acid + metal

When you add an acid to a metal you expect a chemical reaction to occur in which you produce a salt and hydrogen gas.
Your teacher expects you to be able to write a word equation to describe this chemical reaction.
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Hydrogen Emission Spectrum

How many lines are in the emission spectrum of hydrogen?
Where do these lines come from?
What are the Lyman, Balmer, Paschen, Brackett, Pfund and Humphreys Series?
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Formula for Hydrogen?

I admit it. I love TV game shows. Last Friday I watched one of my favourite shows while eating my (late) lunch. I was even moderately successful at answering some of the questions, until the Host asked The Chaser what the chemical formula for hydrogen was. This led to the following exchange:
Chaser: H
Host: Incorrect
Contestants: H one (we will assume they meant H₁)
Host: Incorrect. The correct answer is H two (we will assume he meant H₂)

So, who was right?

Let's take the Host's "correct" answer first.
The Earth's atmosphere contains small amounts of diatomic molecules of hydrogen gas. "Di" means two and "atomic" refers to atoms so hydrogen gas in the atmosphere is made up of molecules in which 2 atoms of hydrogen are bonded together. When we make hydrogen gas in the laboratory we are making these H₂ molecules. So it seems that the Host got it right ..... except ..... the question didn't ask for the formula of hydrogen gas found in the atmosphere!

So let's turn our attention to the Contestants' response.
Is H₁ a plausible chemical formula for hydrogen?
Not really. If there is only 1 atom of an element in the chemical formula, the "1" is trivial and not included in the formula, so H₁ is the same as H which was the Chaser's response!

So, was the Chaser right?
Is H a valid chemical formula for hydrogen?
Hydrogen is a strange atom. It has 1 proton in its nucleus, and 1 electron "orbiting" that nucleus. In fact, this 1 so-called "valence electron" is a feature common to all Group 1 metals (alkali metals), but other properties of hydrogen suggest it is more like a non-metal than a metal. This similarity to the Group 1 metals led to the prediction that it should be possible to create metallic hydrogen. This would be a solid in which the hydrogen atoms (protons in effect) would be held in a 3-dimensional array with delocalised electrons acting as the metallic bonds holding the array together. This metallic hydrogen would, in theory, be an excellent conductor, indeed it would be a "superconductor", which is why the race has been on to create it!

A chemical formula of a covalent molecule tells us how many atoms of each element are covalently bonded together, H₂ has 2 atoms of hydrogen with a covalent bond between them.
But the chemical formula for a 3-dimensional metallic array refers to the ratio of atoms of each element, if only 1 element is present in a metallic array, like that of sodium metal, then the chemical formula is just the symbol for the element, Na, in this case, or H if you are referring to metallic hydrogen.
So, H is a valid chemical formula for metallic hydrogen, if it exists.
But does metallic hydrogen exist?

In January 2017, researchers at Harvard University announced that they had produced metallic hydrogen in the laboratory using immense pressure. So metallic hydrogen, H, can exist.

Back to the game show.
The Host was right, H₂ is the chemical formula for gaseous hydrogen in the atmosphere.
The Chaser was right, H is the chemical formula for metallic hydrogen.
The Contestants were almost right: Chemists don't write H₁ they just write H.

There is a moral to this story.
Be careful when writing questions. The question should not be ambiguous unless you are prepared to accept multiple different answers that are correct.
Be even more careful when answering test and exam questions. If you need to make assumptions to answer the question you MUST state what those assumptions are when you write your answer.

Reference:

1. Ranga P. Dias, Isaac F. Silvera. Observation of the Wigner-Huntington Transition to Metallic Hydrogen. Science, 2017 DOI: 10.1126/science.aal1579

Tutorials Relevant to this Post

Periodic Table of the Elements
Elements and Compounds
Metals and Nonmetals
Molecular Formula

Naming Covalent Compounds
Empirical Formula and Molecular Formula
Trends in Group 1 Elements
Metallic Bonding

Suggested Study Questions

1. Use the Periodic Table of the Elements to find the chemical symbol for each of the following atoms:
• hydrogen
• helium
• carbon
• nitrogen
• oxygen
• chlorine
2. Write a molecular formula for each of the following diatomic gas molecules:
• hydrogen
• nitrogen
• oxygen
• chlorine
3. Give the number of atoms of each element present in the molecular formulae below:
• H₂O
• H₂O₂
• CO
• CO₂
• NH₃
• NO
• NO₂
• N₂O₂
4. Let M represent an atom of an element. Circle the elements below for which the molecular formula of the element at room temperature and pressure could be represented by M
• helium
• sodium
• oxygen
• iron
• gold
• neon
• chlorine
• nitrogen
• hydrogen
5. For the description of each molecule below, write the molecular formula
• one carbon atom and four hydrogen atoms
• one nitrogen atom and three chlorine atoms
• two nitrogen atoms and one oxygen atom
• one nitrogen atom and five oxygen atoms
• two chlorine atoms and two oxygen atoms
• one carbon atom, one hydrogen atom and three chlorine atoms
6. Given the name of each molecule below, write the molecular formula:
• hydrogen chloride
• carbon monoxide
• carbon dioxide
• sulfur dioxide
• sulfur trioxide
• sulfur dichloride
7. Consider the list of compounds with a possible molecular formulae below. Circle the incorrect formulae and justify your answer:
• water, 2HO
• carbon monoxide, C₁O₁
• hydrogen peroxide, H2O₂
• sulfur trioxide: SO₂
• ammonia, NH3
• hydrogen sulfide, H₂S
• carbon dioxide, C₂O
• sulfur dichloride, S₁Cl₂
8. From the list below, circle the elements that belong to Group 1 of the Periodic Table of the Elements:
• sodium
• helium
• oxygen
• lithium
• chlorine
• nitrogen
• carbon
• potassium
• calcium
9. Draw a table with the headings "metal" and "nonmetal". Place each of the following elements in the correct column:
• hydrogen
• helium
• calcium
• carbon
• nitrogen 
• potassium
• oxygen
• chlorine
• sodium
10. From the list below, circle the elements that would exist at room temperature and pressure as an array of "atoms" help together by delocalised electrons:
• hydrogen
• carbon
• sodium
• lithium
• nitrogen
• chlorine
• iron
• gold
• oxygen

Bohr Model of an Atom

What is the Bohr model of the atom?
What is a planetary model of an atom?
What evidence is there that supports Bohr's model of the atom?

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Metallic Hydrogen Superconductor

Superconductors are materials that permit electricity to travel freely, without resistance, so they could dramatically improve the efficiency of power transmission technologies. Metallic hydrogen should be just such a superconductor.

Liquid metallic hydrogen is thought to exist in the high-gravity interiors of Jupiter and Saturn.
Scientists have predicted that electricity would flow, uninhibited, through a material made by compressing hydrogen into a metal. But so far, on Earth, researchers have been unable to use such compression techniques to squeeze hydrogen under high enough pressures to convert it into a metal. University at Buffalo chemists have now proposed an alternative solution for metallizing hydrogen by adding sodium to hydrogen which they think might make it possible to convert the compound into a superconducting metal under significantly lower pressures.

NaH₉, which does not occur naturally on Earth but is expected to be a stable compound, is predicted to become metallic at an experimentally achievable pressure of about 250 gigapascals, about 2.5 million times Earth's standard atmospheric pressure, but less than the pressure at Earth's core which is about 3.5 million atmospheres.

Reference
University at Buffalo (2011, June 13). Under pressure, sodium, hydrogen could undergo a metamorphosis, emerging as superconductor. ScienceDaily. Retrieved June 15, 2011, from http://www.sciencedaily.com­ /releases/2011/06/110613162240.htm

Further Reading
Metals and Non-metals
Kinetic Theory of Gases

Study Questions

1. Draw up a table listing the properties of metals and non-metals.
2. In what ways is elemental hydrogen like a non-metal?
3. In what ways is elemental hydrogen like a metal?
4. Use the Kinetic Theory of Gases to explain what you expect to happen as elemental hydrogen at atmospheric pressure is subjected to increasing pressure.
5. Using the Kinetic Theory of Gases, describe two ways that scientists could, in theory, make solid hydrogen.
6. Using the Kinetic Theory of Gases, explain why hydrogen might exist as a liquid in the interior of the planet Jupiter.
   
7. If 250 gigapascals is about 2.5 million times Earth's standard atmospheric pressure, what does the prefix "giga" stand for?
   
8. Why do you think that chemists suggest adding sodium to hydrogen to create a solid material capable of conducting electricity?
   

Hydrogen from Water Splitting

The production of hydrogen as an alternative fuel to current fossil fuels relies on the creation of a suitably cheap and efficient way to split water using the power of sunlight. Monash University scientists in Australia, working with UC Davis scientists in the USA, have found that a manganese mineral known as birnessite can be used as a catalyst to speed up the splitting of water into hydrogen and oxygen gases.

Birnessite, a soft, black mineral formed from precipitation reactions in lakes, oceans and groundwater, is predominantly an oxide of manganese, but calcium, potassium and sodium are also present in smaller amounts.
The formula for birnessite is (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O
As a catalyst for the water splitting reaction, the manganese in the birnessite cycles between oxidation states. First, when a voltage is applied manganese (II) is oxidized to manganese (IV). Then in sunlight, birnessite goes back to the manganese (II) state.

The water splitting reaction has two steps:

1. Two molecules of water are oxidized to form one molecule of oxygen gas, four protons and four electrons.
   
2. The protons and electrons combine to form two molecules of hydrogen gas

Reference:
Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey, Leone Spiccia. Water-oxidation catalysis by manganese in a geochemical-like cycle. Nature Chemistry, 2011; DOI: 10.1038/nchem.1049

Further Reading
Oxidation States (Numbers)
Oxidation and Reduction
Balancing Half Equations
Electrolysis - Electrolytic Cells
Percentage Composition

Study Questions:

1. What is meant by the term oxidation state (or oxidation number)?
2. What is the oxidation state (or oxidation number) for each of the following:
   
   • Mn³⁺
   • Mn⁴⁺
   • manganese (II)
   • manganese (IV)
     
3. Write equations to represent each of the following:
   
   • The oxidation of manganese (II) to manganese (IV)
   • The reduction of manganese (IV) to manganese (II)
4. For each reaction in question 3 above, identify:
   • the oxidant
   • the reductant
     
5. Write an equation to represent the first step in the water splitting reaction.
6. Write an equation to represent the second step in the water splitting reaction.
7. Use the equations in question 5 and 6 above to write an overall reaction for the water splitting reaction.
8. For each equation in questions 5 and 6,
   • label the reaction as an oxidation or reduction reaction
   • identify the oxidizing agent(s)
   • identify the reducing agent(s)
9. In the formula of birnessite, (Na_0.3Ca_0.1K_0.1)(Mn⁴⁺,Mn³⁺)₂O₄ · 1.5 H₂O, what does the 1.5 H₂O mean?
   
10. Calculate the percentage composition of birnessite.
    
    

Hydrogen Production for Fuel Cells

Only small amounts of hydrogen occur naturally on Earth, yet the US Department of Energy estimates that the USA uses about 9 million tons per year, and, that this is set to grow if the "hydrogen economy" ever eventuates.

About 95% of the hydrogen in use is produced through steam reforming of natural gas, a catalytic process in which steam reacts with methane to yield carbon monoxide and hydrogen. This mixture is known as synthesis gas, or syngas, and is an intermediate in production processes for synthetic fuels, ammonia, methanol and other compounds.

Hydrogen is a high energy density fuel that is being considered as a cleaner source of future energy, particularly for low-temperature fuel-cell powered devices including vehicles. Fuel cells use electrochemical process to convert hydrogen and oxygen into water, producing current that powers a motor. Fuel cell vehicles require highly purified hydrogen such as is produced in the water-gas-shift reaction. This reaction strips residual carbon monoxide from the hydrogen generated through steam reforming of fossil fuels. Water-gas-shift catalysts decrease the amount of carbon monoxide in hydrogen and increase the hydrogen content by harvesting hydrogen from water molecules.

Currently, copper-based catalysts supported on zinc oxide and alumina are in use. Copper is pyrophoric, it can spontaneously ignite when exposed to air, so researchers have been looking for other more stable catalysts.

Platinum supported on cerium oxide is known to work, but platinum is expensive and cerium occurs in only a few places around the world. Scientists have discovered that sodium improves the platinum activity in the water-gas-shift reaction, which can now take place at low temperatures, even on inert materials such as silica. Less platinum is required, so the cost of hydrogen production should decrease.

Reference:
Yanping Zhai, Danny Pierre, Rui Si, Weiling Deng, Peter Ferrin, Anand U. Nilekar, Guowen Peng, Jeffrey A. Herron, David C. Bell, Howard Saltsburg, Manos Mavrikakis, and Maria Flytzani-Stephanopoulos. Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions. Science, 24 September 2010: Vol. 329. no. 5999, pp. 1633 - 1636 DOI: 10.1126/science.1192449

Further Reading
Reaction Rates
Batteries and Fuel Cells

Study Questions

1. What are the 6 most abundant elements on Earth?
2. Natural Gas is the name given to a hydrocarbon. Give the IUPAC name and formula for this compound.
3. Write a balanced chemical reaction for the reaction between steam and natural gas to yield carbon monoxide and hydrogen.
4. Write equations to represent the electrochemical process to convert hydrogen and oxygen into water in a hydrogen fuel cell.
5. Cerium oxide is also known as ceria. Write a possible chemical formula for ceria.
6. Give the systematic name for alumina, and write its formula.
7. Give the systematic name for silica, and write its formula.
   

Hydrogen: Fuel or Foe?

Hydrogen is being viewed as an eventual alternative to fossil fuels. For metals such as steel, aluminium and magnesium, commonly used in automotive and energy technology, hydrogen is less than ideal.

Hydrogen can permeate the metals when filling the tank, or during various manufacturing processes. It can infiltrate the metal lattice through corrosion, during chromium-plating of car parts, or welding, milling or pressing.

Hydrogen can make these metals brittle and their durability deteriorates leading to sudden failure of parts and components such as the fuel tank, parts of the fuel cell, and even ordinary components like ball bearings.

Scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg are studying hydrogen-induced embrittlement in order to find materials and manufacturing processes that are compatible with hydrogen.

Reference:
Fraunhofer-Gesellschaft (2010, August 21). Hydrogen causes metal to break. ScienceDaily. Retrieved August 22, 2010, from http://www.sciencedaily.com­ /releases/2010/08/100816114831.htm

Study Questions

1. What is meant by the term fossil fuels?
2. Give three examples of commonly used fossil fuels.
3. Why are scientists looking at alternatives to fossil fuels?
4. Describe what is meant by a metal lattice.
5. Explain how hydrogen could infiltrate the metal lattice.
6. Explain how this infiltration of hydrogen into the metal lattice could lead to reduced ductility and brittleness.
7. List other physical properties of metals besides ductility and hardness.
8. List some chemical properties of metals.
   
9. How does steel differ from the other metals mentioned in the article above?