Hydrogen Halides

The Reactivity Trend

The reactivity of halogens with hydrogen decreases as you move down Group 7. This means the reactions become slower, require harsher conditions, and are less violent.

• Fluorine ($F_2$): Reacts explosively with hydrogen even in the dark and at very low temperatures ($-200^\circ\text{C}$).
• Chlorine ($Cl_2$): Reacts explosively, but requires the presence of sunlight (UV light) or high temperatures to begin.
• Bromine ($Br_2$): Reacts vigorously, but only when heated steadily in the presence of a catalyst (like platinum).
• Iodine ($I_2$): Reacts very slowly even with strong heating, and the reaction is reversible (it does not go to completion).

This decrease in reactivity occurs because the atomic radius increases down the group, adding more electron shells. The increased shielding from inner electrons weakens the nuclear attraction, making it harder for the halogen atom to attract and share an electron to form a covalent bond.

Dissolving in Water

When hydrogen halide gases escape into moist air, they rapidly react with water vapour to form steamy fumes (tiny droplets of concentrated liquid acid).

If you bubble a hydrogen halide gas directly into liquid water, it readily dissolves. During this process, the molecules undergo dissociation, splitting entirely into hydrogen ions ($H^+$) and halide ions. The presence of these $H^+$ ions in the water is exactly what creates an acidic solution.

For example, hydrogen chloride gas dissolves to form hydrochloric acid:

$$HCl_{(g)} \xrightarrow{\text{H}_2\text{O}} H^+_{(aq)} + Cl^-_{(aq)}$$

Other halogens follow the exact same pattern: hydrogen fluoride forms hydrofluoric acid, hydrogen bromide forms hydrobromic acid, and hydrogen iodide forms hydroiodic acid. Because these solutions are strongly acidic (typically pH 1–2), they will quickly turn blue litmus paper red or Universal Indicator red/orange.

Predicting Astatine's Properties

Because Group 7 elements follow strict, predictable patterns, we can use extrapolation to predict the behaviour of astatine ($At$), even if we have never observed it properly in the lab. Astatine is at the bottom of the group, so it will be the least reactive halogen.

We can predict that astatine will react with hydrogen far slower and far less vigorously than iodine. The reaction would require extreme heat, would be highly reversible, and the resulting compound would have very low thermal stability.

Worked Example

Suggest the balanced symbol equation for the reaction between hydrogen and astatine, including state symbols, and predict the properties of the product.

Step 1: Identify the reactants and their states.

• Hydrogen is a gas ($H_{2(g)}$).

• Following the Group 7 physical state trend (gas $\rightarrow$ liquid $\rightarrow$ solid), astatine is predicted to be a black solid ($At_{2(s)}$).

Step 2: Formulate the product.

• The product will be hydrogen astatide. Following the group pattern for hydrogen halides, it will be a gas ($HAt_{(g)}$).

Step 3: Construct the balanced equation.

$$H_{2(g)} + At_{2(s)} \rightarrow 2HAt_{(g)}$$

Step 4: Predict the product's behaviour in water.

• Like other hydrogen halides, hydrogen astatide gas will dissolve and dissociate in water to release $H^+$ ions, forming an acidic solution (hydroastatic acid).

$$HAt_{(g)} \rightarrow H^+_{(aq)} + At^-_{(aq)}$$