Experiment 7  

Preparation and Properties of Hydrogen Gas

 

 

 

DISCUSSION

Atomic hydrogen, having atomic number 1 and an atomic weight 1.008 amu, is the simplest element.  At ordinary temperatures and pressures it is a gas, composed of diatomic molecules, H2 and is only very slightly soluble in water.  In this form, hydrogen is a colorless, odorless, and tasteless gas.  Hydrogen is the ninth most abundant element in the earth's crust (about 0.9 percent by weight) and the most abundant element in the universe.  The sun and other stars are composed largely of hydrogen.  It is estimated that 90% of the atoms in the universe are hydrogen atoms.  Hydrogen is composed of three isotopes: ordinary hydrogen, 1H, deuterium, 2H or 2D, and tritium 3H or 3T.  Naturally occurring hydrogen is mostly composed of 1H atoms and a very small percentage of deuterium. 

 

The earth’s atmosphere contains only very little molecular hydrogen.  Hydrogen is typically found combined with other elements.  And it is in this form that hydrogen is very common as seen in the many hydrogen containing compounds that exist, both naturally occurring and man-made.  Hydrogen containing compounds can both be ionic and covalent in nature.  Examples include acids, bases, and all organic compounds.  In organic compounds, hydrogen is covalently bonded.  A special group of organic molecules are the biomolecules.  Biomolecules are principally composed of six elements; carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

 

Molecular hydrogen is prepared from compounds through a variety of different chemical reactions.  Three of the most common methods in the laboratory are listed below. 

 

 

Preparation of hydrogen gas from water

Water is the most common and probably most important compound of hydrogen.  Hydrogen is liberated when a direct current of electricity is passed through water containing a small amount of an electrically conducting substance such as sulfuric acid, H2SO4.  The chemical change that occurs can be summarized in a chemical equation, showing the reactant molecules on the left side of the arrow and the products of the reaction on the right side of the arrow.  The net reaction is then written as

 

2 H2O(l)  + electrical energy  ®  2 H2(g)  + O2(g)                                 (1)

 

A chemical reaction needs to be balanced; meaning that the number of atoms on the reactant side have to equal the number of atoms on the product side.  A quick inspection of equation (1) shows that this is indeed the case.  There are 4 hydrogen atoms on the left side and 4 hydrogen atoms on the right side.  Doing the same math for oxygen, you’ll quickly see that there are 2 of each on both sides of the chemical reaction, which results is a perfectly balanced chemical reaction. 

 

In another chemical reaction, hydrogen easily reacts with oxygen to form water.  This reaction is termed a combustion reaction and is the reverse reaction of equation (1).  It can be written as

 

2 H2(g)  + O2(g)  ®  2 H2O(l)                                                                  (2)

 

In today’s experiment, we will use reaction (2) as a simple test for detecting hydrogen, since mixtures of hydrogen and air (or oxygen) burn explosively with a distinctive "popping" or "barking" sound. 

 

 

Preparation of hydrogen gas from active metals

Several of the most active metals-such as lithium, sodium, potassium, rubidium, cesium, magnesium, and calcium-will react with water to produce hydrogen gas.  The net chemical change is summarized in reaction (3) using the reaction of sodium metal with water as an example.  This reaction produces hydrogen gas and sodium hydroxide. 

 

2Na(s)  +2 H2O(l)  ®  2 NaOH(aq)  +  H2(g)                                                       (3)

 

 

Preparation of hydrogen gas from metals with acids

This is the most convenient laboratory method of producing hydrogen.  Several of the metals most commonly used are shown in the reactions below.  In each of these reactions, the metal reacts with an acid to produce a salt and hydrogen gas.  The salts produced in the reactions listed below are magnesium chloride, magnesium sulfate, and zinc phosphate, respectively.

 

Mg(s)  +  2 HCl(aq)  ®  MgCl2(aq)  +  H2(g)                                                                   (4)

Mg(s)  +  H2SO4(aq)  ®  MgSO4(aq)  +  H2(aq)                                                               (5)

3 Zn(s)  + 2 H3PO4(aq)  ®  Zn3(PO4)2(aq)  + 3H2(g)                                                        (6)

 

 

Properties of Hydrogen

In today’s experiment you will collect several bottles of hydrogen and test these bottles for evidence of hydrogen.  Hydrogen is considerably less dense than air and will easily escape these bottles.  We will test the collected hydrogen in each bottle.  Three different observations can be made.

 

(1)  A flame inserted into a bottle of pure hydrogen will go out, because there is no oxygen to support combustion. 

(2)  A flame inserted into a mixture of air and hydrogen will set off a very rapid combustion (burning) of the hydrogen, causing a small explosion.

(3)  A flame brought to the mouth of a bottle of pure hydrogen will cause the hydrogen to burn, but only at the mouth, where the hydrogen is in contact with the air. 

 

Situation (3) is the hardest to detect because the reaction is not explosive, there is little noise, and hydrogen burns with a colorless flame.  Other tests involving the combustion of hydrogen demonstrate that hydrogen is lighter than air and diffuses rapidly. 

 

 

Learn more about hydrogen at

http://pearl1.lanl.gov/periodic/elements/1.html

http://www.praxair.com/praxair.nsf/0/7E8A75E1CC87FA998525655E000B40DA?OpenDocument&Start=1&Count=1000&ExpandView&Menu=&ThemeId=339999&View=LeftNavMenu

http://solstice.crest.org/hydrogen/hydrogen_fuelcell_applications.html

http://www.h2eco.org/

 

 

 


 

MATERIALAS AND EQUIPMENT

Solids: strips of copper, magnesium, and zinc; sodium metal; steel wool; mossy zinc; wood splints.  Liquids: concentrated sulfuric acid (H2SO4).  Solutions: dilute (6 M) acetic acid (HC2H3O2) 0.1 M copper(II) sulfate (CuSO4), dilute (6 M) hydrochloric acid (HC1). dilute (3 M) phosphoric acid (H3PO4) and dilute (3 M) sulfuric acid. 

Pneumatic trough, five wide-mouth gas-collecting bottles.

 

 

 

EXPERIMENTAL PROCEDURE

 

Record all data and observations directly into your notebook. 

 

Caution: Since mixtures of hydrogen and air may explode when ignited, wrap the generator with a towel to prevent flying glass.  Keep burners away from the generator and the delivery tube.

 

 

A.  Preparing Hydrogen from Active Metals (Instructor Demo only)

Record any observations directly in your Data Sheet. 

Fill a test tube half full of water and place it in the test tube rack.  Using a pair of tweezers obtain a piece of sodium and place it on a piece of filter paper.  With a knife, cut a small piece (not larger than a pea), then fold the filter paper over the sodium and press out the kerosene, noting how soft the sodium metal is.  (Sodium is kept in kerosene because it reacts rapidly with water or with the oxygen in air.)  Pick up the sodium with tweezers or tongs and, holding it at arm's length, drop it into the test tube.

 

Immediately bring a flame to the mouth of the test tube and observe the results.  When the reaction has ceased, use a clean stirring rod to place drops of the solution on pieces of red and blue litmus paper to determine whether the solution is acidic or basic.  Acids turn litmus red, bases turn litmus blue.

 

 

B.  Preparing Hydrogen from Acids

1. Using Various Metals

Set up four small test tubes in a test tube rack.  Place one small strip of zinc into the first tube.  In the same way place small samples of copper, steel wool (iron), and magnesium, in this order, into the other three tubes.  Very quickly add a few milliliters of dilute (6 M) hydrochloric acid to each test tube.  Note whether gas is evolved in each case and also note its relative rate of evolution.  Test any gas evolved for evidence of hydrogen by bringing a flame to the mouth of the test tube.

 

2. Using Various Acids.

In the preceding section you found that zinc is one of the metals capable of releasing hydrogen from hydrochloric acid.  In this section you will compare the relative ease with which zinc displaces hydrogen from a variety of acids that differ in strength. 

 

Set up four test tubes in a rack.  Place several milliliters of dilute (6 M) hydrochloric acid into the first tube, dilute (6 M) acetic acid into the second, dilute (3 M) sulfuric acid into the third, and dilute (3 M) phosphoric acid into the fourth.  Now drop a small strip of zinc into each of the four tubes.  The variation in the rates of evolution of hydrogen gas is a measure of the relative strengths of the acids.  Let the reactions proceed for three minutes before making your evaluation.


 

C.  Collecting Hydrogen

Note:  Concentrated sulfuric acid is an extremely hazardous chemical.  It attacks the skin rapidly. If you get it on your skin, wash with water immediately.

 

Construct the gas generator and water trough as shown in Figure 1.  Use a wide-mouth bottle or a 250 mL Erlenmeyer flask, a 2-hole rubber stopper equipped with a thistle tube reaching to within 1 cm of the bottom of the bottle, and a delivery tube.  Clamp the generator to the ring stand.  All connections must be airtight.

 

Weigh about 10 g of mossy zinc chunks into a 250mL Erlenmeyer flask.  Add 4 mL of 0.1 M copper(II) sulfate solution (as a catalyst) and 100 mL of water and make sure that the bottom end of the thistle tube is under water. 

Fill four wide-mouth bottles with water and invert them in the pneumatic trough. 

 

 

 

 

 

 

 

 

 

Figure 1.  Preparing hydrogen

from zinc and sulfuric acid.

 

 

When you are ready to collect the gas, pour (VERY CAREFULLY) about 4 mL of concentrated sulfuric acid through the thistle tube.  Add more acid in 2 to 3 mL increments, as needed, to keep the reaction going.  As each bottle becomes filled with gas, place a glass plate over its mouth while the bottle is still under water; then remove the bottle from the water and store it mouth downward without removing the glass plate.  Set aside, but do not discard, the first bottle of gas collected.  Fill the other three bottles with hydrogen.  When you are done collecting Hydrogen gas, fill the generator with water to cease the reaction.  Open the generator, rinse the remaining zinc thoroughly with water, and return the unreacted zinc to the container provided by the instructor.

 

 

D.  Reaction of Hydrogen

NOTE:  Unless otherwise directed, keep the bottles mouth downward while performing the following tests.  Hold the bottle with tongs or a towel to protect your hands. 

 

In the following procedure, you will test for evidence of hydrogen gas in each of the bottles collected.  Again, record all data and observations directly in your notebook. 

Have a burning splint ready to go before you open the bottle containing the hydrogen gas.  Light any matches away from the hydrogen gas. 

 

1.  Raise the first bottle of gas collected a few inches straight up from the table, and immediately apply a burning splint to the mouth of the bottle.

 

2.  Raise the second bottle straight up a few inches from the table.  Then without delay slowly insert a burning splint halfway into the bottle.  Keeping the mouth of the bottle down, insert the splint in the bottle even though you hear a muffled sound as the flame approaches the mouth of the bottle.  Slowly withdraw the splint until the charred end is in the neck of the bottle; hold the splint there a few seconds, but do not withdraw it completely.  Repeat inserting and withdrawing the splint several times.

 

3.  To demonstrate that hydrogen forms explosive mixtures with air, hold the third bottle mouth upward and remove the glass plate.  After one minute, bring a burning splint to its mouth. 

 

4.  Keeping the cover plate over its mouth, place the fourth bottle of hydrogen (still upside down) mouth to mouth over a bottle of air.  Then remove the cover plate from between them.  Let the two bottles remain in this position for three minutes, then replace the cover plate between them. Leaving the cover plate on the mouth of the lower bottle, raise the top bottle straight up, at least 6 inches, and immediately bring a burning splint to its mouth.  Turn the lower bottle mouth downward, with the cover plate in place.  Bring a burning splint to the mouth of this bottle as you lift it straight up.  Compare the results.

 

 

 

WASTE DISPOSAL

Dispose of any unreacted metal strips by rinsing with water and putting them in the waste baskets or a container provided by your instructor, not in the sink!

 

 

 

PRELABORATORY ASSIGNMENT

Preparation and Properties of Hydrogen

 

1. What is a group of elements?  How many groups are there in the periodic table?

2. What is a period of elements?  How many periods are there in the periodic table?

3. Give the exact location (group and period) of hydrogen and sodium on the periodic table. 

4. Write a balanced reaction for the reaction of sodium with water.  Now write a balanced reaction for the reaction of potassium with water.  What are the names for the products of each reaction?  What do you believe is the difference in reactivity in the two reactions? 

5. Write a balanced equation for the reaction of deuterium, D2, with molecular oxygen.  What is the name of the product?  (Recall that deuterium is an isotope of hydrogen and that it will chemically behave just like molecular hydrogen)

http://www.chemindustry.com/chemicals/search/D/deuterium.asp http://encarta.msn.com/media_461531710_761575299_-1_1/Hydrogen_Isotopes.html

6. What are the possible charges that hydrogen can have in compounds?  Give one example to each of your answers. 

7. a) What is a hydride?  b) Give the Lewis description for the formation of calcium hydride from the elements. 

8.  What is combustion? 


 

DATA AND ANALYSIS: 

Preparation and Properties of Hydrogen Gas

 

Record all observations into your notebook. 

 

A. Preparing Hydrogen from Active Metals

1. Describe what you observed when sodium was dropped into water.

2. Describe what you observed when a flame was brought to the mouth of the test tube.

3. What color did the litmus papers turn?

4. Did the reaction make the solution acidic or basic?

 

B. Preparing Hydrogen from Acids

1. a) Give the names and the symbols of the metals that react with dilute hydrochloric acid.

b) Write the symbols of the four metals in order of decreasing ability to liberate hydrogen from hydrochloric acid. 

2. Write the formulas of the four acids in order of decreasing strength based on the rates at which they liberate hydrogen when reacting with zinc.

 

C. Collecting Hydrogen

1. Why was water added to cover the bottom of the thistle tube?

2. What physical property of hydrogen, other than that it is less dense than water, allows it to be collected in this manner?

 

D. Reaction of Hydrogen

1. What happened when the splint was brought to the mouth of the first bottle of gas collected?

2. Describe fully the results of testing the second bottle of gas.

3. a) Is hydrogen combustible?                      

b) What evidence do you have of this from testing the second bottle of gas?

4. a) Does hydrogen support combustion?

b) What evidence do you have of this from testing the second bottle of gas?

5. What compound was formed during the testing of the first and second bottles of gas?

6. a) What happened when the splint was brought to the mouth of the third bottle of gas?

b) How do you account for this?

7. Why did the first bottle of gas behave differently from the second bottle?

8. Please comment on each of the following:  When testing the fourth bottle:

a) What was the result with the top bottle?

b) What was the result with the lower bottle?

c) How do you account for these results?

9. Hydrochloric acid with metallic zinc was used to generate hydrogen gas.  Was the reaction specific with HCl, or could HBr have been used?