Can you feel the heat?

What are two classifications of reactions in regards to heat utilization?

1. Endothermic reactions
   $Reactants + Energy \rightarrow Products$
   • Heat is removed from the reactant vessel, e.g. temperature drops in the calorimeter.
   • Heat is used to break bonds in the reactants.
2. Exothermic reactions
   $Reactants \rightarrow Products + Energy$
   • Heat is released into the reactant vessel, e.g. temperature increases in the calorimeter.
   • Heat released comes from new bond formation in the products.

What thermodynamic state function is used in evaluating heat transfers in chemical reactions?

Enthalpy ($H$)

What are four various forms of enthalpy used in calculating heat transfers in chemical reactions?

1. $\Delta$H$^{o}_{vap}$
   • Heat required to vaporize one mole of a substance (kJ/mol).
2. $\Delta$H$^{o}_{rxn}$
   • Amount of heat gained or lost in a chemical reaction (kJ/mol).
3. $\Delta$H$^{o}_{f}$
   • Heat required to form one mole of a pure substance from its elements (kJ/ mol).
   • The heat of formation for pure substances is zero.
4. $\Delta$H$^{o}_{comb}$
   • Heat released from the combustion (chemical reaction with O$_{2}$) of one mole of a substance (kJ/mol).

Note: `` $^{o}$'' refers to Òstandard conditionsÓ or 298 K at 1 atm

What is Hess's law?

Hess's law states that enthalpies are additive. In other words, given a reaction it is possible to calculate the $\Delta$H$^{o}_{rxn}$ by subtracting the $\Delta$H $^{o}_{Reactants}$ from the $\Delta$H$^{o}$:

$$\Delta H^{o}_{rxn} = \Delta H^{o}_{Products}- \Delta H^{o}_{Reactants}$$ (39.1)

How can Hess's law be used to derive enthalpies for chemical reactions?

1. Balance chemical reactions.
2. If there are several equations, try to cancel out as many common pairs of factors (e.g. a reactant-product common pair or product-reactant common pair).
3. Use the given $\Delta$H$^{o}$ and add appropriately.
4. If you exchange the positions of the products and the reactants in one of the equations you must change the sign of $\Delta H$.

What is bond dissociation energy?

Bond dissociation energy is the energy released when cleaving a bond.

Table 38.1: Bond dissociation energy: Br$_{2}$

$Br-Br + Energy \rightarrow Br + Br$ - 46 kJ/mol	Bond dissociation energy
$Br + Br \rightarrow Br-Br + Energy$ + 46 kJ/mol	$\Delta$H$_{f}$