Section 2: Thermochemistry

Question 1

Thermochemsitry

Answer 1

• concerns itself with energy: i.e. the conversion of heat to mechanical work (and vice versa), and its relationship to the ‘large scale’ bulk properties of a system that are measurable: Volume, Temperature, Pressure, Heat Capacity, Density etc.
• describes the behaviour of matter and the transformation between different forms of energy on a ‘macroscopic’ scale (i.e. large collections of molecules, not individual molecules).
• is a branch of thermodynamics that investigates the flow of energy into or out of chemical reactions (Heat). From this, we can deduce the energy stored in chemical bonds (amongst other things).

Question 2

System

Answer 2

All the materials involved in the process under study

Question 3

Surroundings

Answer 3

The rest of the universe – or at least those materials outside of the system with which the system interacts.

Question 4

Boundry

Answer 4

The interface between a system and its surroundings

- It is the nature of the boundary that determines whether a system is open, closed or isolated!

Question 5

Open System

Answer 5

Free to exchange both matter and energy with its surrounding (e.g. an open beaker)

Question 6

Closed System

Answer 6

Free to exchange only energy with its surrounding (e.g. a sealed flask)

Question 7

Isolated System

Answer 7

Exchanges neither matter nor energy with its surrounding (e.g. a thermos.. at least for a short time)

Question 8

Energy

Answer 8

(from Greek: “work within”) – Energy is the capacity to do work.
- Energy can change form from potential to kinetic.

Question 9

Work

Answer 9

– Work is done when a force acts through a distance.

Question 10

Kinetic Energy

Answer 10

(“kinetic” means motion in Greek)
– Energy associated with the motion of a body,
or the particles within it.
- 1/2mv^2=J

Question 11

Potential Energy

Answer 11

– Energy due to condition, position, or composition.
– Associated with forces of attraction or repulsion between objects
– Chemical bonding
- W=F•d=J

Question 12

Heat, q

Answer 12

• the quantity of energy, q, transferred between a system and its surroundings (i.e. across a boundary) as a result of a temperature difference.
  •Heat is “energy in transit”, and flows from the warmer to the colder body.
  • Heat flow can occur by different means: conduction, convection or radiation.

Question 13

Do objects contain heat?

Answer 13

NO
•Heat is transitory/temporary: It flows due to a temperature difference, so:
• No temperature difference, No Heat flow.
•We will see that the net effect of heat is to change the internal energy of the system and surroundings

Question 14

Temperature

Answer 14

• A measure of the average energy per particle of the microscopic motions in the system (Temperature is an intensive quantity).
• For a solid, the microscopic motions are principally the vibrations of the constituent atoms about their lattice sites
• In a liquid there are even more ‘degrees of freedom’ for motion, i.e. Translational motion, rotations, intra and intermolecular vibrations

Question 15

Heat and Temperature

Answer 15

• Heat will flow between 2 bodies, A&B, in ‘thermal contact’ until the average energy per particle of the microscopic motions is equal, i.e. until TA = TB

• Why? For heat transfer by conduction or
convection: collisions between atoms at the
surface/boundary.
Consider two billiard/pool balls traveling at quite different speeds. If these two balls collide, is it usually the case that the slower of the two is going even slower after the collision?

Question 16

Zeroth Law of Thermodynamics

Answer 16

• If two systems, A & B, are separately in thermal equilibrium with a third system, C, (i.e. no Heat flow between A ↔ C or B ↔ C) then they are in thermal equilibrium with each other.
• This means that it is possible to define a ‘Temperature’, and to construct a thermometer (i.e. measure the temperature of A or B using a third system, C):
• A gas thermometer might measure the pressure of a fixed volume of gas to indirectly determine T, since: T= P(V/nR)

Question 17

Heat Capacity

Answer 17

• The quantity of heat required to change the temperature of a system by one degree.
• Depends on the system/material
• Molar heat capacity.
- System is one mole of substance.
• Specific heat capacity, c.
- System is one gram of substance
• Heat capacity, C
- Mass × specific heat.

Question 18

c vs. C

Answer 18

Note, ‘c’ is an intensive quantity while ‘C’ is an extensive quantity. C depends on the size of the system, while c only depends on the nature of the system

Question 19

Units of Heat

Answer 19

- units of energy
• Calorie (cal)
– The quantity of heat required to change the temperature of one gram of water by one degree Celsius.
• Joule (J)
– SI unit for heat

1cal= 4.184J

Question 20

Microscopic interpretation of Heat Capacities

Answer 20

cice = 1.96 J/K·g 
cwater = 4.18 J/K·g
 Types of movement:
- translational
- rotational
- vibrational
- electrostatic (intermolecular attracttions)
• In general, the more ways there are to distribute energy throughout the system (i.e. the more microscopic “degrees of freedom”) the higher the heat capacity. No translation or rotation in ice!

Question 21

Conservation of Energy

Answer 21

• In interactions between a system and its surroundings, the total energy remains constant— energy is neither created nor destroyed.
qsystem + qsurroundings = 0
qsystem = -qsurroundings

Question 22

The mechanical equivalent of heat:
James Joule (1818-1889)

Answer 22

• His best-known experiment involved the use of a falling weight to spin a paddle-wheel in an insulated barrel of water, whose increased temperature he measured.
• In 1850, Joule found that 4.2 J were needed to raise the temp. of 1g of water by 1 C°. Quite close to twentieth century estimates!

Question 23

Chemical potential energy

Answer 23

– Contributes to the internal energy of a system. Associated with chemical bonds and intermolecular interactions.
- use no thermometer

Question 24

Heat of reactions, qrxn

Answer 24

– The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature.

Question 25

• Exothermic reactions.

Answer 25

– Produces heat, qrxn < 0

Question 26

• Endothermic reactions.

Answer 26

– Consumes heat, qrxn > 0.

Question 27

Calorimeter

Answer 27

– A device for measuring quantities of heat

Question 28

Bomb Calorimeter

Answer 28

• the BOMB is a closed system
• calorimeter is isolated system
• Determining qrxn is as simple as measuring the increase in calorimeter temperature once the calorimeter heat capacity, C, is known

Question 29

Coffe Cup Calorimeter

Answer 29

• A simple calorimeter.
– Well insulated and therefore isolated.
– Measure temperature change.
qrxn = -qcal

Question 30

Chemical reactions and their ability to do work

Answer 30

• Gas formed pushes against the atmosphere.
• Volumechanges.
• Pressure-volumework.