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10 EXPERIMENT -140 -120 -100 1 400m PYREX Double Replacement Reactions OBJECTIVES You will be able to predict whether a double replacement reaction occurs when two solutions are mixed. You will test your predictions by mix- ing specified pairs of solutions and observing. the results. Then you will write the molecular equation, total ionic equation and net ionic equa- tion for each observed reaction. Successful completion of this exercise requires: 1. your active presence in the laboratory, 2. completion of the experimental procedures outlined here, and 3. completion of the data sheet. The OBSERVATIONS portion and all required i equations of the experimental reactions must be completed during the laboratory period and approved by the laboratory instructor. DISCUSSION A double replacement reaction results when the exchange of components of two compounds occurs. The general representation of double replacement reactions is: AC + BD AD + BC Eq. 1 In this general reaction, A in the reactant AC replaces B in the reactant BD to form a new product AD. In like fashion, B in the reactant BD replaces A in the reactant AC to form a new product BC. Because two replacements occurred, this type of reaction is called a double replacement reaction. Another name commonly used is "metathesis reaction."
EXPERIMENT 10 Double Replacement Reactions If you mix an aqueous solution of silver nitrate, AgNO, and one of sodium chloride, NaCl, immediately a white solid appears in the container. This solid is called a precipi- tate, and the solution is referred to as the reaction mixture. If the solid is filtered out, the liquid that passes through the filter would be called the filtrate. What reaction might have occurred between the two substances, AgNO, and NaCI? If the reaction was a double replacement, the Ag' from the AgNO, could have replaced the Na" in the NaCl forming a new product, AgCl. At the same time, the NO, from the AgNO, could replace the CI from the NaCl and form another product, NaNO,. The resulting chemical equation for this reaction could be: AgNO,(aq) + NaCl(aq) → AgCl(s) + NaNO,(aq) Eq. 2 The symbols (aq) and (s) indicate the state of the substance. The symbol (aq) following a substance means that the substance is dissolved in water. The symbol (s) means solid; and, when this symbol follows the product, the formation of a precipitate is indicated. When a chemical equation shows all species in their molecular forms, as does Equation (2), the equation is called a molecular equation. How could you determine what the white precipitate was if you had not been told? An answer to this question can be obtained by consulting a handbook of chemistry to determine whether either of the products, or both of them, is soluble or insoluble in water. We find that sodium nitrate is soluble in water, but silver chloride is not soluble in water. Hence, the symbol (s) should follow AgCl in Equation (2) and the symbol (aq) should follow NaNO, A comparison of Equation (1) and (2) indicates that the latter is a double replacement reaction. Note that the formation of a precipitate indicated that a chemical reaction had taken place. Now, we can write the chemical equation for the double replacement reaction in the ionic form. Consider the reactants first. When AgNO, and NaCl dissolve in water, they dissociate into Ag", NO,, Na and Cl. As for the products, NaNO, will be present as Na and NO, However, AgCl will be insoluble and is written as a solid or a precipitate. The total ionic equation describing the reaction may be written: Ag' (aq) + NO, (aq) + Na' (aq) + Cl (aq) → AgCl(s) + Na' (aq) + NO, (aq) Eq. 3 Because Na (aq) and NO,(aq) appear on both sides of Equation (3), these ions, called "spectator ions," are often omitted and only those ions reacting are written in the equation. Ag (aq) + Cl(aq) → AgCl(s) Eq. 4 Equation (4) is the net ionic equation for the reaction. In a reaction in which pre- cipitate forms, we can easily isolate the precipitated product by filtration. The solid precipitate is collected on filter paper while the water and the dissolved spectator ions pass through the filter paper. The liquid that passes through a filter is called the filtrate and is often discarded after the filtration is complete. 100
EXPERIMENT 10 Double Replacement Reactions When an aqueous solution of sodium sulfite, Na,SO, and hydrochloric acid HCl, are mixed, an irritating gas is evolved. Assuming a reaction occurs, we can write Equation (5). Eq. S Na,SO,(aq) + 2HCl(aq) → 2NaCl(aq) + H,SO,(aq) But because sulfurous acid, H,SO, is unstable, it decomposes, forming gaseous SO.. In Equation (6), SO, is designated as a gas by the symbol (g). H.SO, (aq) + H₂O()+ $0,(g) The total lonic equation for the reaction is: Eq. 6 2Na (aq) + SO (aq) + 2H (aq) + 2Cl(aq) → 2Na (aq) + 2C1 (aq) + H₂O(l) + SO,(g) Eq. 7 The formation of the gas, SO,, indicates a chemical reaction has taken place. This reaction also meets the requirements of a double replacement reaction as seen by comparing Equations (1) and (5). The net ionic equation of the reaction would be: So, (aq) + 2H(aq) → H₂O(l) + SO,(g) Eq. 8 In some cases, observations suggesting a chemical reaction are not as obvious. If aque- ous solutions of potassium hydroxide, KOH, and hydrochloric acid, HCL, are mixed, the double replacement reaction shown in Equation (9) occurs. HCl(aq) + KOH(aq) → KCH(aq) + H₂O(1) Eq. 9 No precipitate, gas or other visible evidence of reaction is observed. However, heat is produced which indicates that a chemical reaction has taken place. If the reaction mixture were evaporated to dryness, solid KCI would be the residue suggesting that Equation (9) had occurred as written. Equation (9) can be written in the total ionic form as shown in Equation (10). K(aq) + OH(aq) + H(aq) + Cl (aq) →K'(aq) + Cl (aq) + H₂O() Eq. 10 The union of H and OH leads to the very slightly dissociated product, water, which is present as covalently bonded H,O. Note this reaction is also a double replacement reaction. The symbol (1) means liquid state. When this symbol follows a product, it means that the product forms in the liquid state. The net ionic reaction would be: OH (aq) + H¹(aq) → H₂O(1) Eq. 11 Notice and remember: Equation (11) is the net ionic reaction for any Arrhenius acid-base neutralization reaction. (An Arrhenius acid produces hydrogen lons in water solution, while the Arrhenius base produces hydroxide ions.) 101
EXPERIMENT 10 Double Replacement Reactions Driving Forces for Double Replacement Reactions So far, we have indicated that evidence for a double replacement reaction having occurred can be either (a) the formation of a precipitate, (b) the formation of a gas or (c) a temperature change (this may be either a temperature increase, as in the case of reaction (9), or a temperature decrease). So now we can understand that the "driving force" causing a double replacement reac- tion to occur is that at least one of the products will be 1. insoluble in the reaction mixture and will be observed as a precipitate being formed, or 2. insoluble in the reaction mixture and will be observed as a gas being evolved, or 3. soluble in the reaction mixture, but present as a covalently bonded substance (such as H,O) so that very little dissociation into ions occurs. This will be indicated by the observation of a temperature change. In each of these cases, the reacting ions form a product which is essentially removed from the reaction mixture, causing the reaction "to go to completion." YOU SHOULD KNOW THE THREE DRIVING FORCES LISTED ABOVE. In some cases, the mixture of two solutions does not produce a precipitate, a change in temperature or gas evolution. For example, when KCI and NaNO, solutions are mixed, there is no evidence of any chemical reaction. In such case, we can write KCl(aq) + NaNO,(aq) → No Reaction Eq. 12 PROCEDURE A. General Instructions 1. You are to write three equations for each of the reactions you observe in this experiment: (a) the molecular equation; (b) the total ionic equation; (c) the net ionic equation. If you conclude a particular combination of solutions produces no reaction, you can write the formula for the reactants followed by an arrow. and "No Reaction." No further equations are needed if there is no reaction. 2. The reactants for each of the fourteen possible reactions are shown on the next page. Notice that the formulas for the reactants are not given. Since you will need the correct reactant and product formulas in order to write the reaction equations, bring to the lab any aid you wish to help you write these formulas. Your instruc- tor may require you to write the formulas for all reactants prior to lab. 102
Reaction Grid copper) nitrate iron(1) chloride potassium carbonate sodium hydroxide silver nitrate acetic acid hydrochloric acid X X 5. 7. 11. X EXPERIMENT 10 Double Replacement Reactions potassium phosphate 1. 3. X 8. 12. X potassium carbonate 2. 4. X 9. 13. 14. sulfuric acid X X 6. 10. X X B. Experimental Steps Note: Obtain the solutions to be used and a plastic-covered sheet labeled "REACTION GRID." Read the entire experiment before beginning the proce- dural steps indicated. 1. Place one or two drops of the solution indicated by the row label in each of the boxes in the row except those marked with X's. Position the drop so that half of the drop is in the white area while the other half is in the black area of the column. 2. Next add one or two drops of the solution indicated for the column on top of the first drop. Again, skip the boxes marked with X's. Be careful not to touch the tip of the pipet to the drop of solution already on the grid and be careful not to get your hand in the drops already on the grid. 3. Record your observations of precipitate formation or cloudiness (ppt), gas formation or bubbles (gas), or no visible change (no vis. A) in your data sheet. Include the colors of the precipitates that form in your observations. 4. For those solution pairs that produced no visible changes, go to a hood station and repeat the reactions by adding approximately 2 ml. of each solution from the big bottles to two test tubes and pouring the contents of one tube into the other. Mix the contents and feel the test tube after the mixing is complete. If you feel any temperature increase, record "heat" in your data sheet. If no temperature change occurs, write "N. R." for "no reaction" in the appropriate box in your data sheet. 5. Write molecular, ionic, and net ionic reactions for the combinations for which you observed reactions on the reaction grid or in the test tubes. 103
EXPERIMENT 10 Double Replacement Reactions 6. Clean off the reaction grid with a tissue. Discard the tissue in the solid chemical waste container. Solutions in the test tubes can be flushed down the sink with water. Wash the test tubes and place them upside down to drain in the rack at your station. 7. From your observations on each reaction attempted, conclude whether a reaction did or did not occur and indicate this in the appropriate place on the data sheet. 8. On a separate sheet(s) of paper, in a NEAT and ORGANIZED manner, write the (a) molecular equation, (b) total ionic equation, and (c) net lonic equation for each of the reactions that did occur, including the states (s, 1. g. aq) of the substances. You will receive a zero for Experiment 10 if you do not include the pages with the equations written out. Consult Table A to help you identify a precipitate or gaseous product. If no reaction occurred between reactants "A" and "B," write A+B NR. C. Example Equations-(a) molecular; (b) total ionic; (c) net ionic 1. Solutions used: Ca(NO), and Na,CO, Observation: white ppt. a. Ca(NO),(aq) + NaCO,(aq) →CaCO,(s) + 2NaNO,(aq) b. Cal (aq) + 2NO, (aq) + 2Na (aq) + CO2(aq) → CaCO,(s) + 2Na (aq) + 2NO, (aq) c. Ca (aq) + CO² (aq) → CaCO,(s) 2. Solutions used: LICI and Al(NO), Observation: no observable change a. LiCl(aq) + Al(NO), → N.R. 3. Solutions used: Na CO, and HNO, Observation: gas bubbles The molecular equation we should write would be a. Na,CO,(aq) + 2HNO,(aq) → 2NaNO,(aq) + H₂O(l) + CO.(g) This is because any H,CO, that is produced from CO, and Hions is unstable and quickly decomposes according to the equation 104 H_CO_(aq) — H_O+CO_(g) The formation of the CO,(g) explains the observed gas bubbles in the reaction. b. 2Na (aq) + CO, (aq) + 2H(aq) + 2NO, (aq) → 2Na (aq) + 2NO, (aq) + H₂O(1) + CO₂(g) c. CO,¹- (aq) + 2H(aq) → H₂O(l) + CO.(g)
EXPERIMENT 10 Double Replacement Reactions Table A: Solubility rules for ionic compounds. Chloride Bromide lodide Fluoride Soluble compounds 1. All compounds of the 1A metals are soluble. 2. All compound containing the ammonium ion are soluble. 3. The following ions generally form soluble compounds Nitrate no exceptions Acetate Chlorate Perchlorate Sulfate no exceptions few common exceptions few common exceptions except compounds containing calcium, strontium, barium, Pb, or silver ions except compounds containing silver, Pb, Cu", or Hg, ions except compounds containing silver, Pb, Cu", or Hg, ons except compounds containing silver, Pb, Cu", or Higions except compounds with Pb or the alkaline earth metal cations Insoluble compounds 4. The following ions generally form insoluble compounds: Sulfide except compounds with ammonium ions, alkali metal cations, cal- cium, strontium, or barium ions Carbonate except compounds with the ammonium ion or group 1A Phosphate except compounds with the ammonium ion or group 1A Hydroxide except compounds with group 1A, ammonium, calcium, strontium, or barium ions 5. Hydrogen sulfide and hydrogen cyanide are gases. 6. Carbonic acid breaks down to form CO (g) and H,OH,CO, should not appear as a product in a double replacement reaction. 105
EXPERIMENT DOUBLE REPLACEMENT REACTIONS OBSERVATIONS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 10 13. Nome Lecture Instructor Formulas of Reactants Chem 1110110 Section Approved Observations (ppt, gas, etc.) white ppt gas bubbles Did a reaction occur? blue ppt blue ppt yellow-orange ppt yellow pat gas bubbles gas bubbles no visible change/ test tube got warm. no change no change no visible change/ test tube got warm white ppt tan ppt 14. Note: Write molecular, total ionic, and net ionic equations for the above reactions on separate sheets of paper and turn those in with your data sheets. You will receive a zero for Experiment 10 if you do not include the pages with the equations written out. 107