(1) Thermochemistry

Question 1

Define thermochemistry.

Answer 1

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

Both the energy of the substance(s) undergoing the change and the surrounding environment must be considered.

Question 2

Define a system.

Answer 2

Systems are a set of substances or reactants and products being studied (substances undergoing change).

Often is represented by a chemical equation.

Question 3

Define surroundings.

Answer 3

All matter around the system that is capable of absorbing/releasing energy.

Question 4

Name the 3 types of chemical systems.

Answer 4

Open: energy and matter can move in/out of the system from/to its surroundings.

Closed: energy can move in/out of the system but matter cannot.

Isolated: ideal system where neither energy nor matter can move in/out of the system.

Question 5

Name the 2 types of energy in a chemical system.

What is the equation for the total energy of a system?

Answer 5

Kinetic energy (Ek) and potential energy (EP).

Total energy (internal energy): E = Ek + EP

Question 6

Define potential energy and the two types.

Answer 6

Potential energy (EP): the energy stored by associations within or between atoms.

Nuclear potential energy: energy of forces that hold the nucleus together.
–> interaction of protons and neutrons in nucleus of an atom

Electronic potential energy: energy of interactions of electrons with nuclei
–> occurs both within and between atoms
–> between atoms results in formation of chemical bonds (bond energy)

Question 7

Define kinetic energy.

Answer 7

Kinetic energy (Ek): energy of motion of a particle
–> related to the mass and velocity of the particle (EK = 1/2mv^2)

Question 8

List the 4 types of kinetic energy and what they are.

Answer 8

electron energy (EE): energy of movement of electrons within an atom

vibrational energy (Ev): energy of movement of connected atoms back and forth, along a chemical bond. Only type of kinetic energy in solids, occurs in all 3.

rotational energy (ER): energy of rotating an atom or molecule around an axis (whole molecule). Molecules gain rotational energy when they enter a liquid state, and gas state.

translational energy (ET): caused by molecules moving from one place to another. First appears when a substance is liquid, but is much more pronounced in gaseous state.

Question 9

Define thermal energy.

Answer 9

Thermal energy: a type of kinetic energy available from a substance, as a result of motion of molecules. Transfer of thermal energy occurs during changes in matter.

Question 10

Define temperature.

Answer 10

Temperature: the avg. kinetic energy of particles in a sample of matter.

Question 11

Define heat (q).

Answer 11

Heat (q): the amount of energy transferred between substances during a physical, chemical, or nuclear change. Heat can be transferred from the system to the surroundings or the surroundings to the system. (kJ or J)

Question 12

State the relationship between thermal energy and substances.

Answer 12

transfer of heat results in changes of thermal energy.
- thermal energy of the originated substance decreases, thermal energy of the destination substance increases.

transfer of thermal energy results in change in temperature.
- transfer increases kinetic energy of recipient.
- decreases kinetic energy of original substance

Question 13

Define exo and endothermic reactions.

Answer 13

exothermic: overall heat is transferred from system to surroundings (release)
- thermal NRG of system decreases
- products have a lower potential energy than reactants, more stable
- synthesis

endothermic: overall heat is transferred from surroundings to system (absorption)
- thermal energy of system increases
- products have a higher potential energy than reactants, less stable
- decomposition

Question 14

What are the 3 types of energy changes?

Answer 14

1. physical
2. chemical
3. nuclear

Question 15

Define physical changes and provide an example.

Answer 15

changes of state require/release energy because intermolecular attractions must be broken/formed.

Freezing/melting, boiling/condensation, only breaking or forming forces of attraction.

Question 16

Provide examples of physical changes for melting and freezing.

Answer 16

Melting ice: hydrogen bonds between water molecules must be broken. Thermal energy must be added (from surroundings). Endothermic.

Freezing water: energy is removed from system, hydrogen bonds can form to make crystals. Energy must be TRANSFERRED from water to surroundings, such as a freezer. exothermic.

Question 17

Explain physical changes for dissolving.

Answer 17

Dissolving: intermolecular forces are broken and replaced by others.
–> decomposition of NaOH to ions is exothermic
–> decomposition of NH4NO3 to ions is endothermic

• in both, energy is required to break the initial bonds
• new ion-dipole attractions are formed between ions and H2O molecules
• endo or exo depends on relative quantities of energy required/released
• for NaOH, less energy is required to break the existing bonds than is released when ion-dipole attractions are formed, so it is exothermic (releases more than absorbed)

Question 18

Define chemical changes.

Answer 18

Chemical changes: reactant bonds are being broken and product bonds are being formed as atoms/ions are separated/combined/rearranged

bond breaking: requires energy, endothermic

bond forming: reverse of breaking, releases energy, exothermic
- molecule being formed is more stable/has lower potential energy

Question 19

Explain how to determine of the overall reaction is endo or exothermic.

Answer 19

1. add up the energy required to break the reactant bonds
2. add up the energy released when product bonds form

if more energy is required to break the old bonds than is released from the new ones, the system would have lost potential energy; endothermic.

Question 20

Define nuclear changes.

Answer 20

Nuclear changes: rearrangements within the nuclei of one or more atoms.
- strong nuclear forces that hold nucleus together
- some mass is converted to energy (E = mc^2)

since the speed of light (c) is so large, even a tiny amount of matter converted to energy results in a large amount of energy
–> a lot of energy is involved (heat from the sun, despite being 1.5 * 10^8 km away)