Keen observation of the rates of reaction about the series of steps leading to the formation of products is called the reaction mechanism . The reaction between H_(2) and I_(2) to form hydrogen iodide was originally postulated as a simple one step reaction H_(2) + I_(2) to 2 HI Rate = k [H_(2)][I_(2)] but the formation of HI has been explained on the basis of the following mechanism ({:( I_(2) , to , 2 I "(fast)" , ... (i) ), ( H_(2) + I , to , H_(2) I "(fast)" , ... (ii)) , (H_(2) I + I , to , 2HI "(slow)" , ... (iii) ):})/("overall" H_(2) + I_(2) to 2 HI For the reaction 2NO(g) + 2H_(2)(g)to N_(2)(g) + 2H_(2)O(g) the rate expressions can be written in the following ways (d[N_(2)])/(dt) = k_(1) [NO] [H_(2)] , (d[H_(2)O])/(dt) = k [NO] [H_(2)] - (d[NO])/(dt) = k'_(1) = [NO] [H_(2)] , - (d[H_(2)])/(dt)= k''_(1) [NO][H_(2)] The relationship between k , k_(1) , k'_(1) , k''_(1)is