Thermochemistry The study of the energy changes that accompany physical, chemical or nuclear transformations of matter.

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Thermochemistry The study of the energy changes that accompany physical, chemical or nuclear transformations of matter.

Heat & Energy

Energy in Chemical Reactions

Exothermic Reactions Overall heat is transferred from the system to the surroundings

Products have a lower potential energy than the reactants

Temperature of the surroundings increase

Thermal energy of the system decreases and thermal energy of the surroundings increases

Endothermic Reactions Overall heat is transferred from the surroundings to the system

Products have a higher potential energy than the reactants

Temperature of the surroundings decrease

The thermal energy of the system increases and thermal energy of the surroundings decreases

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

Key Terms

Heat Amount of energy transferred between substances during a physical, chemical or nuclear change.

Temperature The average kinetic energy of the particles in a sample of matter.

Thermal Energy Yype of kinetic energy available from a substance as a result of motion of its molecules.

Types of Energy Changes There are 3 types of energy changes

Nuclear Change Arise from rearrangements within the nuclei of one or more atoms.

A lot of energy is involved in nuclear reactions Far greater than physical and chemical changes

In this type of reaction some mass is converted to energy (E = mc^2)

Chemical Change Reactant bonds are being broken and product bonds are being formed as the atoms are being separated, combined or rearranged.

How to determine if reaction is Endothetmic or Exothermic

Add up the energy released when the “new” bonds form

- 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 required to break the “old” bonds

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

Bond breaking requires energy so it is endothermic

Some energy must be added to allow reactant bonds to start breaking

Physical Change Changes of state require energy or release energy overall because intermolecular attractions must be broken.

Example Ice melting to water

It will be an endothermic proccess

Some of the hydrogen bonds between water molecules must be broken so thermal energy must be added

Physical & Chemical Systems A System is the set of substances or reactants and products being studied

Types of Chemical Systems

3 types of chemical systems

Isolated System ideal system where neither energy nor matter can move into or out of the system

Closed System Energy can move into or out of the system but matter cannot

Open System Both energy and matter can move into or out of the system

Surroundings All of the matter around the system that is capable of absorbing or releasing energy

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

Kinetic Energy the energy associated with the motion of a particle

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.

Rotational Energy - Associated with the rotating of an atom or molecule around an axis - Gain rotational energy when they enter a liquid state

Vibrational Energy - Associated with movement of connected atoms back and forth along a chemical bond. - This is the only kinetic energy in a solid

Electron Energy - Associated with the movement of electrons within an atom

Potential Energy The energy stored by associations within or between atoms

Electronic Potential - Associated with the interaction of electrons with nuclei - Occurs both within and between atoms

Nuclear Potential - Associated with forces that hold the nucleus together - Interaction of protons and neutrons