chemistry

Magnesium hydride

Magnesium hydride is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Magnesium hydride rather than just read about it. In short: Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.

Key takeaways

Magnesium hydride belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Magnesium hydride to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Magnesium hydride from memory before moving on to harder problems.

Reference excerpt

Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium. For comparison, one cubic meter can contain 45 kg of hydrogen pressurized at 700 atm, 70 kg of liquid hydrogen, or up to 106 kg of hydrogen bound in magnesium hydride. Magnesium hydride is also investigated for use in thermobaric weapons and incendiary weapons, standalone or as a mixture with a solid oxidizer; China tested a (non-nuclear) "hydrogen bomb" using the substance. It can be also used in emulsion explosives as a source of bubbles and additional fuel. It can be added to improve heat release of aluminized explosive compositions and to improve burn rate of propellants.

Preparation In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500 °C) with MgI2 catalyst:

Mg + H2 → MgH2 Lower temperature production from Mg and H2 using nanocrystalline Mg produced in ball mills has been investigated. Other preparations include:

the hydrogenation of magnesium anthracene under mild conditions: Mg(anthracene) + H2 → MgH2 the reaction of diethylmagnesium with lithium aluminium hydride product of complexed MgH2 e.g. MgH2.THF by the reaction of phenylsilane and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THF or TMEDA as ligand.

Structure and bonding The room temperature form α-MgH2 has a rutile structure. There are at least four high pressure forms: γ-MgH2 with α-PbO2 structure, cubic β-MgH2 with Pa-3 space group, orthorhombic HP1 with Pbc21 space group and orthorhombic HP2 with Pnma space group. Additionally a non stoichiometric MgH(2-δ) has been characterised, but this appears to exist only for very small particles (bulk MgH2 is essentially stoichiometric, as it can only accommodate very low concentrations of H vacancies). The bonding in the rutile form is sometimes described as being partially covalent in nature rather than purely ionic; charge density determination by synchrotron x-ray diffraction indicates that the magnesium atom is fully ionised and spherical in shape and the hydride ion is elongated. Molecular forms of magnesium hydride, MgH, MgH2, Mg2H, Mg2H2, Mg2H3, and Mg2H4 molecules identified by their vibrational spectra have been found in matrix isolated samples at below 10 K, formed following laser ablation of magnesium in the presence of hydrogen. The Mg2H4 molecule has a bridged structure analogous to dimeric aluminium hydride, Al2H6.

Reactions MgH2 readily reacts with water to form hydrogen gas:

MgH2 + 2 H2O → 2 H2 + Mg(OH)2 At 287 °C it decomposes to produce H2 at 1 bar pressure. The high temperature required is seen as a limitation in the use of MgH2 as a reversible hydrogen storage medium:

MgH2 → Mg + H2

References

Illustrations

Worked examples

Example 1 — a first encounter with Magnesium hydride

Start with the simplest possible case. Write down what Magnesium hydride claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Magnesium hydride before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Magnesium hydride ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Magnesium hydride

In research: Magnesium hydride appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Magnesium hydride in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Magnesium hydride is common in secondary-school and first-year university syllabi. It links to neighbouring topics Magnesium compounds, Metal hydrides, so understanding it makes those chapters shorter.
In everyday life: Look for Magnesium hydride outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Magnesium hydride in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Magnesium hydride means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Magnesium hydride out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Magnesium hydride in simple terms?

Magnesium hydride is the chemical compound with the molecular formula MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.

Why does Magnesium hydride matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Magnesium hydride?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Magnesium hydride.