Kinetics and mechanism of the reaction between sodium carbonate and silica

The kinetics and mechanism of the reaction between sodium carbonate and silica were studied using thermogravimetric analysis to monitor the percent reaction versus time. A modified form of the rate constant in the Ginstling and Brounsthein model has been shown to describe the sodium carbonate - silica reaction of dried samples. This equation is: $\text{(a)K}\text{=1−}\text{2x}\text{3}\text{−(1−}\text{x}\text{)}{}^{\mathrm{2,3}}$$\text{were}\text{K=}\text{2DC}\text{1−}\text{R}{}_{\mathrm{n}}\text{k}{}_1\text{M}\text{+}\text{Q}\text{2}\text{+}\text{1}\text{2}\text{1}\text{2}\text{−1}\text{k}{}_2\text{R}{}^2{}_{\mathrm{SiO2}}$ t is the time, D is the diffusion coefficient of the rate controlling species, C, is the concentration gradient, K₁ and K₂ are constants, R_n is the sodium carbonate particle size, R_SiO2 is the silica particle size, and M and Q are the total volumes of the sodium carbonate and silica, respectively. Surface diffusion was shown to cover all silica particles rapidly and supply Na⁺ and O⁼ ions to the reaction interface. The activation energy of the process was found to be 30 k-cal/mole. The presence of water vapor caused the diffusion rate to increase rapidly and the rate controlling step became chemical processes which occurred at the phase boundary. This process was determined to have an activation energy of 35 k-cal/mole.