Thermochemistry
The study of the energy changes that accompany physical, chemical or nuclear
transformations of matter.
Physical & Chemical Systems
A System is the set of substances or reactants and products being studied
Energy in chemical systems
Two Types of Energy
The total energy of the system is equal to the sum of the kinetic and potential energy
Potential Energy
The energy stored by associations within or between atoms
Nuclear Potential
- Associated with forces that hold the nucleus together
- Interaction of protons and neutrons
Electronic Potential
- Associated with the interaction of electrons with nuclei
- Occurs both within and between atoms
Kinetic Energy
the energy associated with the motion of a particle
Electron Energy
- Associated with the movement of electrons within an atom
Vibrational Energy
- Associated with movement of connected atoms back and forth along a
chemical bond.
- This is the only kinetic energy in a solid
Rotational Energy
- Associated with the rotating of an atom or molecule around an axis
- Gain rotational energy when they enter a liquid state
Translational Energy
- Caused by the molecules moving from one place to another.
- First appears when a substance is a liquid but is much more pronounced in the gaseous state.
Types of Chemical Systems
Surroundings
All of the matter around the system that is capable of absorbing or releasing energy
3 types of chemical systems
Open System
Both energy and matter can move into or out of the system
Closed System
Energy can move into or out of the system but matter cannot
Isolated System
ideal system where neither energy nor matter can move into or out of the system
Types of Energy Changes
There are 3 types of energy changes
Physical Change
Changes of state require energy or release energy overall because intermolecular attractions must be broken.
Example
Ice melting to water
Some of the hydrogen bonds between water molecules must be broken so thermal energy must be added
It will be an endothermic proccess
Chemical Change
Reactant bonds are being broken and product bonds are being formed as the atoms are being separated, combined or rearranged.
Bond breaking requires energy so it is endothermic
Some energy must be added to allow reactant
bonds to start breaking
Bond forming is the REVERSE of breaking, so it must RELEASE energy so it is exothermic
Some energy is released when the bonds that hold the molecule together are formed
How to determine if reaction is Endothetmic or Exothermic
Add up the energy required to break the “old” bonds
- If energy to break old bonds is greater than the reaction is Endothermic
- If energy to break new bonds is greater than the reaction is Exothermic
Add up the energy released when the “new” bonds form
Nuclear Change
Arise from rearrangements within the nuclei of one or more atoms.
In this type of reaction some mass is converted to energy
(E = mc^2)
A lot of energy is involved in nuclear reactions
Far greater than physical and chemical changes
Heat & Energy
Key Terms
Thermal Energy
Yype of kinetic energy available from a substance as a result of motion of its
molecules.
Temperature
The average kinetic energy of the particles in a sample of matter.
Heat
Amount of energy transferred between substances during a physical, chemical or nuclear change.
Relationships
When a change occurs heat is transferred between the system and its surroundings.
Heat Transfer leads to transfer of thermal energy
Transfer of thermal energy results in a change in the temperature
Energy in Chemical Reactions
Endothermic Reactions
Overall heat is transferred from the surroundings to the system
The thermal energy of the system increases and thermal energy of the surroundings decreases
Temperature of the surroundings decrease
Products have a higher potential energy than the reactants
Exothermic Reactions
Overall heat is transferred from the system to the surroundings
Thermal energy of the system decreases and thermal energy of the surroundings increases
Temperature of the surroundings increase
Products have a lower potential energy than the reactants