Enthalpy and heat of reaction Flashcards
What is heat?
disorganised, random movement
What is work?
Organised in one direction
What is a state function?
Quantity that depends only on the thermodynamic state of the system and not on how that state was prepared
What are examples of state functions?
H, U , Cvm and Cpm
What are not state functions?
Heat and work
What is the equation for enthalpy?
H = U + pV (Enthalpy = energy + (pressure x volume)
What is the equation for enthalpy at a constant pressure?
ΔH = ΔU + pΔV
How does ΔH relate to first law?
ΔH = q + w = q - pΔV
What is the change in enthalpy for a reactant at a constant pressure is equal to?
Heat
(ΔH)p = q
What is the change in enthalpy for a reactant at a constant volume equal to?
Heat
(ΔH)v = q
What is the molar heat capacity?
The heat required to raise the temperature of one mole of substance by one kelvin
What is the equation for molar heat capacity at a constant volume?
Cvm = (δUm / δT)v
What is the equation for the molar heat capacity at a constant pressure?
Cpm = (ΔHm/δT)p
What does “m” mean in the molar heat capacity equations?
Per mole
What is the equation for a slope?
Δy / Δx
If Δx = 0, what can you write the exact slope as?
δy / δx
What can you do if y depends on another variable z as well as x?
Specify the slope in the direction of constant Z as (δy/δx)ᴢ
What principle does calorimetry run on?
If water gains heat, the chemical system must have lost heat
What is ΔғH?
The standard enthalpy of formation
What is the definition of the standard enthalpy of formation?
The enthalpy change when one mole of substance is made from its elements in their reference states
What is ΔrxnH?
Enthalpy of reaction
How do you calculate enthalpy of reaction?
ΔrxnH = Σ vΔfH (products) - Σ vΔfH (reactants)
What do you need to take into account when you work out ΔrxnH?
The stochiometry of the reaction
What does Kirchoff’s law describe?
Describes the enthalpy of a reaction’s variation with temperature changes
