Saved Bookmarks
| 1. |
Explain the measurement of Delta H calorimetetry |
|
Answer» Solution :The energy changes associated with reactions are measured at constant volume. Under these conditions, no work is done as the reaction is CARRIED out at constant volume in the bomb CALORIMETER. Even for reactions involving gases, there is no work done as `Delta V=0` therefore `w=0`. Temperature change of the calorimeter produced by the completed reaction is then converted to `q_V`, by using the known heat capacity of the calorimeter with the help of `=q= C Delta T` . (b) `Delta H` measurement of: Measurement of heat change at constant PRESSURE can be done in a calorimeter shown in Fig. We know that `Delta H =q_(p)` at constant pressure and, therefore, heat absorbed or evolved, `q_(p)` at constant pressure is also called the heatof reaction or enthalpy of reaction, `Delta_(r) H`. In an exothermic reaction, heat is evolved, and system loses heat to the surroundings. Therefore, `q_p` will be negative and `Delta _(r) H` will also be negative. Similarly in an endothermic reaction, heat is absorbed, `q_(p)` is positive and `Delta _(r) H` will be positive.  FIGURE : Calorimeter for measuring heat changes at constant pressure (atmospheric pressure). At constant pressure heat changes `Delta H = pm q_(p)` For endothermic reaction `q_(p) = pm ve "and" Delta H = +ve` For exothermic reaction `q_(p) = - ve and Delta H= - ve` The heat absorbs or produced `q_(p)` at constant pressure is called energy of reaction `Delta_(r) H`. |
|