Thermal Chemistry

Question 1

What is the first law of thermodynamics and the equation associated with it?

Answer 1

The first law of thermodynamics states that energy can be converted from one form to another but it is never destroyed.

ΔU=Q-W

ΔU - Change in internal energy
Q - Heat energy
W - Work energy

Question 2

What is the second law of thermodynamics?

Answer 2

Entropy is always increasing in the universe

ΔS≥0

Entropy is a measurment of disorder or randomness in a system

Δ - is the capital greek letter delta and, in chemistry, means “change in”

Question 3

What are the products of complete combustion?

Answer 3

Carbon dioxide (CO₂) and water (H₂O)

Requires oxygen gas (O₂) and fuel (usually a hydrocarbon)

Question 4

What are the products of incomplete combustion?

Answer 4

Carbon monoxide (CO) and soot (C_(s))

Requires oxygen gas (O₂) and fuel (usually a hydrocarbon)

Question 5

What is complete combustion?

Answer 5

Complete combustion occurs in the presence of sufficient molar amounts of oxygen to react with the fuel being combusted

Question 6

What is incomplete combustion?

Answer 6

Combustion that occurs without a sufficient amount of oxygen present

Question 7

In thermodynamics, what is an open system?

Answer 7

An open system allows for the free exchange of heat energy and matter into and out of the system.

Red squiggley line represents heat energy
Blank solid line represents matter

Question 8

In thermodynamics, what is a closed system?

Answer 8

A closed system still allows for the free exchange of energy into and out of the system; however, does not allow for matter to flow into or out of the system.

Red squiggley line represents heat energy
Blank solid line represents matter

Question 9

In Thermodynamics, what is an isolated system?

Answer 9

An isolated system does not allow for the exchange of energy or matter from the system to the universe, or from the universe to the system.

Red squiggley line represents heat energy
Blank solid line represents matter

Question 10

What is the relationship between entropy and temperature?

Does increasing temperature cause an increase or decrease in entropy?

Remember: Entropy is a measurement of disorder/randomness

Answer 10

Entropy and temperature have directly proportionally relationship. Consider the Gibb’s free energy eq. ΔG=ΔH-TΔS, temperature and entropy are multiplied so that this term increases in magnitude as either temperature or entropy increases (and vice versa).

ΔG - Gibbs free energy
ΔH- Enthalpy
T - Temp. (kelvin)
ΔS - Entrop

Question 11

What is enthalpy?

Also, what is the SI unis associated with enthalpy?

Answer 11

Enthalpy is the measure of heat energy. The SI unit for enthalpy is the joule (J)

The joule (J) is the SI unit for meausring energy

Question 12

What is entropy?

Also, what are the units of entropy?

Answer 12

Entropy is the measurement of disorder or randomness of a thermodynamic system.

The units of entropy are joules/kelvin (J/K)

joules - measurement of energy
kelvin - measurment of temperature (Temp. in Kelvin = Temperature in Celcius + 273.15)

Question 13

What is temperature?

Answer 13

Temperature is the average kinetic energy in a system.

Question 14

What is Gibb’s Free Energy?

What is the equation for Gibb’s free energy?

Also, what are the units for ΔG?

Answer 14

Gibbs free energy combines enthalpy (heat energy) and entropy (the measurement of disorder in a system) into a single term to determine whether or not a chemical reaction will be spontaneous (occur without any additional energy input into the chemical reaction)

ΔG=ΔH-TΔS

ΔG - Gibbs free energy
ΔH- Enthalpy
T - Temp. (kelvin)
ΔS - Entropy

ΔG is reported in joules or kilojoules (KJ)

Question 15

What variable determines if a chemical reaction is spontaneous or not?

Answer 15

Gibb’s free energy

Gibb’s free energy will negative if the reaction is spontaneous

The more negative the Gibb’s free energy, the more spontaneous the reaction. A Gibb’s free energy of -800 KJ will spontaneously occur much much faster than a spontaneous reaction with a Gibb’s free energy of -0.1 KJ.

Question 16

What does ΔG=0 indicate for a chemical reaction?

Answer 16

The reaction has reached an equilibrium state

Question 17

What does it mean if a reaction is endergonic?

Answer 17

The reaction has a positive ΔG value and is not spontaneous

Question 18

What does it mean if a reaction is endothermic?

Answer 18

The reaction has a positive ΔH value, meaning that the reaction requires heat energy to proceed in the forward direction

Question 19

What does it mean if a reaction is exergonic

Answer 19

The reaction has a negative ΔG value and the reaction is spontaneous in the forward direction

Question 20

What does it mean if a reaction is exothermic?

Answer 20

The reaction has a negative ΔH value and the reaction release heat when proceeding in the forward direction.

Question 21

if ΔG=ΔH-TΔS, and a negative ΔG value means that the reaction is spontaneous, what are the factors that result in a spontaneous chemical reaction?

Think about the following variables and their relationship to ΔG
ΔH
T
ΔS

Answer 21

The following will increase the likelihood that a reaction is spontaneous:

• A reaction is exothermic (produces/releases heat)
• Temperature is high (the higher the temp. the more likely the reaction will be spontaneous)
• The reaction results in an increase in entropy (correlate with the second law of thermodynamics)

Question 22

What is the difference between specific heat and heat capacity?

Answer 22

Heat capacity refers to the amount of thermal energy (enthalpy) required to raise the temperature of a substance by 1°C. While specific heat (sometimes referred to as specific heat capacity) is the amount of thermal energy required to raise 1 g of a substance by 1°C

Specific heat is mass depedent (substance must be one gram)

So specific heat is the heat capacity for one gram of a given substance.

Question 23

What is the specific heat of water?

Use the units of J/g·°C

Answer 23

4.186 J/g·°C

The specific heat of water

The amount of joules (heat energy) required to raise one gram of water (remember the density of water is 1 g/ml) by one degree C.

Question 24

What would the enthalpy diagram of an exothermic reaction look like?

Answer 24

For an exothermic reaction, the net enthalpy of the products is less than the reactants because heat energy is released

Question 25

What would the enthalpy diagram of an endothermic reaction look like?

Answer 25

For an endothermic reaction, the net enthalpy of the products is more than the reactants because energy is gained by the chemical system.