What are internal energy change and enthalpy change? How are these releated? Under what conditions both become equal?

Define the terms internal energy change and enthalpy change. How are they related?

Predict the enthalpy change, free energy change and entropy change when ammonium chloride is dissolved in water and the solution becomes colder.

Free energy change is related to enthalpy and entropy changes as:

What is total internal reflection ? Under what conditions it takes place ?

Enthalpy change equal internal energy change when

In a reaction P+Q rarr R+S , there is no change in entropy, Enthalpy change for the reaction (DeltaH) is "12 kJ mol"^(-1) . Under what conditions, reaction will have negative value of free energy change?