Hydrogen is the most reactive gaseous molecule which can react with any other acid easily. Let us focus on the reaction mechanism between HNO3 and H2.
Hydrogen or H2 is a covalent gaseous molecule where combustion enthalpy is very high so it can easily react with any other molecule vigorously. Hydrogen is not combustible but helps in combustion. Nitric acid is strong mineral acid where the counter anion is nitrate.
Nitric acid is reacted with hydrogen without any catalyst and temperature. Let us discuss the mechanism of the reaction between nitric acid and hydrogen, the reaction enthalpy, the type of reaction, product formation, etc. in the following part of the article.
1. What is the product of HNO3 and H2?
NH3 (Ammonia) is formed as a major product along with water as a by-product when concentrated HNO3 and H2 are reacted together. But when diluted acid is reacted with H2 then it gives nitrogen dioxide along with water.
- HNO3(concentrated) + H2 = NH3 + H2O
- HNO3(diluted) + H2 = NO2 + H2O
2. What type of reaction is HNO3 + H2?
HNO3 + H2 reaction is an example of a base formation reaction, single displacement reaction, and redox and precipitation reaction. Here, the base is formed in the course of the reaction.
3. How to balance HNO3 + H2?
HNO3 + H2 = NH3 + H2O this reaction is not balanced yet, we have to balance the equation in the following way-
- First, we label all the reactants and products by A, B, C, and D as there are four different molecules obtained for this reaction and the reaction looks like this,
- A HNO3 + B H2 = C NH3 + D H2O
- Equating the coefficients for the same type of elements by rearranging them.
- After the rearrangement of coefficients of the same elements by their stoichiometric proportion, we get,
- H = A = 2B = 3C = 2D, N = A = C, O = 3A = 2D,
- Using the Gaussian elimination and equating all the equations we get, A = 1, B = 4, C = 1, and D =3,
- The overall balanced equation will be,
- HNO3 +4 H2 = NH3 + 3 H2O
- 2HNO3 +H2 = 2NO2 + 2H2O
4. HNO3 + H2 titration
To estimate the quantity of nitrate or strength of acid we can perform a titration between H2 and HNO3.
Apparatus used
We need a burette, conical flask, burette holder, volumetric flask, and beakers for this titration.
Titre and titrant
HNO3 versus H2, HNO3 acts as a titrant taken in the burette and the molecule to be analyzed is H2 taken in a conical flask as liquid form.
Indicator
The whole titration is done in an acidic medium or acidic pH so the best suitable indicator will be phenolphthalein which gives perfect results for this titration at given pH.
Procedure
The burette is filled with standardized HNO3. H2 is taken in a conical flask along with respective indicators. HNO3 is added dropwise to the conical flask and the flask is shaken constantly. After a certain time, when the endpoint arrives, the indicator changes its color and the reaction is done.
We repeat the titration several times for better results and then we estimate the nitrate quantity by the formula V1S1 = V2S2. The titration is done under -2530C.
5. HNO3+ H2 net ionic equation
The net ionic equation between HNO3 + H2 is as follows,
H+(aq.) + NO3–(aq.) + H2(g) = NH3(g) + H+(l) + OH–(l)
- HNO3 will be ionized into H+ and nitrate as a counter ion as it is a strong electrolyte.
- H2 remains undissociated because it exists in gaseous form and cannot be ionized.
- In the product part NH3 also exists as an undissociated form due to the gaseous form.
- Water ionized into proton and hydroxide ion in a liquid state.
6. HNO3+ H2 conjugate pairs
In the reaction, HNO3 + H2 conjugate pairs will be the corresponding de-protonated and protonated form of that particular species which are listed below-
- Conjugate pair of HNO3 = NO3–
- Conjugate pair of OH– = H2O
- Conjugate pair of NH3 = NH4+
7. HNO3 and H2 intermolecular forces
HNO3 + H2 has the following intermolecular forces,
- The intermolecular force present in HNO3 is the strong electrostatic force between protons and nitrate ions.
- In H2, covalent force, van der waal’s force, and London dispersion force are present. It is also an electrostatic and ionic force present in water, also van der waal’s attraction.
| Molecule | Acting force |
| HNO3 | Electrostatic, van der waal’s Dipole interaction, Covalent |
| H2 | Van der waal’s force, covalent force, London dispersion force |
| NH3 | Covalent force, van der waal’s attraction, ionic interaction |
| H2O | H-bonding, covalent force, ionic interaction |
| NO2 | Van der waal’s interaction, dipole interaction, London dispersion force |
8. HNO3 + H2 reaction enthalpy
HNO3 + H2 reaction enthalpy is -598.33 KJ/mol which can be obtained by the formula: enthalpy of products – enthalpy of reactants. Here the change in enthalpy is negative. The enthalpy of 2nd reaction between diluted HNo3 and H2 is -2.52KJ/mol.
| Molecule | Enthalpy (KJ/mol) |
| H2(g) | 0 |
| HNO3(aq.) | -207.36 |
| NH3(aq.) | -80.29 |
| H2O (vapor) | -241.8 |
| NO2(g) | 33.18 |
and Products
9. Is HNO3 + H2 a buffer solution?
The reaction between HNO3 + H2 gives a buffer solution of NH3 and H2O which can control the pH of the reaction upon the addition of acid.
10. Is HNO3 + H2 a complete reaction?
The reaction between HNO3 + H2 is complete because it gives two major ones: a strong base and other is a water molecule.
11. Is HNO3 + H2 an exothermic or endothermic reaction?
The reaction of HNO3(Conc.) + H2 is exothermic in terms of thermodynamics first law. This reaction released more energy and temperature to the surroundings, where δH is always more negative.
The reaction between diluted HNO3 and H2 is also an exothermic reaction where the δH is lower negative.
12. Is HNO3 + H2 a redox reaction?
HNO3 + H2 reaction is a redox reaction because N gets reduced and hydrogen gets oxidized in this reaction. Here HNO3 acts as a reducing agent whereas H2 acts as an oxidizing agent.
HNO3 + H2 Reaction
13. Is HNO3 + H2 a precipitation reaction
The reaction between HNO3 + H2 is a precipitation reaction because NH3 gets precipitated in the solution at ammonium hydroxide in certain pH which can be removed by heating the solution.
14. Is HNO3 + H2 reversible or irreversible reaction?
The reaction between HNO3 + H2 is reversible because it produced water and base. The equilibrium shifts towards the left-hand side because on the left side gaseous molecule is present where the entropy increases and the reaction will be reversible.
HNO3 + H2 <–> NH3 + H2O
15. Is HNO3 + H2 displacement reaction?
The reaction between HNO3 + H2 is an example of a single displacement reaction. Because in the above reaction H+ was displaced by N from HNO3.
Conclusion
The reaction between HNO3 and H2 gives us base ammonia along with water in the concentrated form of acid but with a diluted form of acid, it forms nitrogen dioxide. This reaction is a base formation and reversible reaction.
Hi……I am Biswarup Chandra Dey, I have completed my Master’s in Chemistry from the Central University of Punjab. My area of specialization is Inorganic Chemistry. Chemistry is not all about reading line by line and memorizing, it is a concept to understand in an easy way and here I am sharing with you the concept about chemistry which I learn because knowledge is worth to share it.
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