1
Key Things to Remember
Heat lost = Heat gain
-Qlost = +Qgained
- Exothermic rxn = + Endothermic rxn
Q=mcΔT (Heat Flow, where Q is thermal energy in J, m is mass in g, c is specific heat capacity in J/g°C,
and ΔT is change in temperature in °C)
ΔH=Q/n (Molar heat of reaction, where ΔH is molar heat of rxn in J/mol or kJ/mol, Q is Heat Flow in J or
kJ and n is number of moles in mol)
Enthalpy change (∆H) is the energy exchanged between a system and its environment during a physical
change or a chemical reaction at constant pressure.
ΔH = Hproducts - Hreactants (measured in kJ)
Enthalpy Diagrams of a reaction
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Always examining it with respect to ΔH = Hproducts - Hreactants
o In endothermic rxn, the enthalpy of the products is greater than the enthalpy of the
reactants, requiring thermal energy to be absorbed
 The potential energy stored in the chemical bonds is greater in the products
than in the reactants
o In exothermic rxn, the enthalpy of the reactants is greater than the enthalpy of the
products, releasing thermal energy
 The potential energy stored in the chemical bonds is greater in the reactants
than in the products
Knowing the thermochemical equation and the enthalpy change, you can draw a enthalpy
diagram
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2
Thermochemical equations
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In any chemical reaction, there exists a endothermic portion and an exothermic portion
o The overall reaction being endothermic or exothermic is determined on the sum of each
of the endothermic and exothermic portions of the reaction!
In all reactions (weather physical or chemical) you have breaking and forming of bonds
o Breaking bonds = requires energy = endothermic
o Making bonds = releases energy =exothermic
(Dissolution of a solvent in a solute to form a solution)
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Stage 1 is endothermic; the solvent particles attracted to each other have to separate
Stage 2 is endothermic, the solute particles attracted to each other have to separate
Stage 3 is a rearrangement of solute and solvent particles, which is exothermic
Therefore, ∆H is the energy balance between energy absorbed and released!
o ∆H= ∆H1 + ∆H2 + ∆H3
 If ∆H1 + ∆H2 is greater than ∆H3 → endothermic
 If ∆H1 + ∆H2 is less than ∆H3 → exothermic
o ∆H = ∆Hbonds broken + ∆Hbonds formed
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3
Ways to Calculate ΔH
1. Calorimetry experiments
a. ΔH=Q/n
2. Hess' Law
a. the enthalpy change (ΔH) for the overall reaction will be equal to the sum of the
enthalpy changes for the individual steps
3. Bond Energies/Energy Balance
a. ∆H = ∆Hbonds broken + ∆Hbonds formed
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