Hess’ law also known as Hess’s law of constant heat summation states, “at constant temperature, heat energy changes ( ∆ H r ) accompanying a chemical reaction will remain constant, irrespective of the way the reactants react to form product”. The first law of thermodynamics states that the total energy of the substances before and after any (physical or chemical) change should be equal. According to the law, the total energy of the reactant should be equal to the total energy of the product. Any difference in the energy between the reactants and products is also fixed at a particular temperature and will not change with the path followed by the reactants to form products. Hence, heat energy also can be considered as a reactant or product of the reaction and included in the reaction. Hence, exothermic reactions can be written as: A + B → C + D + ∆ H Similarly, endothermic reactions become: A + B + ∆ H → C + D

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 19 - Lattice Energy>EXAM-STYLE QUESTIONS>Q 6

MEDIUM

AS and A Level

IMPORTANT

Earn 100

Explain, in terms of differences of lattice energies and enthalpy changes of hydration, why magnesium sulfate is more soluble in water than calcium sulfate.

Important Questions on Lattice Energy

EASY

AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 19 - Lattice Energy>BEFORE YOU START>Q 1

Draw a fully labelled reaction pathway diagram (energy profile diagram) for an endothermic reaction. Explain, how this diagram shows that the reaction is endothermic?