Enthalpy
_________________ is the total _________________ of a system
It is represented by a ____.
If the pressure remains _________________, the _________________ increase of a sample of matter equals the
_________________ as _________________ that is received.
This remains true even during a _________________ reaction or a change of _________________ occurs.
It also indicates the _________________ _________________ energy of the particles in a sample
So to calculate Enthalpy, we have to visit molar heat capacity
_________________ _________________ _________________ of a pure substance is the _________________ as
________________ needed to increase the temperature of _________________ of a substance by _______________
It is symbolized by _____
It has a unit of ____________
Formula: ___________________________
_________________ = (amount in _________________) (_________________ _________________ capacity)
(change in _________________)
Note: moles are used so you may have to do some gram to mole problems. (Use molar mass)
_________________ on _________________ has many Molar Heat Capacities of Elements and Compounds.
Molar Enthalpy Change
Since enthalpy “equals the energy as heat that is received”, we can rewrite the heat equation to:
______________________
_________________ _________________ change = (_________________ _________________ capacity) (change in
_________________)
Even though enthalpy is an important quantity, the only way to measure energy is through a _________________.
Therefore, the _________________ value of H cannot be determined, but _________________ can be determined
The enthalpy change for one mole of a pure substance is known as the _________________ _________________
change.
Examples:
- The molar heat capacity of tungsten is 24.2 J/(mol.K). Calculate the energy as heat needed to increase the
temperature of 0.40 mol of tungsten by 10.0 K.
- Suppose a sample of water increased in temperature by 3.5 K when the sample absorbed 856 J of energy as heat.
Calculate the number of moles of water if the molar heat capacity is 50.5 J /(mol.K). What is the mass of that water?
Molar Heat Capacity is related to Specific Heat
_________________ _________________ is the energy as heat needed to raise the _________________ of one
_________________ of substance by one _________________.
Symbolized by ___
This is why an iron gets hot faster than a pot of water
_________________ _________________(g/mol) x_____ (J/K.g) = _____ (J/K.mol)
Example Problem
- If the molar heat capacity of nitrogen (N2) is 29.1 (J/K.mol), what is nitrogen’s specific heat?
- The molar heat capacity of Al(s) is 24.2 J/K.mol. Calculate the molar enthalpy change when Al(s) is cooled from
128.5 C to 22.6 C.
- Lead has a molar heat capacity of 26.4 J/K.mol. What molar enthalpy change occurs when lead is cooled from 302
C to 275 C?
Another Formula for specific heat
or q=mcpT
cp = q/(mT)
Example: Calculate the specific heat of a substance if a 35 g sample absorbs 48 J as the temperature is raised
from 293 K to 313 K
Two more heat equations
Heat of _________________ (Lf) - Heat involved in melting or freezing a substance
Heat of _________________ (Lv) - Heat involved in evaporation or condensing a substance
As a phase change occurs, all energy is being used for the change so the temperature will not change.
So, during a phase change, heat equals the _________________ (m) times a _________________ (L) for that material
(_________________)
Example: If the heat of fusion of water is 80 cal/g, what is the amount of heat energy required to change 15.0 grams of
ice at 0C to 15.0 grams of water at 0C?
How do they all fit together? (Look at Figure 17 from the book)
As heat is added, temperature increases linearly (_________________)
Except, when a phase change occurs, because all the heat is going into changing the phase (____________, note no __)