EIGHT BOTTLES 1 Chemists often do "detective work" in the lab. In order to figure out the answer to a question or to solve a mystery, chemists perform a variety of tests and analyses, then interpret the resulting data. Your goal in this lab is to do just such an analysis. You will identify the contents of eight different bottles. Each bottle contains one ionic compound (a salt), one acid or one base dissolved in water. Since ionic compounds and acids form ions when dissolved in water, each bottle contains freely floating ions in solution. For example, an aqueous solution of sodium chloride, NaCl, contains Na + ions and Cl ions in solution. When dissolved in the water, the solid NaCl crystal lattice is broken apart, ion by ion. Each ion is then surrounded by a group of water molecules. The oxygen atoms of the water molecules tend to congregate near cations (positive ions), while the hydrogen atoms of the water tend to congregate near anions (negative ions). Ions surrounded by water molecules are said to be "hydrated ions." H H H O Na + O Cl O H H H Hydrated sodium ion and chloride ion When two aqueous solutions that contain ions are combined, the most likely event is a double displacement reaction. During such a reaction, there is an exchange of ions between the two compounds. Sometimes a solid, a liquid, and/or a gas is formed during this exchange. Sometimes nothing forms during the combination no solid, no liquid, no gas. For such a combination, we say no reaction has occurred. If a reaction does occur, the product can usually be determined by examining solubility rules. For example, in the case of combining AgNO3(aq) and NaCl(aq), a white solid is formed. The formulas of the products are determined by exchanging the Ag + and Na + ions with NO3 and Cl as follows: AgNO3(aq) + NaCl(aq) AgCl(?) + NaNO3(?) [Eqn. 1] Which product is the observed solid? A table of water solubilities indicates that AgCl does NOT dissolve in water, but NaNO3 does dissolve in water. Therefore, the products should be labelled as AgCl(s) and NaNO3(aq).
2 A very useful form of Eqn. 1 is called the net ionic equation of the reaction. Such an equation shows the simplest summary of the reaction that occurred when the solutions were combined. The net ionic equation for Eqn. 1 is: Ag + (aq) + Cl (aq) AgCl(s) [Eqn. 2] This summary tells the reader that Ag + ions came from solution 1, Cl ions came from solution 2, and when combined, they formed the solid AgCl crystal lattice. The Na + ions and NO3 ions really did not do much of anything. They began as freely floating ions in their respective solutions, and they ended that way. That's not a change at all! Ions that begin and end in the same state are called spectator ions. Spectator ions are NOT included in a net ionic equation. Often when beginning to write a net ionic equation, all a student has are the formulas of the reactants. To complete a net ionic equation, follow these five steps. 1) Write the formulas of the products. 2) Balance the equation. 3) Assign correct state symbols (s, l, g, aq) to the reactants and products. To do so, you will need to consider solubility rules, and whether a substance is a strong or weak acid or base. 4) Rewrite the equation in ionic form. Show ions of aqueous strong electrolytes separately. For example: NaCl(aq) = Na + + Cl NaNO3(aq) = Na + + NO3 BaCl2(aq) = Ba 2+ + 2 Cl HCl(aq) = H + + Cl 2 NaCl(aq) = 2 Na + + 2 Cl A weak electrolyte, such as acetic acid, HC2H3O2, does not ionize completely and is thus not written in ionic form. 5) Cross out spectator ions in the full ionic equation. What's left is the net ionic equation. When applied to the reaction between AgNO3 and NaCl, the process looks like this: Steps 1-3 result in the... FULL EQUATION: AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) Step 4 gives the... FULL IONIC EQUATION: Ag + + NO3 + Na + + Cl AgCl(s) + Na + + NO3 (all ions here are assumed to be aq) Step 5 concludes with the... NET IONIC EQUATION: Ag + (aq) + Cl (aq) AgCl(s)
Similarly, for the reaction between KOH and HCN, the process looks like this: 3 Steps 1-3 result in the... FULL EQUATION: KOH(aq) + HCN(aq) KCN(aq) + H2O(l) Step 4 gives the... FULL IONIC EQUATION: K + + OH + HCN(aq) K + + CN + H2O(l) (remember, HCN is a weak electrolyte) Step 5 concludes with the... NET IONIC EQUATION: OH - (aq) + HCN(aq) CN + H2O(l) Some combinations of ions form products which immediately decompose. Often this decomposition is accompanied by the formation of a gas. Some of the gases have recognizable odors which can aid in identification of the unknown solutions. Five compounds that decompose when they are produced during a double displacement reaction are ammonium hydroxide (NH4OH), carbonic acid (H2CO3), sulfurous acid (H2SO3), thiosulfuric acid (H2S2O3) and nitrous acid (HNO2): NH4OH H2CO3 H2SO3 HNO2 NH3(g) + H2O(l) CO2(g) + H2O(l) SO2(g) + H2O(l) NO(g) + H2O(l) + HNO3(aq) The first four equations are balanced as shown. The last decomposition, for nitrous acid, must be balanced this way: 3 HNO2 2 NO(g) + H2O(l) + HNO3(aq). When a reaction includes a product that decomposes, follow the same 5-step procedure to write its net ionic equation. However, after step 2 (balancing the equation) replace the decomposed compound with the new compounds that it forms. Then proceed as usual. For example, consider the reaction between hydrochloric acid (HCl) and sodium carbonate (Na2CO3). FULL EQUATION: 2 HCl(aq) + Na2CO3(aq) 2 NaCl(aq) + H2CO3(aq) decomposes 2 HCl(aq) + Na2CO3(aq) 2 NaCl(aq) + CO2(g) + H2O(l) new compounds made
4 FULL IONIC EQUATION: NET IONIC EQUATION: 2 H + + 2 Cl + 2 Na + + CO3 2 2 Na + + 2 Cl + CO2(g) + H2O(l) (all ions here are assumed to be aq) 2 H + (aq) + CO3 2 (aq) CO2(g) + H2O(l) Some double displacement reactions do not form a precipitate or a gas. In a neutralization reaction an acid and a base react to form water and a salt that is water soluble. Although no change is observed, a reaction has occurred. For example HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)
PROCEDURE 5 SAFETY NOTE Handle all solutions with care. Avoid skin and eye contact. Some solutions are harmless, but some contain potentially toxic or damaging compounds: AgNO3, H2SO4, HNO3, NaOH and BaCl2. To be safe, treat each unknown as if it were harmful. If a solution does spill on you, wash the affected area with copious amounts of water and notify your instructor of the solution number. Part I - Known Solutions That Form Precipitates Or Gases The purpose of this procedure is to become acquainted with the appearance of precipitates that form when small amounts of solutions are combined. 1. Combine the following solutions by placing 5-10 drops of each on a watch glass. In each case, a precipitate should form. Identify the precipitate that formed. Record detailed observations (color, appearance, etc.) on the data sheet. NOTE: As you work, make sure you avoid contaminating solutions. A. silver nitrate (AgNO3) and barium chloride (BaCl2) B. silver nitrate (AgNO3) and potassium iodide (KI) C. silver nitrate (AgNO3) and ammonium chloride (NH4Cl) D. barium chloride (BaCl2) and sulfuric acid (H2SO4) HOW TO AVOID CONTAMINATION The dropper bottle tip should NEVER touch the watch glass, or a solution which is on the watch glass. dropper tip watchglass solution
6 2. Combine the following solutions by placing 5-10 drops of each on a watch glass. In each case, a gas should form. Carefully smell immediately after mixing. Record your observations (odor, etc.) on the data sheet. NOTE: As you work, make sure you avoid contaminating solutions. Part II - Unknown Sets A. sodium hydroxide (NaOH) and ammonium chloride (NH4Cl) B. sulfuric acid (H2SO4) and sodium nitrite (NaNO2) C. sulfuric acid (H2SO4) and sodium sulfite (Na2SO3) The purpose of this part of the lab is to identify the contents of the given bottles. You will work with four bottles at a time. Make every possible combination and record your observations. Then, use solubility rules and other information discussed in this experiment to determine which compound is in which bottle. After the first set of four bottles, check your identification with your instructor before going on to the next set of four bottles. 1. BOTTLES 1, 2, 3 AND 4 In these four bottles are the following compounds, listed here alphabetically: barium chloride (BaCl 2 ) potassium iodide (KI) silver nitrate (AgNO 3 ) sulfuric acid (H 2 SO 4 ) Combine the solutions two at a time by placing 5-10 drops of each on a watch glass. First bottles 1 and 2, then 1 and 3, then 1 and 4. Next 2 and 3, 2 and 4 and finally 3 and 4. Look for signs of precipitate formation or gas formation. Record your observations on the data sheet. Then figure out the contents of each bottle. 2. BOTTLES 5, 6, 7 AND 8 In these four bottles are the following compounds, listed here alphabetically: ammonium chloride (NH 4 Cl) nitric acid (HNO 3 ) silver nitrate (AgNO 3 ) sodium hydroxide (NaOH) Combine the solutions two at a time by placing 5-10 drops of each on a watch glass. First bottles 5 and 6, then 5 and 7, then 5 and 8. Next 6 and 7, 6 and 8 and finally 7 and 8. Look for signs of precipitate formation or gas formation. Record your observations on the data sheet. Then figure out the contents of each bottle.
7 SOLUBILITY RULES Cl Br I OH NO2 NO3 SO3 2 SO4 2 S2O3 2 Na + Sbl Sbl Sbl Sbl Sbl Sbl Sbl Sbl Sbl K + Sbl Sbl Sbl Sbl Sbl Sbl Sbl Sbl Sbl H + Sbl Sbl Sbl H2O! Dec Sbl Dec Sbl Dec NH4 + Sbl Sbl Sbl Dec Sbl Sbl Sbl Sbl Sbl Ag + Ins Ins Ins Ins s sbl Sbl Ins s sbl Ins Ba 2+ Sbl Sbl Sbl Sbl Sbl Sbl s sbl Ins s sbl Ca 2+ Sbl Sbl Sbl Ins Sbl Sbl Ins s sbl s sbl Sbl = soluble in water Ins = insoluble in water s sbl = slightly soluble (depending on amounts used, may or may not see precipitate) Dec = decomposes in water Trends Worth Noting 1. All compounds containing Na +, K + or NH4 + as the cation are soluble in water. 2. All compounds containing NO3 - as the anion are soluble in water. 3. Most compounds containing Cl -, Br - and I - as anions are soluble in water (exceptions are compounds with Ag +, Pb +2, or Hg2 +2 as the cation). 4. Most compounds containing SO4 2- as the anion are soluble in water (exceptions are compounds with Pb +2, Hg2 +2, Sr +2, or Ba +2 as the cation). Colors of Solids Many ionic solids are white. Some (especially those containing a transition metal as the cation) have a color besides white. Here are some colors of interest: AgBr = pale yellow (may appear as cloudy white when using small amounts) AgI = pale yellow AgCl = bright white (turns gray-lavender on exposure to light for a few minutes) AgOH = brown (forms as AgOH but decomposes in solution to Ag2O) AgNO2 = white Ag2S2O3 = brown BaSO4 = white BaSO3 = white
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9 EIGHT BOTTLES Data Sheet Name Partner Part I OBSERVATIONS 1. A. B. C. D. 2. A. B. C. Part II OBSERVATIONS AND CONCLUSIONS 1. 1 + 2 1 + 3 1 + 4 2 + 3 2 + 4 3 + 4 BOTTLE 1 2 3 4 DISSOLVED COMPOUND
10 2. 5 + 6 5 + 7 5 + 8 6 + 7 6 + 8 7 + 8 BOTTLE 5 6 7 8 DISSOLVED COMPOUND
11 EIGHT BOTTLES Post-lab Assignment On a separate sheet of paper, write the net ionic equation for each of the reactions you carried out in Part II of this lab. Label the reactions by number combination (for example, 1+2, 1+3, etc.). Follow the 5-step procedure outlined in the handout. For each, write (1) the full equation, (2) the full ionic equation and finally (3) the net ionic equation. If no reaction occurred, simply write NR (no equations required). Your paper should look like this: Bottle Combination 1+2 Full Eqn: Full Ionic Eqn: Net Ionic Eqn: 1+3 Full Eqn: Full Ionic Eqn: Net Ionic Eqn: 1+4 Full Eqn: Full Ionic Eqn: Net Ionic Eqn: Etc.
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EIGHT BOTTLES Pre-lab Questions 13 Name Section 1. Four bottles are numbered 1-4, with no other labels on them. One bottle contains BaCl2(aq), another one contains KI(aq), a third one contains AgNO3(aq), and the final bottle contains H2SO4(aq). However, there is no indication which bottle contains which solution. You do the following experiments: Mixing bottle 2 and 3 gives a yellow precipitate. Mixing bottle 2 and 4 gives a white precipitate that turns lavender. Indicate which solution is in each of bottles 1,2,3, and 4. 2. Use the 5-step procedure described in this experiment to write full, full ionic and net ionic equations for the following reactions. Use the solubility rules to determine the states of the products. If no reaction occurs, write NR (for "no reaction.") A. AgNO3(aq) + CaBr2(aq) FULL EQUATION: FULL IONIC EQUATION: NET IONIC EQUATION: B. H2SO4(aq) + KCN(aq) FULL EQUATION: FULL IONIC EQUATION: NET IONIC EQUATION: C. HC2H3O2 (aq) + Ba(OH)2(aq) Last revised 9/28/2016 DN FULL EQUATION: FULL IONIC EQUATION: NET IONIC EQUATION: