Enthalpy Flashcards
Enthalpy change
The energy change per mole at constant pressure
Standard conditions
298K and 100KPa pressure
Enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states
Enthalpy change of combustion
The enthalpy change when one mole of substance is completely burned in oxygen
Why shouldn’t you balance the product in enthalpy equations
Enthalpy measures the change in only 1 mole
How is standard conditions written
ΔH°
(line through circle)
What reaction has negative enthalpy change, which has positive
Exothermic - Negative enthalpy change
Endothermic - Positive enthalpy change
Enthalpy Change equation
q =m X c X (delta)T
q= enthalpy change (normally KJmol^-1)
m = mass (g)
c = specific heat capacity (water is 4.18 KJmol^-1)
T = Temperature (kelvin
Hess’s law
The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
Follows law of conservation of energy, 2 reactions to form the same products will have the same enthalpy change, shown in thermochemical cycle
Enthalpy change of formation
equation
Bond enthalpy products - Bond enthalpy reactants
Only use if given Enthalpy change of formation data
Enthalpy change of combustion
Bond enthalpy reactant - Bond enthalpy products
Almost always this equation
Hess law equation
ΔHr = ΔH2 – ΔH1
Reactants to products = Reactants to intermediate - Products to intermediate
