Chemistry 11 Unit 1:Stoichiometry 10/30/2016 /20

Transcription

1 Lab #6 Reaction of a Metal with Hydrochloric Acid THE AIM OF THIS EXPERIMENT: Name: Partners: In this experiment, you will react hydrochloric acid with magnesium to produce H 2 gas, and to determine the molar ratio between magnesium and hydrogen for this reaction. INTRODUCTION: The hydrogen gas produced from the reaction of magnesium with hydrochloric acid is collected over water. Therefore the gas is not dry, but contains water vapour. Dalton's law of partial pressure states that the total pressure of a mixture of gases is the sum of their partial pressures. To find the pressure of only the hydrogen, the vapour pressure of the water at the given temperature is subtracted from the total pressure of the gas within the buret (See Eq. 1). /20 PH Proom P 2 H 2O (1) where P room is the barometric pressure on your lab day. can be found from Table1. The moles of gas can be obtained by the ideal gas law, using your results and converting the data to the correct units. Similarly, we can use the molar volume at the room temperature. P H2 O PV nrt (2) where P = P H2 in atm (from Eq. 1) V = volume in litres (V(total) from Eq.3) R = gas constant, L-atm/mol K PV nrt PV nrt n n V RT n P Molar Volume RT P T = room temperature in K WASTE DISPOSAL Magnesium ribbon Mg dispose of any extra in solid inorganic waste Hydrochloric Acid HCl wash down the sink with lots of water

2 Procedure 1. Rinse the gas buret with distilled water. 2. 3/4 fill a 1000 ml beaker with tap water. Set up a stand and buret clamp by the beaker. 3. Weigh to 2 decimal places a piece of magnesium ribbon (mass should be between 0.02g and 0.03g) The mass we used was g 4. Make a copper cage containing the magnesium ribbon, making sure that there is at least 8 cm of copper wire left at the top. (See Figure 1). 5. Measure out ~15 ml of 3 mol/l HCl using a graduated cylinder. Using a buret funnel, pour the HCl into the buret. SLOWLY fill the buret to the top with distilled water. While filling, rinse any acid that may be on the sides of the buret, so that the final liquid in the top of the buret will contain very little acid. DO NOT stir up the acid layer in the bottom of the buret. 6. Holding the copper cage by the handle, insert it into the buret until the cage is positioned between the 1 ml and 2 ml marks. Hook the copper wire over the edge of the buret and clamp it there by inserting the rubber stopper. When properly set up, the buret will contain no air bubbles and the water will completely fill the hole in the stopper as well as the buret. 7. Cover the hole in the stopper with your finger and invert the buret into your beaker of water. 8. Clamp the buret in place. The acid, being more dense than water, will stream down through the water and will eventually react with the metal. (Figure 2) 9. After the reaction stops, wait about 5 minutes to allow the buret and its contents to come to room temperature. Bubbles clinging to the side of the buret can be dislodged by tapping the buret. 10. Cover the hole in the stopper with your finger and transfer the buret to a large container in the lab which is almost filled with water at room temperature. Once the stopper is immersed, remove your finger from the hole. Raise or lower the buret until the level of the liquid inside and outside the buret are the same. This permits you to measure the volume of gases in the buret (hydrogen and water vapour) at the air pressure of the room.

3 11. Read the volume with your eye at the same level as the bottom of the meniscus. Record the volume of the gas to the nearest 0.05 ml. This is V (r). ml H 2 convert to liters L H Discard the contents of the buret and rinse it well with water. 13. Record the volume of gas, the mass of your magnesium used on the Lab White Board 14. Measure the room temperature. C (Convert C to K by adding K C ) K 15. Measure atmospheric temperature. hpa (Convert to mm Hg, another unit of pressure) 760 mm Hg hpa 1013 kpa = mm Hg 16. The pressure of hydrogen in the atmospheric pressure minus the pressure attributed to water in the container. The pressure in the gas buret must be the pressure from the hydrogen and the water vapour. In order to find the pressure of hydrogen we write: P H2 = P atmosphere - P water Find P H2. (See Table attached / in the lab for the P water ) mm Hg H 2 O 17. Convert this pressure of Hydrogen from mm Hg to atm of pressure. mm Hg H 2 mm Hg 1.00 atm 760 mm Hg atm H Calculate the molar volume at this temperature: (T is temp from (14), in K & P is atmospheric, atm (17) ) Molar Volume ( P Latm molk )T L /mol

4 19. Given the molar volume at the temperature in the lab you found in question 18 (This is the value that you will use as if you were using 22.4L at 0 C. Since your teacher didn t want to freeze you we had to do the experiment at room temperature meaning the molar volume needed to change), Find the number of moles of hydrogen gas produced in this experiment. Show your work. This is n experiment [1 pts] 20. Write the balanced equation for the reaction of magnesium metal with HCl(aq) producing Magnesium chloride and hydrogen gas. [1 pt] 21. Assuming that the amount of HCl is in excess, determine the number of moles of hydrogen gas that should have been produced from the mass of magnesium you used and record in question (3). Show your work. This is n theoretical. [1 pt] 22. Determine the percent error for your experiment. [1 pt] n % Error H,experiment n H 2 n H,theoretical,theoretical

5 Analysis 1. What is meant by molar volume? [1 pt] 2. What is meant by stoichiometry? [1 pt] 3. The % error for this experiment can be quite high. Why is using a 2-decimal place scale (accurate to 0.01g) instead of a 3-decimal place scale (accurate to 0.001g) when measuring the magnesium a possible explanation of this? [1 pt] 4. Why did we have to recalculate the molar volume for this lab and not just use 22.4 L/mol? [1 pt] 5. If you captured 32.00ml of Hydrogen gas under the same conditions used during this lab, how many grams of magnesium were used to produce this volume of H 2? [3 pts]

6 6. Sodium metal will react with Hydrochloric acid in the following manner: Na(s) + HCl(aq) NaCl(aq) + H 2 (g) (a) Balance the chemical equation [1 pt] (b) If 0.250g of Na(s) are used with excess HCl(aq), how many moles of hydrogen gas can be expected? [2 pts] (c) If the atmospheric pressure is 1.00atm, and the experiment was conducted at 24 C: (i) What is the molar volume at this temperature? [1 pt] (ii) How many ml of hydrogen should be produced? [2 pts] (d) At 24 C and 1.00 atm of pressure you want to fill a blimp with Hydrogen. (Which is a bad plan since Hydrogen is extremely flammable). If the volume of the blimp is Liters and you want to fill it with Hydrogen gas (H 2 (g) ) by the above reaction, how many grams of sodium would you need to fill the blimp with H 2 (g)? [3 pts]

7 Table 1 VAPOUR PRESSURE OF WATER AT VARIOUS TEMPERATURES Temperature Pressure ( C) (mm Hg)