Now, since reactions are reversible and enthalpy is a state function, hess's law tells us the change in enthalpy of the four reaction is equal to the negative change in enthalpy of the reverse reaction. So if we look at this reversible reaction here, a plus B is equal to C plus D, where the change in enthalpy is negative ten. And the reverse reaction, according to Hess's Law, is C plus D equal A plus B, and this is negative of this. So it's positive ten kilojoules per mole. The last thing we should mention is the mathematical formula for entropy, which basically states that change in entropy is the heat or change in energy over time, over temperature, and temperature is in Kelvin. Step one.