Journal Entry:

What is the value of ᐃH^०₂₉₈ for reaction Z?

(A)  90 kJ mol^-1

(B)  -90 kJ mol^-1

(C) 172 kJ mol^-1

(D) -172 kJ mol^-1

Learning Intentions

●     We will learn how to determine enthalpy of a reaction using Hess's Law and Heats of Formation.

●     We will learn how to use Hess’s Law to answer AP free response questions about energy of reactions.

●    We will learn that when a reaction is reversed, the sign of the enthalpy of reactions is changed, and when two or more reactions are summed to obtain an overall reaction, the enthalpies of reactions are summed to obtain the net enthalpy of reaction.

Closing Task:

You can apply Hess's Law and other enthalpy equations to AP released questions.

Content Standards being covered:

Chemical system undergo three main processes that change their energy: heating/cooling, phase transitions, and chemical reactions. (E.K. 5.B.3.)

At the particulate scale, chemical processes can be distinguished from physical processes because chemical bonds can be distinguished from intermolecular interactions. (E.K. 5.D.2)

 The net energy change during a reaction is the sum of the energy required to break bonds in the reactant molecules and the energy release in forming the bonds of the product molecules. The net change in energy may be positive for endothermic reactions where energy is required, or negative for exothermic reactions where energy is released. (E.K. 5.C.2)