Lecture: Hydrogen and the Electromotive Series
1. The history of hydrogen
a. Cavendish discovered hydrogen in 1766 and called it “flammable air”.
b. Later it was recognized as an element that was a component of water.
c. Lavoisier named it hydrogen (water former) in 1783 when he created water with it.
2. The preparation of hydrogen
a. Only minute amounts exist free in nature as H2.
b. Water, hydrocarbons, some acids and some bases serve as sources of hydrogen.
c. Electrolysis of water is one method- discussed previously
d. Reactions with metals and water
i. Potassium, sodium, and calcium can displace hydrogen from water at ordinary temperatures in that order of decreasing activity. Then hydroxides of the metals are formed.
ii. Magnesium, zinc, and iron will displace hydrogen from water provided the metals are heated and the water is in the form of steam. The oxides of the metals are formed.
e. Reactions with metals and acids
i. Zinc will react with dilute sulfuric acid to release H2 and produce soluble ZnSO4.
ii. Zinc will react with hydrochloric acid (concentrated or dilute) to produce H2 and soluble ZnCl2.
iii. Magnesium will do the same but much more vigorously.
iv. Impurities such as water can be removed by passing the gas through CaCl2 or KOH while solid impurities are removed with a glass wool plug.
v. Oxidizing acids will form water and remove electrons from the metal.
1. Concentrated sulfuric acid will produce water and H2S or SO2 when reacting with a metal like zinc.
2. Nitric acid reacts to produce water and one or more nitrogen compounds
f. Reactions with metals and strong bases
i. Zinc, aluminum, and tin can displace hydrogen from the water solutions of metal hydroxides such as NaOH and KOH.
Electromotive Series of Metals
1. The electromotive series is a list of elements with the most electropositive elements at the top and the least electropositive elements at the base. In other words, the elements that most easily give up electrons are at the top of the list and as one goes down the list, the elements tend to hold their electrons more strongly. A metal higher up in the list can displace a metal from a compound that is lower down the list.
2. Zn + CuSO4 → Cu + ZnS4 The zinc atoms give their electrons up to the copper ions. The zinc ions that result move into the solution while the copper atoms plate out onto the zinc strip. (Zinc is above copper on the list.)
3. Magnesium will replace the zinc in zinc sulfate and the iron in iron II sulfate but nickel metal will not replace the zinc in zinc sulfate.
4. Metals below hydrogen in the Electromotive Series cannot liberate hydrogen from water or acids but they can react with oxidizing acids to produce salts, water, and reduction products of the oxidizing acid. 3 Cu + 8 HNO3 → 2 NO + 3 Cu(NO3)2 + 4 H2OThe fact that copper is oxidized to its metal ion Cu++ by nitric acid indicates that nitric acid is a good oxidizing agent (electron-acceptor).
5. The rate of reaction between metals and acids depends upon several factors. In general, a more electropositive metal will react more rapidly with a given acid than a less electropositive metal. Some acids, like sulfuric and hydrochloric, are strong acids and react rapidly with metals while weak acids, like acetic acid and phosphoric acid, react slowly with metals.
The Physical Properties of Hydrogen
1. The pure gas is odorless, colorless, and tasteless. It does not easily dissolve in water. It is the lightest substance known.
2. There are two isotopes of hydrogen, deuterium and tritium. Deuterium is 1 part in 5000 of all hydrogen while tritium is unstable and occurs in very small amounts.
The Chemical Properties of Hydrogen
1. Hydrogen exists as very stable, covalently bonded, diatomic molecules. The energy required to break this bond is equal to 103,000 calories per g. mol. wt. of hydrogen gas.
2. The covalent bond strength of hydrogen gas is said to be 103,000 cal/g. mol.
Types of Hydrogen Compounds
Compounds of hydrogen with non-metals.
1. Hydrogen will combine with all of the non-metals except the inert gases.
2. The bonds are covalent. For example, hydrogen will combine with oxygen if a spark is applied to the mixture to produce the covalent compound of water where the hydrogen is sharing electrons to have an electron pair.
3. Hydrogen will combine with the halogen gases under different conditions.
a. Fluorine gas- violent even at low temperatures and in the dark.
b. Chlorine gas- violent when heated, exposed to a spark or sunlight.
c. Bromine gas- reacts above 400oC and accelerated by a catalyst like platinum.
d. Iodine gas- reacts above 600oC since it is the least reactive halogen.
4. Hydrogen gas reacts with nitrogen to produce ammonia only at high temperatures and pressure and with a catalyst.
5. Hydrogen gas reacts with sulfur to produce hydrogen sulfide gas at high temperatures (bubble gas through molten sulfur).
Compounds of hydrogen with highly electropositive metals
1. The alkali and alkaline-earth metals react at high temperatures with hydrogen to form salts called hydrides.
Ca + H2 yields CaH2
2. Hydrides are composed of the positive metal ion and the negative hydride ion held together in a crystal lattice by ionic attraction.
3. Ionic hydrides are unstable in water and react to produce the hydroxide of the metal and hydrogen gas.
MgH2 + H2O yields Mg(OH)2 + H2
4. The bonds are ionic bonds. The hydrogen becomes hydride by taking on an electron from the metal.
Reactions of Hydrogen with Compounds
Oxides of Metals
1. When hydrogen gas is passed over an oxide of a metal that has been heated, the hydrogen reduces the metal to its metallic state and combines with the oxygen to make water.
Na2O + H2 yields 2Na + H2O (The sodium is reduced and the hydrogen is oxidized.)
2. This process works for the metals below chromium in the electromotive series. The higher the metal in the Series, the higher the required temperature for the process to work.
With Other Compounds
1. Hydrogen will produce methyl alcohol when mixed with carbon monoxide gas at high temperature and pressure in the presence of a catalyst (mixture of zinc oxide and chromium (III) oxide).
2. Vegetable oil can be mixed with hydrogen in the presence of a catalyst (nickel) to convert it into a hardened fat (hydrogenation).
Calculating Equivalent Weights
This equals the amount of a substance that will combine with 1 gram of oxygen
The grams of any element combined with an amount oxygen / the grams of oxygen
The gram equivalent weight of the element is the amount of the element that will combine with 8 grams of oxygen
Multiply the answer above by 8 grams
Do the same for hydrogen but in the second step, multiply by 1.008 grams
Demo: Preparation and reactivity of hydrogen
Introduction to Electrochemistry—Electrolysis of Water
Build a simple electrochemical cell to introduce the basic principles of electrochemistry. All you will need are mechanical pencil leads, a 9-V battery, and a U-tube. The purpose of this experiment is to investigate the chemical reaction that takes place when an electric current is forced through water. Compare the amount of gas and indicator color changes at each electrode to identify the oxidation and reduction half-reactions and to determine the overall reaction. Safe and economical, this introductory-level experiment is also a great “classifying matter” activity!
Quiz/Test:
Lab:
Metal Activity and Reactivity—Oxidation and Reduction Reactions
The usefulness of metals depends on both their physical properties
and their relative reactivity. A ranking of metals based on their relative
reactivity and ease of oxidation is called an activity series. In this microscale
experiment, students compare the activity of metals by investigating the reactions
of metals with metal cations. The single-replacement reactions will occur
in one direction only, with a more active metal always replacing a less active
metal ion. Use this experiment as a simple introduction to oxidation–reduction
reactions and to help students develop critical thinking skills as they rank
metals from most active to least active.