HF is an ionic, polar weak acid with a dipole moment value of 1.86 D. Ca(OH)2 is strong basic slaked lime with pH value of 9. Let us see how Ca(OH)2 reacts with HF in this article.
Hydrogen fluoride (HF) is highly soluble in water and forms hydro fluoric acid that reacts with metal and emits hydrogen gas. HF is an electric insulator that reacts with the glass and melts it. Calcium hydroxide (Ca(OH)2) with a hexagonal crystal structure is 100 % ionized in water. It has the Ksp value of 5.5 * 10-6.
In this article, we will discuss important facts about the HF + Ca(OH)2 reaction such as the reaction enthalpy, the heat required, the product formed, the type of reaction, the type of intermolecular forces between their compounds, etc.
What is the product of HF and Ca(OH)2
Calcium fluoride (CaF2) and water (H2O) are produced as the main products in HF +Ca(OH)2 reaction.
2 HF (aq) + Ca(OH)2 (s) → CaF2 (s) + 2 H2O (l)
What type of reaction is HF +Ca(OH)2
- HF +Ca(OH)2 is an acid – base (neutralization) reaction where HF is a weak base and Ca(OH)2 is a strong base.
- HF +Ca(OH)2 is a salt metathesis (double displacement) reaction.
- HF +Ca(OH)2 is an exothermic reaction.
How to balance HF +Ca(OH)2
The unbalanced chemical equation for HF +Ca(OH)2 is,
HF(aq) + Ca(OH)2(s) → CaF2(s) + H2O(l)
To get the balanced equation, we should follow the steps given below.
- The number of hydrogen atoms is 3 on the reactant side and 2 on the product side. So we multiply HF and H2O with a coefficient of 2 on the reactant and product side, respectively so that number of hydrogen becomes 4 on both sides.
- 2HF (aq) + Ca(OH)2 (s) → CaF2 (s) + 2H2O (l)
- Similarly, the number of oxygen atoms is 2 on the reactant side while it is 1 on the product side so we multiply H2O with a coefficient of 2 on the product side so that the number of oxygen becomes the same i.e., 2
- 2HF (aq) + Ca(OH)2 (s) → CaF2 (s) + 2H2O (l)
- Hence, the net – balanced chemical equation is
- 2 HF (aq) + Ca(OH)2 (s) → CaF2 (s) + 2 H2O (l)
HF + Ca(OH)2 titration
Quantitative estimation of HF is estimated by performing the titration of HF against Ca(OH)2 because HI is acid and Ca(OH)2 acts as a strong base, so titration of this reaction is carried out through an acid-base reaction.
Apparatus used
A burette, Pipette, measuring flask, glass funnel, clamp stand, measuring cylinder, volumetric flask, and beakers are required for this titration.
Indicator
Bromothymol blue dye is used as an indicator for this titration.
Procedure
- A standard amount of HF is filled into the burette and at the same time, the aqueous solution of Ca(OH)2 with the respective indicator is taken in a conical flask.
- Then HF is added dropwise into the conical flask very carefully. The constant shaking of the Ca(OH)2 solution, provides the accurate endpoint.
- The procedure is repeated at least three times until a constant endpoint comes where the indicator changes its color.
- After the successful titration, the strength of hydrogen fluoride is measured by the formula V1N1 = V2N2.
HF +Ca(OH)2 net ionic equation
The net ionic equation for HF + Ca(OH)2 is
2H+(aq) +2F–(aq) + Ca(OH)2 (s) = CaF2 (s) + 2H2O (l)
To get this ionic equation we should follow the steps given below
- Write the solubility equation for HF + Ca(OH)2 by labeling the state or phase (s, l, g or aq) of each substance in the balanced molecular equation of HF + Ca(OH)2
- 2 HF (aq) + Ca(OH)2 (s) → CaF2 (s) + 2 H2O (l)
- Break all the ionic substances which are in an aqueous state into their corresponding ions to get the complete ionic equation
- 2H+(aq) + 2F– (aq) + Ca(OH)2 (s) = CaF2 (s) + 2H2O (l)
- To get the net ionic equation, remove spectator ions from the reactant side and the product side of the complete ionic equation.
- But here no spectator ion is present. So, the net ionic equation is
- 2H+(aq)+2F–(aq)+Ca(OH)2(s) = CaF2(s)+2H2O(l)
HF + Ca(OH)2 conjugate pairs
- The conjugate acid of HF acid is F–
- The conjugate base of H2O is OH–
- Ca(OH)2 and CaF2 do not contain conjugate pairs (acid/base) because in both these compounds proton is not present that can be released.
HF and Ca(OH)2 intermolecular forces
Each compound in HF + Ca(OH)2 → CaF2 + H2O reaction contain the following inter molecular forces:
- Dipole – dipole interaction is present between the HF molecules due to its polar nature.
- Ca(OH)2 contains coulomb force and ionic forces.
- Electrostatic forces are present in CaBr2 molecules.
- Hydrogen bonding is present in H2O molecules.
HF + Ca(OH)2 reaction enthalpy
The enthalpy of the HF + Ca(OH)2 reaction is negative 112.21 kJ/mol. The enthalpy of the formation of reactants and products is as follows:
| Compound | Standard Formation Enthalpy (ΔfH°(kJ/mol)) |
| HF | -332.36 |
| Ca(OH)2 | -1002.82 |
| CaF2 | -1208.09 |
| H2O | -285.83 |
- ΔH°f = ΣΔH°f (products) – ΣΔH°f (reactants) (kJ/mol)
- ΔH°f = [2*(332.36) + 1002.82) – (1208.09 + 2*(285.83)) (kJ/mol)
- ΔHf = -112.21 (kJ/mol)
Is HF + Ca(OH)2 a buffer solution
HF + Ca(OH)2 is a mixture of buffer solution because it provides CaF2 and H2O which is an aqueous solution of a weak acid (HF) and conjugate base ( Ca(OH)2) of the weak acid (HF).
Is HF + Ca(OH)2 a complete reaction
HF + Ca(OH)2 is a complete reaction because HF completely neutralizes Ca(OH)2 into its base (CaF2).
Is HF + Ca(OH)2 an exothermic or endothermic reaction
HF + Ca(OH)2 is an exothermic reaction as the reactants emit 112.21 kJ/mol energy during the reaction and become unstable.
Is HF + Ca(OH)2 a redox reaction
HF + Ca(OH)2 is not a redox reaction because the oxidation states of each element remain the same before and after the completion of the reaction.
Ca+2 (O-2 H +1) 2 + 2 H +1 F -1 = Ca +2 F2 -1 + 2 H2+1 O -2
Is HF + Ca(OH)2 a precipitation reaction
HF + Ca(OH)2 is a precipitate reaction because in this reaction CaF2 is produced in the solid form which is precipitated out through the reaction mixture with the help of centrifugation.
Is HF + Ca(OH)2 reversible or irreversible reaction
HF + Ca(OH)2 is an irreversible reaction. Because at the same reaction conditions, the reactants can not be formed back.
Is HF + Ca(OH)2 displacement reaction
HF + Ca(OH)2 is a double displacement reaction because Ca2+ from Ca(OH)2 and F– from HF displace each other’s positions to form different products.
Conclusion
From this article, it is clear that a strong base like calcium hydroxide reacts with weak acid HF through a double displacement process and emit heat into the surrounding. On the other hand, the formation of CaFr2 results in the removal of two hydroxide ions from Ca(OH)2 which confirm the diacid basic nature of Ca(OH)2.
Hi, I am Kavita Singhal, Ph.D. in Chemical Sciences. My subject area of interest is Physical and Inorganic Chemistry with special emphasis on Electrochemistry, Polymer Chemistry, Nano Chemistry, Corrosion Study, Cyclic voltammetry, Supercapacitance, and Organometallic Chemistry.
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