Hydrofluoric acid (HF) is a Bronsted acid, while aluminium hydroxide (Al(OH)3) is a Bronsted base. Let us go into the details about the reaction between HF and Al(OH)3.
HF is a colorless strong inorganic acid. It can be dissolved in water. HF is used in industries like electronics, glass etching and fluorescent light bulbs. Al(OH)3 is an amphoteric white amorphous powder. It is soluble in alkaline and acidic solutions but water-insoluble.
In this article, we will learn more details about the reaction between HF and Al(OH)3, like the product formed, the balancing method, type of reaction etc.
What is the product of HF and Al(OH)3?
Aluminium fluoride (AlF3) and water (H2O) are formed when aluminium hydroxide reacts with hydrofluoric acid.
Al(OH)3+ 3HF ——-> AlF3 + 3H2O
What type of reaction is HF and Al(OH)3?
HF + Al(OH)3 is classified as an acid-base reaction (neutralization reaction) since a strong acid neutralizes a strong base to produce salt and water.
How to balance HF and Al(OH)3?
The steps to balance HF + Al(OH)3 are as follows –
- The general equation is HF + Al(OH)3 = AlF3 + H2O.
- Count the number of each element involved in the reaction, in both the sides, i.e., reactant and product sides of the equation.
| Elements | Reactant side | Product side |
|---|---|---|
| H | 4 | 2 |
| F | 1 | 3 |
| Al | 1 | 1 |
| O | 3 | 2 |
- The product side is devoid of 2 moles of fluorine, 2 moles of hydrogen atom, and 1 mole of oxygen atom.
- To equalize the number of elements in the reactant and product side, we add 3 moles of HF on the reactant side and 3 moles of water on the product side.
- Therefore, the overall balanced reaction is given by –
- 3HF + Al(OH)3 = AlF3 + 3H2O.
HF + Al(OH)3 titration
An acid-base titration between hydrofluoric acid and aluminium hydroxide can be performed to measure the strength of HF. The procedure for titration is explained below.
Apparatus
Burette, burette holder, pipette, conical flask, beakers, and wash bottle.
Indicator
Phenolphthalein, an acid-base indicator is used to detect the end point of the titration.
Procedure
- Aluminium hydroxide solution of known concentration is taken in a burette, while hydrofluoric acid is taken in a conical flask.
- Add 1-2 drops of phenolphthalein indicator into the conical flask.
- Al(OH)3 is slowly added from the burette into the conical flask with constant shaking until the colorless solution turns light pink.
- The above titration is repeated for 3 concordant readings.
- The strength of HF is calculated using the formula, SAl(OH)3 VAl(OH)3 = SHF VHF
HF + Al(OH)3 net ionic equation
The net ionic equation between HF + Al(OH)3 is –
3H+ (aq) + Al(OH)3 (s) = Al3+ (aq) + 3H2O (l)
The following steps are ensured to derive the net ionic equation:
- Balance the given molecular equation to get –
- 3HF + Al(OH)3 = AlF3 + 3H2O
- Indicate the ionic form of each of the substances existing in aqueous form and capable of complete dissociation in water. The complete ionic equation of HF + Al(OH)3 is –
- 3H+ (aq) + 3F– (aq) + Al(OH)3 (s) = Al3+ (aq) + 3F– (aq) + 3H2O (l)
- Cancel the spectator ions (3F–) on both sides of the equation to get the net ionic equation.
- 3H+ (aq) + Al(OH)3 (s) = Al3+ (aq) + 3H2O (l)
HF + Al(OH)3 conjugate pairs
- HF and F– are the conjugate acid-base pairs.
- OH– and H2O are the conjugate acid-base pairs.
HF + Al(OH)3 intermolecular forces
- Hydrogen bonding and dipole-dipole interactions exist as intermolecular forces in HF. The latter is more predominant due to the presence of highly electronegative fluorine atom.
- London dispersion forces exist in aluminium hydroxide.
HF + Al(OH)3 reaction enthalpy
HF + Al(OH)3 standard reaction enthalpy is -250.17 KJ/mol. The values of enthalpy of formation are listed below –
| Reactants and Products | Enthalpy in KJ/mol |
|---|---|
| HF (aq) | -332.63 |
| Al(OH)3 (s) | -1284 |
| AlF3 (aq) | -1531 |
| H2O (l) | -285.8 |
- ∆Hf°(reaction) = ∆Hf°(products) – ∆Hf°(reactants)
= -1816.8 – (-1566.63)
= -250.17 KJ/mol
Is HF + Al(OH)3 a buffer solution?
HF + Al(OH)3 is not a buffer solution because HF is a strong acid, and a strong acid is not involved in buffer formation.
Is HF + Al(OH)3 a complete reaction?
HF + Al(OH)3 is a complete reaction, as aluminium fluoride formed is completely soluble in water.
Is HF + Al(OH)3 an exothermic reaction?
HF + Al(OH)3 is an exothermic reaction, as the reaction enthalpy is negative, and the reaction moves forward due to the heat produced.
Is HF + Al(OH)3 a redox reaction?
The reaction between HF + Al(OH)3 is not a redox reaction, as the oxidation states of the elements remain the same.
Is HF + Al(OH)3 a precipitation reaction?
HF + Al(OH)3 is not a precipitation reaction as AlF3 formed is completely soluble in water.
Is HF + Al(OH)3 an irreversible reaction?
HF + Al(OH)3 is an irreversible reaction because the product, aluminium fluoride formed is water soluble, so a backward reaction does not occur.
Is HF + Al(OH)3 a displacement reaction?
The reaction between HF + Al(OH)3 is an example of a double displacement reaction, because the anions and cations exchange their places to form salt and water.
Conclusion
This article compiles the reaction between HF and Al(OH)3. Aluminium fluoride formed in this neutralization reaction, is used to produce an optical film of low-index, fluoroaluminate glass and as an additive in aluminium manufacturing.
Read more about following HF facts
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