Section 2: Thermochemistry Flashcards
Thermochemsitry
- 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).
System
All the materials involved in the process under study
Surroundings
The rest of the universe – or at least those materials outside of the system with which the system interacts.
Boundry
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!
Open System
Free to exchange both matter and energy with its surrounding (e.g. an open beaker)
Closed System
Free to exchange only energy with its surrounding (e.g. a sealed flask)
Isolated System
Exchanges neither matter nor energy with its surrounding (e.g. a thermos.. at least for a short time)
Energy
(from Greek: “work within”) – Energy is the capacity to do work.
- Energy can change form from potential to kinetic.
Work
– Work is done when a force acts through a distance.
Kinetic Energy
(“kinetic” means motion in Greek)
– Energy associated with the motion of a body,
or the particles within it.
- 1/2mv^2=J
Potential Energy
– Energy due to condition, position, or composition.
– Associated with forces of attraction or repulsion between objects
– Chemical bonding
- W=F•d=J
Heat, q
- 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.
Do objects contain heat?
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
Temperature
- 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
Heat and Temperature
• 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?
Zeroth Law of Thermodynamics
- 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)
Heat Capacity
• 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.
c vs. C
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
Units of Heat
- 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
Microscopic interpretation of Heat Capacities
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!
Conservation of Energy
• In interactions between a system and its surroundings, the total energy remains constant— energy is neither created nor destroyed.
qsystem + qsurroundings = 0
qsystem = -qsurroundings
The mechanical equivalent of heat: James Joule (1818-1889)
- 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!
Chemical potential energy
– Contributes to the internal energy of a system. Associated with chemical bonds and intermolecular interactions.
- use no thermometer
Heat of reactions, qrxn
– The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature.
• Exothermic reactions.
– Produces heat, qrxn < 0
• Endothermic reactions.
– Consumes heat, qrxn > 0.
Calorimeter
– A device for measuring quantities of heat
Bomb Calorimeter
- 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
Coffe Cup Calorimeter
• A simple calorimeter.
– Well insulated and therefore isolated.
– Measure temperature change.
qrxn = -qcal
Chemical reactions and their ability to do work
- Gas formed pushes against the atmosphere.
- Volumechanges.
- Pressure-volumework.
