4.1 Gas Evolution - Pre-Lab Questions

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1. Using the periodic table and your knowledge of reactions, write the balanced equation forthe reaction of calcium (Ca) metal with an aqueous solution of hydrochloric acid (HCl).

2. If you weighed out 0.38 g of calcium and reacted it with an excess amount of HCl, how

many moles of H2(g) would you expect to produce?

3. If the water temperature in your experiment was measured as 23.8°C and the barometric

pressure was given as 753 mmHg, what volume of H2(g) would you expect to produce ifyou reacted 0.38 g of calcium metal with excess HCl?

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4.2 Gas Evolution - Introduction

Strong acids are very reactive and produce hydrogen gas when combined with metals.Hydrochloric acid (HCl) is a strong, monoprotic acid, which yields one hydrogen ion (H+) forevery molecule of HCl put into solution. When hydrochloric acid reacts with a metal, the amountof hydrogen generated depends upon the metal used. Zinc is a transition metal that forms ionswith a charge of +2. When zinc reacts with hydrochloric acid, the following reaction occurs:

Zn(s) + 2HCl(aq) ® ZnCl2(aq) + H2(g)

Because the charge on the metal ion is +2, the salt that forms has a 1:2 ratio of zinc ion tochloride ions, and the balanced reaction requires two HCl molecules as a result. If the metal ionhad a +3 charge, the ratio would have been 1:3 and three HCl molecules would be required andso on. The ratio between the ions forming the salt affects the overall stoichiometry of thereaction.

RelevanceGas laws in combination with reaction stoichiometry are often used to save lives. One lifesavingdevice that is highly dependent on the use of gas laws is the automobile airbag. Airbagdeployment is created chemically rather than mechanically. This is because a mechanicaldeployment would be too slow. The explosive creation of nitrogen gas from sodium azide createsthe filled airbag in a fraction of a second. Gas laws allow the construction of a completely full, butnot overfull, bag of gas that keeps the driver and passenger(s) of a car from serious injury duringa collision. These calculations require both a knowledge of reaction stoichiometry and the idealgas law. The reaction used in this experiment, while not explosive, requires the same type ofcalculations.

BackgroundIn order to determine the stoichiometry of a reaction by experimental methods, you must have away to determine the amounts of both the reactants and products. For solids, this means mass;for solutions, concentration; and for gases, volume at known temperature (T) and pressure (P). Inthe reaction above, the mass of the zinc is easily obtained using an analytical balance. However,the mass of the products are not easily obtained. ZnCl2 is in solution and provides no easymethod by which to measure its concentration. The hydrogen gas on the other hand, can bemeasured if it is collected as it is produced. Use of the ideal gas law will then provide a methodby which the mass can be calculated.

Collecting the GasThe gas produced from the reaction of metal filings and HCl(aq) can be collected using a side-arm flask and a graduated cylinder (Figure 1). The reactants are placed into the side-arm flaskand the stopper is quickly replaced. The side-arm flask is connected to a hose that carries the H2gas produced to an inverted graduated cylinder filled with water. As the gas enters the cylinder,the water is pushed out. Once the reaction is complete, the volume of the gas can be determinedby recording the volume of gas in the graduated cylinder. The mass of the gas can then bedetermined with the measured volume and the Ideal Gas Law.

Figure 1. Set up for hydrogen gas collection.

Calculation of Moles of GasThere are two methods for calculating the moles of gas present in a sample given its volume. Ifthe volume of a gas sample is at STP (0°C and 1 atm), then simply dividing the value expressedin liters by 22.4 liters/mole will produce the number of moles of gas in that sample. If yourexperiment was not done at STP, calculate the number of moles of gas present in a sample byusing the ideal gas law and the volume, pressure, and temperature of the gas sample.

PV = nRT

n = PV RT

The only complication in this method is the presence of water vapor in the cylinder, which mustbe taken into account.

Water VaporIf a gas sample is collected over water or collected by displacement of water, water vapor will bepresent. You need to correct for the pressure of the water vapor. The vapor pressure of water isdependent on the temperature of the water it resides above. A table of the vapor pressure ofwater at common room temperatures is provided below.

Vapor Pressure of Water at Selected Temperatures

Table 1. Data from NIST Standard Reference Database 69: NIST Chemistry WebBook

T (°C) Pwater (torr) T (°C) Pwater (torr)

15 12.67 25 23.57

16 13.51 26 25.01

17 14.40 27 26.53

18 15.38 28 28.13

19 16.34 29 30.81

20 17.38 30 31.58

21 18.49 31 33.44

22 19.66 32 35.40

23 20.89 33 37.45

24 22.19 34 34.61

Reference: E.W. Lemmon, M.O. McLinden and D.G. Friend, "Thermophysical Properties of Fluid Systems" in NIST Chemistry WebBook, NIST

Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, Gaithersburg

MD, 20899, http://webbook.nist.gov, (retrieved June 12, 2016).

Correction for Water Vapor PressureTo correct for the pressure of the water vapor, first determine the pressure of the wet hydrogengas in the inverted cylinder. When the water level in the cylinder is at exactly the same level asthe water level in the trough of water, the pressure inside the cylinder exactly equals the currentatmospheric pressure in the room. Determine the pressure from a barometer (or the value will beprovided to you by your instructor). You also need to know the temperature of the gas, so recordthe temperature of the water bath. Once you have the atmospheric pressure/wet hydrogenpressure and the temperature, you will calculate the “dry” hydrogen gas pressure by subtractingthe water vapor pressure from the atmospheric pressure.

Example: A sample of hydrogen is collected over water at 24°C. The pressure of the wethydrogen is determined to by 764 torr. What is the pressure of the dry hydrogen?

Step 1: Determine vapor pressure from Table 1. The vapor pressure of water at 24°C is22.19 torr.

Step 2: Calculate the pressure dry hydrogen = 764 torr − 22.19 torr = 741.8 torr

Once the gas pressure has been corrected for the water vapor pressure, you have all theinformation (P, V, and T) you need to calculate the moles of gas.

Mole RatioIf this experiment is repeated several times, using different amounts of metal and an excess ofHCl each time, you can plot the moles of hydrogen produced versus the moles of metalconsumed and the slope will be the stoichiometric ratio of hydrogen to zinc. In the example givenin the introduction, one mole of hydrogen is generated for every mole of zinc consumed (1 mole

H2:1 mole Zn). The slope of the graph would be one. Since one mole of hydrogen gas under labconditions is approximately 24.5 L, you will work with much smaller quantities.

The Experiment Each pair of students will complete the reaction six times, each time using a different mass ofmetal so that the plot will have a range of x-values. Use a spreadsheet to calculate the number ofmoles of hydrogen produced. Plot a graph where moles of metal will be plotted on the x-axis, andthe moles of hydrogen will be plotted on the y-axis.

Safety Notes: The hydrochloric acid used in this experiment can damage your eyesand skin. Wear safety goggles at all times during this lab. If acid splashes into youreyes, use the eyewash station for a minimum of fifteen minutes, followed by anevaluation by a physician. If acid splashes on your skin, wash it off immediately withcool water while your lab partner notifies your instructor. If you spill any acid on thebench or floor, notify your instructor who will neutralize it with sodium bicarbonate.The hydrogen gas generated in today’s reaction is flammable when exposed tooxygen. Do not use any flames during this experiment.

4.3 Gas Evolution - Procedure

Part I: Setting Up Your Cylinder

1. Collect a trough, 200-mL or 250-mL graduated cylinder, 125-mL side-arm flask, stopper,rubber tubing, ring stand, and clamp.

2. Fill the trough with water until it is about ~2 cm from the top.

3. Attach the clamp to the ring stand.

4. Completely submerge the graduated cylinder in the water trough and tilt the openingslightly upward to get rid of any trapped air bubbles.

5. Once the cylinder is full of water, invert it in the trough without allowing the top to surface.This will keep the water in the cylinder and you should have no trapped air.

6. While holding the cylinder in place, clamp it to the ring stand for support. Be sure the top ofthe cylinder remains below the surface of the water.

7. Collect a 125-mL side-arm flask, stopper, and a length of rubber tubing. Make sure thetubing is long enough to fit several cm into the cylinder and still be able to be attached toyour side-arm flask.

8. Carefully insert the rubber tubing into the graduated cylinder. Insert it far enough that it willnot easily fall out of the cylinder.

Part II: Starting the Reaction

1. Using a 25-mL graduated cylinder, measure 20 mL of 1.0 M HCl.

2. Carefully pour the 1.0 M HCl into your side-arm flask.

3. Collect your assigned samples of magnesium and weigh out six varying samples (between0.05 and 0.40 g) to the nearest 0.001 g. Be sure you can identify each sample with its mass.Record the masses:

Mass #1___________Mass #2___________Mass #3___________Mass #4___________Mass #5___________Mass #6___________

Remember you need to use six different masses; do not use the same mass six times.

4. Add the metal to the acid and very quickly place the stopper on the mouth of the flask.

5. Wait a minute after the bubbling stops and adjust the cylinder so the water levels inside andoutside the cylinder are equal.

6. Record the volume on the graduated cylinder.

7. Record the temperature of the water bath. Since the hydrogen bubbled through the water, itwill be the same temperature as the water bath. Record the temperature of the water bath atthe end of each trial as the temperature may change during the class period.

8. Disconnect the side-arm flask and pour the aqueous metal chloride and remaining acid intothe proper waste container.

9. Rinse the flask once with water and pour that water into the waste container as well.

10. Collect another 20 mL of 1.0 M HCl and carefully pour it into the flask for the second trial.

11. Refill the inverted graduated cylinder as before and conduct the experiment again with anew mass of metal.

12. You will repeat the experiment for a total of six trials.

Part III: Determine the Atmosphere Pressure

1. Record the atmospheric pressure on the barometer or obtain the value from your instructor.One reading will be sufficient, as the barometric pressure rarely changes significantlyduring a single lab session.

Clean Up At the end of the experiment, wash your flask carefully with dish soap and rinse thoroughly withwater. Wipe the bench top with a damp paper towel to ensure that no drops of acid remain.

4.4 Gas Evolution - Data Sheet

1. Write the balanced equation for the reaction between your metal and 1.0 M HCl:

2. Record the atmospheric pressure: ___________

3. Complete the following tables with your experimental data.

Trial MassMetal (g)

MolesMetal (mol)

Temperature ofWater (K)

Volume ofHydrogen Gas (L)

Water VaporPressure (atm)

1

2

3

4

5

6

Trial Corrected H2 Pressure (atm)

1

2

3

4

5

6

4. Provide an example of each calculation you completed to determine the data in the tablesabove.

5. Plot a graph of the moles of metal vs. the moles of H2 gas produced.

4.5 Gas Evolution - Post-Lab Questions

Name: Date:

Instructor: Section/Group:

Show all work for full credit.

1. Below is a plot of the theoretical data from the reaction of magnesium metal and 1.0 M HCl(aq). On the blankgraph paper provided (see next page), plot the moles of metal vs moles of hydrogen gas calculated for yourexperiment. How closely do your calculated values and graph compare to the expected values? Describe anysimilarities or differences between the two graphs.

4.6 Gas Evolution - Graph Paper