Mechanical measure of affinity

Velocity of Reaction and Affinity, Mechanical Measure of Affinity. 

In order to measure the moving force which causes a reaction, it is in general suitable to make use of a measurable opposing force, that may be gradually applied 

This method is most obviously applicable to reactions which are accompanied by an evolution of gas, so we will first take one from the researches of Tammann and Nernst, in which the action of zinc on sulphuric acid, with evolution of hydrogen, 

In this way it was found, amongst other cases, that a pressure of 18 atmospheres stops further evolution of hydrogen in 0 13 normal sulphuric acid in contact with zinc, and 1-3 normal in respect of zinc sulphate. And that a higher pressure of hydrogen reversed the reaction, causing precipitation of zinc, was shown hy leaving the apparatus to stand at the atmospheric temperature, after it had previously been warmed to inci eaae the pressure. 

The circumstances are similar, but simpler, with a hydrated salt. Frowein, e. g. (p. 57), found that zinc sulphate (ZnSO. 7H2O) at 18° has a maximum pressure of 8 4o6 mm., in accordance with the fact that a higher external pressure produces the reverse reaction  

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