Foundations — Preparation, properties, uses of dihydrogen

Before you can read the parent note, you need to earn every symbol it throws at you. This page defines each one from absolute zero — plain words, then a picture, then why the topic can't do without it. Read top to bottom; each block leans on the one above.

1. The atom and the molecule: $\text{H}$ vs ${\text{H}}_2$

Look at the figure: a lone atom on the left (one proton, one electron cloud), and on the right two atoms overlapping their clouds into a shared pool — that shared pool is the chemical bond. The whole topic is about making that right-hand picture happen.

2. State tags: $\text{(s)}$, $\text{(l)}$, $\text{(g)}$, $\text{(aq)}$

Before any reaction, we label what physical form each substance is in. These little tags appear everywhere below, so we earn them first.

3. Charge symbols: ${\text{H}}^{+}$, ${\text{H}}^{-}$, and the electron $e^{-}$

Hydrogen can appear in three different "states of ownership" of its electron. This is the single most important idea for the whole chapter, so we go slowly.

In the figure, watch the electron count on the central proton:

• ${\text{H}}^{+}$ (red): zero electrons — naked, positive.
• $\text{H}$ (white): one electron — neutral, balanced.
• ${\text{H}}^{-}$ (green): two electrons — crowded, negative.

4. Oxidation and reduction: gaining vs losing $e^{-}$

The parent writes two "half-reactions" (we use $z$ for the metal's electron count to avoid clashing with $n$ = moles later): $\text{M}\to {\text{M}}^{z+}+z{e}^{-}\text{(metal loses electrons — oxidised)}$ $2{\text{H}}^{+}+2e^{-}\to {\text{H}}_2\text{(hydrogen gains electrons — reduced)}$

The symbol $z$ here just means "however many electrons this particular metal gives" — $2$ for zinc ($\text{Zn}\to {\text{Zn}}^{2+}$), $3$ for aluminium ($\text{Al}\to {\text{Al}}^{3+}$). See Redox Reactions for the full machinery.

5. The "will it go?" number: reduction potential $E^{\circ }$

The parent claims some reactions happen and others don't. What decides? A single measured number.

The figure is a vertical ladder of $E^{\circ }$ values. Electrons fall downhill — they leave the species with the lower $E^{\circ }$ and go to the one with the higher $E^{\circ }$.

• ${\text{H}}^{+}/{\text{H}}_2$ sits at $E^{\circ }=0.00\text{V}$ (the agreed zero of the ladder — the standard hydrogen electrode).
• ${\text{Zn}}^{2+}/\text{Zn}$ sits below it at $E^{\circ }=-0.76\text{V}$.

Because zinc is lower on the ladder, its electrons happily fall up to the hydrogen — so zinc + acid → ${\text{H}}_2$ is spontaneous. That is exactly the parent's claim, now with a picture behind it.

6. Counting particles: the mole, $n$, and molar mass $M$

Atoms are too small to weigh one at a time, so chemists count in giant fixed bundles.

7. Gas volume at STP: the $22.4\text{L/mol}$ rule

8. Energy of reaction: enthalpy $\Delta H$

The figure shows two energy hills: for the endothermic step the products sit higher than the reactants (energy climbed up, so heat went in); for the exothermic step products sit lower (energy fell, heat came out).

9. Arrows and labels above them

10. Electricity's bookkeeping: $Q$, $I$, $t$, $z$, $F$

For electrolysis the parent uses Faraday's law. Here is each symbol, earned.

Prerequisite map

This map shows the flow: atoms and state tags feed the charge states; those split into the "will it go?" ladder ($E^{\circ }$) and the energy bookkeeping ($\Delta H$); counting ($n$, $M$, molar volume) and electricity ($Q$, $z$, $F$) supply the calculations — and all of them pour into the parent topic.

Equipment checklist

Cover the right side and test yourself. If any answer surprises you, reread that section.

Next: with every symbol now grounded, return to the parent note and read the preparation equations as pictures of electrons moving. Related prerequisite pages: Redox Reactions, Hydrides - Ionic, Covalent, Metallic, Electrolysis and Faraday's Laws, Thermodynamics of Chemical Reactions.