Other standard enthalpy changes

There are possibly as many standard enthalpy changes as there are types of reactions. Some common ones other than those mentioned in previous articles include:

    1. Standard enthalpy of hydrating an anhydrous salt (not to be confused with standard enthalpy of hydration), e.g.

CuSO_4(s)+5H_2O(l)\rightarrow CuSO_4\cdot 5H_2O(s)\; \; \; \; \; \; \; \Delta H_{hydt}^{\: o}=-78.2\: kJmol^{-1}

    1. Standard enthalpy of precipitation, e.g.

AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)\; \; \; \; \; \Delta H_{ppt}^{\: o}=-65.8\; kJmol^{-1}


Calculate the enthalpy of precipitation of PbBr2 when 150.0 mL of 0.500 M Pb(NO3)2 is added to 80.0 mL of 1.000 M NaBr in a calorimeter with the temperature rising from 298.15 K to 299.28 K (assuming that the solution’s specific heat capacity is 4.200 Jg-1K-1 and its density is 1.0 g/ml).


0.0800 moles of NaBr precipitates 0.0400 moles of PbBr2. Using eq5 from a basic level article,

\Delta H_{ppt}=-(4.200)(150.0+80.0)(299.28-298.15)=-1092\: J

\Delta H_{ppt}=\frac{-1092}{0.0400}=-27.3\: kJmol^{-1}



Next article: Hess’s law
Previous article: Standard enthalpy change of lattice energy
Content page of intermediate chemical energetics
Content page of intermediate chemistry
Main content page

Leave a Reply

Your email address will not be published. Required fields are marked *