Classification. Barium nitrate dissolves in water to produce dissolved barium ions and dissolved nitrate ions.

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1 Classification When dissolved into water, many compounds dissociate into their individual ions. For example, sodium sulfate dissolves in water to produce dissolved sodium ions and dissolved sulfate ions. Na 2 SO 4(aq) 2Na + (aq) + SO 4 2 Barium nitrate dissolves in water to produce dissolved barium ions and dissolved nitrate ions. Ba(NO 3 ) 2(aq) Ba 2+ (aq) + 2NO 3 The compounds that produce ions when dissolved in water can be classified into five categories: strong acids weak acids neutral salts weak bases strong bases. All acids, when dissolved in water, form solutions with low ph values. The lower ph value reflects increased concentration of hydronium ions (H 3 O + (aq)) in the solution due to dissociation of the acid. The dissociation of nitric acid can be shown as H 2 O (l) + HNO 3(aq) H 3 O + (aq) + NO 3 Strong acids dissociate completely and lower ph more than weak acids, which do not dissociate completely. Dissociation equations for strong acids are drawn with single arrows () as above. Dissociation equations for weak acids are drawn with double arrows ( ) as in H 2 O (l) + HC 2 H 3 O 2(aq) H 3 O + (aq) + C 2 H 3 O 2 All bases, when dissolved in water, form solutions with high ph values. The higher ph value reflects increased concentration of hydroxide ions (OH ) in the solution through dissociations that are shown as H 2 O (l) + NH 3(aq) NH 4 + (aq) + OH 1

2 Or, when the compound contains hydroxide ions in the formula, Sr(OH) 2(s) Sr +2 (aq) + 2OH As with weak acids, dissociation equations for weak bases use the double arrow ( ). Neutral salts do not significantly change the ph of water when they dissociate. They form neither hydronium ions nor hydroxide ions when dissolved in water. Dissociation equations for soluble neutral salts are drawn with single arrows (). Procedure 1 Prepare the ph probe for data collection using the LoggerPro system. 1. Open LoggerPro 3.3 by double clicking on the LoggerPro icon on the desktop. 2. Select the correct probe by clicking on the button on the top left of the screen that looks like a calculator. This will open a box titled LabPro:1on the screen. 3. Click on the white box for channel 1 Choose Sensor ph/mv/orp Amplifiers ph. Notice that LoggerPro has put a picture of a ph probe in the channel 1 box. Close this box. 4. To start calibrating the ph probe click on Experiment Calibrate LabPro: 1 CH1: ph. This will open a box titled Sensor Settings. Click on the calibrate now button. 5. Carefully rinse the ph probe with distilled water. Place the ph probe in either ph 4 or ph 10 buffer solution. Wait for the voltage reading to stabilize and record the buffer ph value in the Enter Value: box. 6. Repeat step 5 with the other ph buffer solution (the one you didn t use in step 5). 7. Click OK to exit calibration screen. You must describe this procedure carefully in your notebook; this procedure will be repeated many times in Chemistry lab. Calibrate the ph probe with the two buffer solutions provided in lab. Be sure to slowly swirl the solution being monitored with the probe, and allow enough time for the probe reading to stabilize before recording the final value. Measure and record the ph of each of the solutions listed in Data Table 1 in your notebook. Be sure to rinse off the probe with water into a waste beaker between samples to prevent crosscontamination! Each of the solutions is 0.1 M, which means that they all have the same amount of the dissolved compound (solute) per liter of solution. The chemical identity of the solute will change. To finish the data table, classify each compound as a strong acid, weak acid, weak base, or strong base. This classification will require a judgment on your part. There is at least one example of each type of compound. To decide how the solute affected the water s ph, compare it to water that has been sitting in the hood overnight. Dissolved CO 2 from the air will make the ph of even pure water drop below 7. 2

3 Data Table 1 Compound ph of solution water HCl NH 3 (NH 4 OH) HNO 3 NaOH HC 2 H 3 O 2 HBr H 2 SO 4 KOH H 2 C 2 O 4 HClO 4 HNO 2 HI Complete Data Table 2 in your notebook. Strong Acids Weak Acids Weak Bases Strong Bases Procedure 2 Salts, which are composed of ions, can act as weak acids, weak bases, or they can be neutral salts. All of the solutions in this procedure are made by dissolving salts in water. Measure and record the ph of each of the solutions listed in Data Table 3 in your notebook. Each of the solutions is 0.1 M, which means that they all have the same amount of the dissolved salt (solute) per liter of solution. 3

4 To finish the data table, classify each compound as a weak acid, weak base, or neutral salt. Again, to decide how the solute affected the water s ph, compare it to water that has been sitting in the hood overnight. Compound water KCl NH4Cl ph of solution NaNO3 NaC2H3O2 CH3NH3Cl NaBr KHSO4 NaHC2O4 KClO4 NaNO2 N 2 H 4 KI Complete Data Table 4 in your notebook. Data Table 4 Neutral Salts Weak Acids Weak Bases 4

5 Questions 1. Write the chemical equations for the dissociation reaction of all the compounds listed in Data Table 2. Be sure to use the correct type of arrow in the equations, include the liquid and aqueous state designations, and ionic charges. 2. Draw a molecular representation for the dissociation of HCl in water. Show 10 water reactants and 6 HCl reactants. 3. Draw a molecular representation for the dissociation of KOH in water. Show 10 water reactants and 6 NH 3 reactants. 4. Draw a molecular representation for the dissociation of KI in water. Show 10 water reactants and 6 KI reactants. 5