2-Molar Enthalpies - Science-with

Let’s try a question from Friday...
 When 50 mL of 1.0 mol/L hydrochloric acid is neutralized
completely by 75 mL of 1.0 mol/L sodium hydroxide in a
polystyrene cup calorimeter, the temperature of the total
solution changes from 20.2°C to 25.6°C. Determine the
enthalpy change that occurs in the chemical system.
ΔrH = Q
= mc Δt
=(50+75)g x 4.19 J/g•°C x (25.6-20.2)°C
=2.83 kJ
The enthalpy change for the neutralization of HCl in this context is
recorder as -2.83 kJ because it’s an exothermic reaction.
Molar Enthalpies
 Sometimes it is better to know the enthalpy change per
unit chemical amount (i.e. per mole) so that you can
relate the enthalpy change to a chemical reaction
equation that is balanced in moles.
 Molar enthalpy of reaction is the enthalpy change in a
chemical system per unit chemical amount (per mole)
of a specified chemical undergoing change in the system
at constant pressure.
Enthalpy change
of reaction
Molar enthalpy
of reaction
ΔrH = nΔrHm
Note: the ‘r’s can be
switched with a ‘c’s
for combustion
Chemical amount
Where do I find molar quantities
 Molar quantities are often referenced or memorized
 Atomic molar masses are from the periodic table
 molar volume of any gasses at STP is 22.4 L/mol
 We can use these to predict, say, the enthalpy of
combustion (ΔcH)
Sample problem
 Predict the change in enthalpy due to the combustion of
10.0 g of propane used in a camp stove.
1) First you have to determing the change in enthalpy for
the combustion ΔcH
ΔcH = nΔcHm
To calculate ΔcH we need to know the chemical amount and
molar enthalpy of combustion ΔcHm
n= 10.0 g x 1/44.11g/mol (molar mass)
Molar enthalpy of combustion of propane is: -2043.9 kJ/mol
ΔcH = nΔcHm
= 10.0 g x 1/44.11 g/mol x (-2043.9 kJ/mol)
= - 463 kJ
Another problem
 Predict the enthalpy change due to the combustion of
10.0g of butane in a camp heater. The molar enthalpy of
combustion to produce water vapour is -2657.3 kJ/mol
ΔcH = nΔcHm
= 10.0 g x 1/58.14g/mol x (-2657.3 kJ/mol)
= - 457 kJ
Δ H = Q therefore
nΔcHm = mcΔt
You can use this to determine an unknown
but be careful, in calorimetry, the right
hand side is all about water and the left
is about the fuel.
Questions 1-7 and 11 on page 494

similar documents