The enthalpy relates to the energy of the bonds broken and made during a chemical reaction
Enthalpy changes can be measured and calculated but enthalpy cannot
For an exothermic reaction ∆H is negative; for an endothermic reaction ∆H is positive
The enthalpy change is the heatenergychange at constantpressure
Standard enthalpy values are the ΔH values for enthalpy changes of specific reactions measured under standardconditions
Standard conditions are 100kPa pressure and a stated temperature
Standard enthalpy of reaction (ΔrH ) is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction
Standard enthalpy of formation (ΔfH ) is the enthalpy change when 1mole of a compound is formed from its constituent elements with all reactants and products in standardstates under standard conditions
Standard enthalpy of combustion (ΔcH ) is the enthalpy change when 1mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions
q = change in heat energy in joules
m = mass in grams of the substance to which the temperature change occurs
c = specific heat capacity
ΔT = temperature change in ºC or kelvin (K).
Energetics is the study of energy transfers between reacting chemicals and their surroundings
Thermochemistry is the study of energy transfers in chemical reactions
Bond breaking requires energy & bond making releases energy
System: the chemical reaction
Surroundings: everything around or outside the chemical reaction
Boundary: separates the system from the surroundings
fill in the blanks
A) Boundary
B) System
C) Surroundings
Dissipated: energy irreversibly lost by the system
Enthalpy: the energy content of a system under constant pressure
Enthalpy: the energy content of a system under constant pressure
Calorimetry is used to physically measure changes in enthalpy
Bomb calorimetry:
A sample of a compound in a sealed vessel is burned and measured the temperaturechange
Often the calorimeter will just determine the temperature change in the vessel and you will have to calculate the enthalpychange of combustion
Bomb calorimetry can be inaccurate due to:
Heat lost to the surroundings.
Any incompletecombustion that may take place.
Loss of some reactant that evaporates before it combusts.
Bond enthalpy is the enthalpy change when one mole of bonds is broken in the gas phase
There are two main reasons mean bond enthalpies are inaccurate:
Mean bond enthalpies are only valid in the gas phase
Bond enthalpies depend on the particular molecule, whereas mean bond enthalpies are quoted generally
Hess's law:
The enthalpychange in a reaction is independent of the route taken and depends only on the initial and final states
Mean bond enthalpy is the heat change needed to break a covalentbond. It is a mean over different molecules
mean bond enthalpy = Σ(bonds broken) − Σ(bonds formed)
bond enthalpies are meanvalues from a range of compounds
Water has a knowndensity so therefore, a volume of 50.0 cm3 could be measured out instead of a mass from a balance
Hess's law is that enthalpy change is independent of the route taken