Thermodynamics and Thermochemistry Flashcards
What is heat?
A thermal energy
This is a dynamic property that can only be defined during a transformation. It is not temperature (which is a static property defined for a state in a system)
What is internal energy?
The average total mechanical energy (kinetic + potential) of the particles that make up the system.
What is the first law of thermodynamics?
When a system absorbs an amount of heat Q from the surroundings and does a quantity of work W on the same surrounds, its internal energy changes by the amount:
ΔE = Q - W
This is basically stating that energy is conserved for isolated systems.
If heat is absorbed by the system Q > 0
If heat is released by the system Q < 0
If work is done by system on surroundings W > 0
If work is done by surroundings on system W < 0
What is 1 calorie?
The amount of thermal energy required to raise the temperature of water by 1 degree Celsius
How many joules is 1 calorie?
1 cal = 4.184 J
How can you obtain the energy per particle in chemical reaction energy exchanges?
Divide energy by Avogadro’s number ( 6.02 x 10^23 mol^−1)
How can Fahrenheit be converted into celsius?
(X°F - 32) x 5/9 = X°C
or
°F = 9/5°C + 32
What are three ways that heat can be transferred between a system and its surroundings?
- Conduction (contact between system and surroundings)
- Convection (Circulation of hot liquid/gas through system)
- Radiation (no contact between heat source and system)
What are state functions?
Path-independent functions, where the change in ___ (usually energy) between the initial state and final state is independent of the properties of the intermediate state.
Of the following fundamental energy functions (internal energy E, heat Q and work W), which are state functions and which are not?
State function
- Internal energy E
Not state functions
- Heat Q
- Work W
Q and W depend on the path taken to the final state. Q and W can be imposed on a system, but E cannot. E responds to Q and W inputs/outputs.
What are three consequences for internal energy being a state function?
- If the changes in the internal energy during the intermediate transformations are known, they can be used to calculate the change for the entire process from initial to final state.
- The change in the internal energy to a final state is the opposite to go back from a final to initial state
- If we start from an initial state and go back to initial state through a series of intermediate transformations, the change in the internal energy for the entire process is zero
Give the mathematical definition for enthalpy. How is this reduced to its most simple definition?
ΔH = ΔE + PxV
Where P and V are pressure and volume of the system. Because the majority of chemical reactions involve constant P and work is in a mechanical nature (W = PxV), the change in enthalpy during a chemical reaction reduces to:
ΔH = ΔE + PxV = (Q - W) + PxV = Q
ΔH = Q (for most chemical reactions)
Change of enthalpy is a direct measure of the heat that evolves or is absorbed during a reaction carried out at constant pressure
ΔH is +/- for exothermic/endothermic reactions
Exothermic: -ΔH
Endothermic: +ΔH
What are standard conditions for standard enthalpies (ΔH°)?
- Standard pressure: 1 atm
- Standard temperature is 25°C
- Standard physical state is natural state of element
These standards must be true for the standard enthalpy change (ΔH°) for a given reaction.
What is the enthalpy of formation?
The enthalpy of formation of a given compound is defined as the enthalpy change that accompanies the formation of the compound from its constituting elements
This is under standard temperature, pressure and natural elemental physical state
If the reaction of formation is carried out at constant pressure, the change in the enthalpy represents the amount of heat released or absorbed during the reaction.
ΔH°(reaction) = ΣΔH°form(products) - ΣΔH°form(reactants)
What is bond dissociation energy (bond energy)?
The change in enthalpy when a particular bond is broken in the diatomic molecules of 1 mole of gas
We can calculate the enthalpy change of any reaction using the sum of bond energies of teh reactants and the products in the following way:
ΔH° (reaction) = ΣBE(reactants) - ΣBE(products)
The standard enthalpy of formation of a mole of any ‘element’ is what?
ZERO
What is the specific heat capacity (C) for water? Why?
1
Because 1 calorie can heat 1 gram of water two 1 degrees celsius
If a block of ice is allowed to melt in a bucket of warm water, how can the heat of fusion of ice (melting) be determined?
Measure the temperature drop in the bucket of water and apply the law of conservation of energy
Q = mL
Where L is the latent heat, which is a constant.
What is the second law of thermodynamics?
The second law of thermodynamics allows the determination of the preferred direction of a given transformation. Transformations which require the smallest amount of energy and lead to the largest disorder (entropy) of the system are the most spontaneous
What is entropy?
A state function that measures the degree of disorder in a system. For instance, the entropy of ice is lower than the entropy of liquid water since ice corresponds to an organize crystalline structure (virtually no disorder).
Spontaneous reactions proceed if the system is gaining entropy
What is Gibbs free energy?
A state function, which can be used as a criterion for spontaneity. This function is defined as:
G = H- TS
Where H is enthalpy
T is temperature
S is entropy
What is change in Gibbs free energy (formula)
When is a reaction spontaneous? At equilibrium?
ΔG = ΔH - T ΔS
A reaction is spontaneous if ΔG is negative
It is in a state of equilibrium (spontaneous in both directions) if ΔG = 0
What is the change in enthalpy for the reaction below at 25 degrees celsius?
2HCl (g) → H2 (g) + Cl2 (g)
if ΔHf of HCl at 25 degrees is -92.5 kJ/mol
2HCl (g) → H2 (g) + Cl2 (g)
The formation of one mole of HCl liberates 92.5 kJ of heat. Therefore, the formation of two moles of HCl liberates twice as much (-185.0 kJ).
The question asks for the enthalpy change (ΔH) for the reverse reaction, the sign is simply changed to get the answer: 185 kJ
True or false? Boiling point is dependent both on the number of solute particles and the type of solute particles (eg. ionic, etc.).
False
Boiling point is a colligative property that only relies on solute particle numbers (boiling point elevation).
A compound that dissociates into three solutes will have a higher boiling point than one that dissociates into two solutes.
What is a minimum boiling azeotrope? Maximum boiling azeotrope?
A solution where the boiling point is lower than the boiling point of either pure component. This is due to the attraction between like molecules being HIGHER than the attraction between unlike molecules (aka the components)
A maximum boiling azeotrope has a boiling point that is higher than the boiling point of either pure component. This happens when attraction between the components in solution is greater than the attraction between individual components to themselves.
What is an adiabatic system?
A system where no energy enters or leaves.
What type of heat transfer is occurable in a vacuum?
Radiation is the only method of heat transfer that can occur in a vacuum, as electromagnetic waves can certainly propagate through a vacuum.
Convection relies on energy transfer through mass movement of heated material. Conduction requires serial vibration of molecules through a medium, and hence, requires molecules.
