Hess's Law

The Hess's Law calculator computes the sum of enthalpy changes for a reaction based on the changes in series of steps.

INSTRUCTIONS: Choose units and enter the following:

• (ΔH⁰_a) Enthalpy change for step A of a reaction series
• (ΔH⁰_b) Enthalpy change for step B of a reaction series
• (ΔH⁰_c) Enthalpy change for step C of a reaction series
• (ΔH⁰_d) Enthalpy change for step D of a reaction series

Overall Enthalpy Change(ΔH⁰_rxn):  The calculator returns the enthalpy change in kilojoules per mole (kJ/mol).  However this can be automatically converted to compatible units via the pull-down menu.

The Math / Science

The Hess’s Law formula is a summation of enthalpy changes for a reaction. G. H. Hess published this equation in 1840 and discovered that the enthalpy change for a reaction is the same whether it occurs via one step or several steps. Hess's Law Formula is:

H⁰_rxn= H⁰_a + H⁰_b + H⁰_c + H⁰_d

where:

• H⁰_rxn is the overall enthalpy change of a reaction
• H⁰_a is the enthalpy change for step A of a reaction series
• H⁰_b is the enthalpy change for step B of a reaction series
• H⁰_c is the enthalpy change for step C of a reaction series
• H⁰_d is the enthalpy change for step D of a reaction series

All inputs have default units of kilojoules per mole (kJ/mol).

If H⁰_rxn is positive, then the reaction is endothermic, which means the reaction requires the absorption of heat to proceed to completion. However, if H⁰_rxn is negative, then the reaction is exothermic, and the reaction proceeds to completion by generating heat. 

Chemistry Calculators

• R - Gas Constant: 8.3144626181532 J/(K⋅mol)
• Boyle's Law Calculator:  P₁ • V₁ = P₂ • V₂
• Charles Law Calculator: V₁• T₂ = V₂ • T₁
• Combined Gas Law Calculator:   P•V / T= k
• Gay-Lussac Law:  T₁•P₂ =T₂•P₁ 
• Ideal Gas Law: P•V = n•R•T
• Bragg's Law:   n·λ = 2d·sinθ
• Hess' Law:  ΔH⁰_rxn=ΔH⁰_a+ΔH⁰_b+ΔH⁰_c+ΔH⁰_d
• Internal Energy: ΔU = q + ω 
• Activation Energy:  E_a = (R*T₁⋅T₂)/(T₁ - T₂) ⋅  ln(k₁/k₂)
• Arrhenius Equation:  k = Ae^E_a/(RT)
• Clausius-Clapeyron Equation:  ln(P₂/P₁) = (ΔH_vap)/R * (1/T₁ - 1/T₂)
• Compressibility Factor: Z = (p*V_m)/(R*T)
• Peng-Robinson Equation of State:  p = (R*T)/(V_m - b) - (a*α)/(V_m² + 2*b*V_m - b²)
• Reduced Specific Volume:  v_r = v/(R* T_cr / P_c)
• Van't Hoff Equation:  ΔH⁰ = R * ( -ln(K₂/K₁))/ (1/T₁ - 1/T₂)

See Also

Khan Academy: Hess's Law

ChemWiki: Hess's Law

References

Whitten, et al. "Chemistry" 10th Edition. Pp. 564