When a liquid freezes and then melts the steps are exothermic?

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

> b takes place in the gas phase in an isothermal tubular reactor. the feed is one mole of a per one mole of c, an inert. the entering temperature and pressure are 427°c and 10 atm, respectively. the gas constant r = 0.08206 atm · l/mol · k.