Asubstance has a boiling point of 78 °c. which of the following is true about the substance?
when the substance is heated, its temperature will start rising above 78 °c after 50% of the liquid has changed to vapors.
when the substance is heated, its temperature will start rising above 78 °c after 100% of the solid has changed to liquid.
when the substance is cooled, its temperature will fall below 78 °c after 100% of the vapor has changed to liquid.
when the substance is cooled, its temperature will fall below 78 °c after 50% of the liquid has changed to solid.

When the substance is cooled, its temperature will fall below 78 °C after 100% of the vapor has changed to liquid.

Explanation:

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.