For this type of reaction the constitutive relation for the molar production rate is given by $$\hat{\zeta}=k\prod\limits _{\alpha}\left(c^{\alpha}\right)^{\nu_{R}^{\alpha}}\,.$$ The constitutive form of the specific forward reaction rate must be selected from the list of materials given in Section 4.11.4↓. The units of $\hat{\zeta}$ are [n/L$^{\mathrm{3}}\cdot$ t] and those of $c^{\alpha}$ are [n/L$^{\mathrm{3}}$ ].

Example:

Consider the forward reaction that produces solid copper sulfide from solid copper and solid sulfur, $$\mbox{Cu}+\mbox{S}\to\mbox{CuS}$$ All three species are modeled explicitly in the mixture as solid-bound molecules, with id's 1 (Cu), 2 (for S) and 3 (for CuS). The chemical reaction material is given by:

<reaction name="copper sulfide production" type="mass-action-forward">
  <vR sbm="1">1</vR>
  <vR sbm="2">1</vR>
  <vP sbm="3">1</vP>
  <forward_rate type="constant reaction rate">
    <k>1e-3</k>
  </forward_rate>
</reaction>