Thermodynamics and Thermochemistry

Question 1

What is heat?

Answer 1

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)

Question 2

What is internal energy?

Answer 2

The average total mechanical energy (kinetic + potential) of the particles that make up the system.

Question 3

What is the first law of thermodynamics?

Answer 3

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

Question 4

What is 1 calorie?

Answer 4

The amount of thermal energy required to raise the temperature of water by 1 degree Celsius

Question 5

How many joules is 1 calorie?

Answer 5

1 cal = 4.184 J

Question 6

How can you obtain the energy per particle in chemical reaction energy exchanges?

Answer 6

Divide energy by Avogadro’s number ( 6.02 x 10^23 mol^−1)

Question 7

How can Fahrenheit be converted into celsius?

Answer 7

(X°F - 32) x 5/9 = X°C

or

°F = 9/5°C + 32

Question 8

What are three ways that heat can be transferred between a system and its surroundings?

Answer 8

• Conduction (contact between system and surroundings)
• Convection (Circulation of hot liquid/gas through system)
• Radiation (no contact between heat source and system)

Question 9

What are state functions?

Answer 9

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.

Question 10

Of the following fundamental energy functions (internal energy E, heat Q and work W), which are state functions and which are not?

Answer 10

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.

Question 11

What are three consequences for internal energy being a state function?

Answer 11

• 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

Question 12

Give the mathematical definition for enthalpy. How is this reduced to its most simple definition?

Answer 12

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

Question 13

ΔH is +/- for exothermic/endothermic reactions

Answer 13

Exothermic: -ΔH
Endothermic: +ΔH

Question 14

What are standard conditions for standard enthalpies (ΔH°)?

Answer 14

1. Standard pressure: 1 atm
2. Standard temperature is 25°C
3. Standard physical state is natural state of element

These standards must be true for the standard enthalpy change (ΔH°) for a given reaction.

Question 15

What is the enthalpy of formation?

Answer 15

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)

Question 16

What is bond dissociation energy (bond energy)?

Answer 16

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)

Question 17

The standard enthalpy of formation of a mole of any ‘element’ is what?

Answer 17

ZERO

Question 18

What is the specific heat capacity (C) for water? Why?

Answer 18

1

Because 1 calorie can heat 1 gram of water two 1 degrees celsius

Question 19

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?

Answer 19

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.

Question 20

What is the second law of thermodynamics?

Answer 20

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

Question 21

What is entropy?

Answer 21

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

Question 22

What is Gibbs free energy?

Answer 22

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

Question 23

What is change in Gibbs free energy (formula)

When is a reaction spontaneous? At equilibrium?

Answer 23

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

Question 24

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

Answer 24

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

Question 25

True or false? Boiling point is dependent both on the number of solute particles and the type of solute particles (eg. ionic, etc.).

Answer 25

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.

Question 26

What is a minimum boiling azeotrope? Maximum boiling azeotrope?

Answer 26

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.

Question 27

What is an adiabatic system?

Answer 27

A system where no energy enters or leaves.

Question 28

What type of heat transfer is occurable in a vacuum?

Answer 28

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.