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05Class 11 Chemistry

Thermodynamics formulas

First law, enthalpy, entropy and Gibbs energy formulas for Class 11 Chemistry Thermodynamics. Watch the sign convention, since it differs from Physics.

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All Chemistry formulas

First law

First law
ΔU=q+w\displaystyle \Delta U = q + w

w is the work done on the system.

Expansion work
w=−pextΔV\displaystyle w = -p_{\text{ext}}\Delta V
Reversible isothermal work
w=−2.303 nRTlog⁡V2V1\displaystyle w = -2.303\,nRT\log\frac{V_2}{V_1}
Enthalpy
ΔH=ΔU+ΔngRT\displaystyle \Delta H = \Delta U + \Delta n_gRT
Heat capacities
Cp−CV=R\displaystyle C_p - C_V = R

Thermochemistry

From formation enthalpies
ΔrH∘=∑ΔfHproducts∘−∑ΔfHreactants∘\displaystyle \Delta_rH^\circ = \sum\Delta_fH^\circ_{\text{products}} - \sum\Delta_fH^\circ_{\text{reactants}}
From bond enthalpies
ΔrH=∑BEreactants−∑BEproducts\displaystyle \Delta_rH = \sum\text{BE}_{\text{reactants}} - \sum\text{BE}_{\text{products}}

Note this order is reversed.

Entropy and Gibbs energy

Entropy change
ΔS=qrevT\displaystyle \Delta S = \frac{q_{\text{rev}}}{T}
Gibbs energy
ΔG=ΔH−TΔS\displaystyle \Delta G = \Delta H - T\Delta S

A process is spontaneous when ΔG is negative.

Link to equilibrium
ΔG∘=−2.303 RTlog⁡K\displaystyle \Delta G^\circ = -2.303\,RT\log K

Where students slip up

  • Reversing the bond enthalpy formula, which is reactants minus products.
  • Counting all moles in Δn. Δnᵍ includes gases only.
  • Mixing units in ΔG = ΔH − TΔS. ΔH is usually in kJ, and ΔS in J/K.

Try it on your weakest chapter.

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