a gas is formed as a product of this reaction. Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g) 47