A convention has been adopted to simplify free-energy calculations for biochemical reactions. The standard state is defined as having a ph of 7 Consequently, when Ht is a reactant, its activity has the value I(corre- sponding to a pH of 7)in equations 6 and 9. The activity of water also is taken to be I in these equations. The standard free-energy change al pH 7, denoted by the symbol ago, will be used throughout this book. The kilocalorie(abbreviated kcal) will be used as the unit of energy