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Calculate Delta H Rxn for Each of The Following 2h2s

Reviewed by Calculator Editorial Team

Calculating the enthalpy change (ΔH) for reactions involving hydrogen gas (H₂) is essential in chemistry. This guide explains how to determine ΔH rxn for hydrogen reactions using standard enthalpies of formation.

How to Calculate ΔH rxn for Hydrogen Reactions

The enthalpy change for a reaction (ΔH rxn) can be calculated using the standard enthalpies of formation (ΔHf°) of the products and reactants. For reactions involving hydrogen gas (H₂), you'll need the standard enthalpies of formation for all compounds in the reaction.

Steps to Calculate ΔH rxn

  1. Write the balanced chemical equation for the reaction.
  2. Find the standard enthalpies of formation (ΔHf°) for all reactants and products.
  3. Calculate the total enthalpy of the reactants (sum of ΔHf° × coefficients).
  4. Calculate the total enthalpy of the products (sum of ΔHf° × coefficients).
  5. Determine ΔH rxn by subtracting the total enthalpy of reactants from the total enthalpy of products.

Note: ΔH rxn is typically reported in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Make sure all enthalpies are in the same units before performing calculations.

Formula Used

The standard formula for calculating ΔH rxn is:

ΔH rxn = ΣΔHf°(products) - ΣΔHf°(reactants)

Where:

  • ΔH rxn = Enthalpy change for the reaction
  • ΔHf°(products) = Sum of standard enthalpies of formation of products
  • ΔHf°(reactants) = Sum of standard enthalpies of formation of reactants

For reactions involving hydrogen gas (H₂), you'll need to account for the standard enthalpy of formation of H₂, which is 0 kJ/mol by definition.

Worked Example

Let's calculate ΔH rxn for the following reaction:

2H₂(g) + O₂(g) → 2H₂O(l)

Given the following standard enthalpies of formation:

  • ΔHf°(H₂) = 0 kJ/mol
  • ΔHf°(O₂) = 0 kJ/mol
  • ΔHf°(H₂O) = -285.8 kJ/mol

Calculation steps:

  1. Total enthalpy of reactants: (2 × 0) + (1 × 0) = 0 kJ
  2. Total enthalpy of products: 2 × (-285.8) = -571.6 kJ
  3. ΔH rxn = -571.6 - 0 = -571.6 kJ

The enthalpy change for this reaction is -571.6 kJ, indicating it's an exothermic process.

Standard Enthalpies of Formation
Compound State ΔHf° (kJ/mol)
H₂ Gas 0
O₂ Gas 0
H₂O Liquid -285.8

FAQ

What is the standard enthalpy of formation for H₂?

The standard enthalpy of formation for H₂(g) is defined as 0 kJ/mol because it's the standard state for hydrogen gas.

How do I find standard enthalpies of formation?

Standard enthalpies of formation can be found in chemistry reference books, online databases like NIST, or in educational resources.

What units are used for ΔH rxn?

ΔH rxn is typically reported in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).

Is ΔH rxn always negative for hydrogen reactions?

No, ΔH rxn can be positive (endothermic) or negative (exothermic) depending on the specific reaction and the enthalpies involved.