Thermochemistry Test Review

The three phases of matter, i. e The three phases of matter, i.e., solids, liquids, and gases, have different properties. Drawing and interpreting particle diagrams are favorite activities here.

Energy can exist in different forms, e. g Energy can exist in different forms, e.g., chemical, light, heat, nuclear. Energy conversion questions are common: chemical potential energy to kinetic energy (exothermic rxns); chemical potential energy to electrical (voltaic cells); electrical to chemical potential energy (electrolytic cells) Nuclear energy to kinetic energy (fission and fusion)

Heat is a transfer of energy (usually thermal energy) from a body of higher temperature to a body of lower temperature. Thermal energy is the energy associated with the random motion of atoms and molecules. Thermal energy = kinetic energy.

Temperature is the measure of the average kinetic energy of the particles in a sample of material. Temperature is not a form of energy. The bolded sentence above is tested somewhere on every single exam!!

Real and Ideal Gases Real gases have significant attractive forces between them and/or are not separated by great distances relative to their size. High pressures and low temperatures will create these conditions for gases. Under conditions of life on earth, H2O behaves as a real gas, while N2 and O2 behave ideally.

KMT Questions

Particles are in constant motion except at absolute zero (zero Kelvin) Particles are in constant motion except at absolute zero (zero Kelvin). Kinetic molecular theory describes the relationships of pressure, volume, temperature, velocity, and frequency and force of collisions.

The concepts of kinetic and potential energy can be used to explain physical properties that include: fusion (melting), solidification (freezing), vaporization (boiling, evaporation), condensation, sublimation, and deposition.

Sublimation questions are common, deposition is rare.

Heating and Cooling Curves Most questions are variations on heating and cooling curves. The horizontal lines are the phase changes. Be sure you know which one (melting-freezing or boiling-condensing) is which on the diagram.

At the melting point, solid and liquid phases are in equilibrium with each other. At the normal boiling point, liquid and gas phases are in equilibrium.

Particle diagrams are commonly used on the regents to demonstrate phase changes.

Phase Changes of Water Physical changes of water are given special attention. Information can be found on Table B. You should know when and how to use the “heat” equations on Table T. For phase changes, q=mHf and mHv. For liquid water temperature increases and decreases, q=mcΔT is used.

The structure and arrangement of particles and their interactions determine the physical state of a substance at a given temperature and pressure.

Intermolecular Forces Intermolecular forces created by the unequal distribution of electrons result in varying degrees of attraction between molecules. Hydrogen bonding is an example of a strong intermolecular force.

Small molecules that have equal or symmetrical charge distributions are non-polar, and are gases to very low temperatures and pressures (that is, they behave ideally under most conditions). Larger molecules, even those with symmetrical charge distributions, have larger attractive forces and higher boiling points and melting points.

The boiling point (distillation) is used to separate crude oil (a mixture of hydrocarbons) into fractions based on molecular size. Unequal distribution of charge is a euphemism for a polar molecule. These molecules have dipoles [partial (+) and (–) parts of the molecule]. For similar molecules, the larger the dipole, the higher the melting point and boiling point are.

Hydrogen Bonding Hydrogen “bonding” is the strongest dipole force. Only N, O, and F form hydrogen bonds. Although it may seem obvious, hydrogen bonds are only between these elements and hydrogen: N – H , O – H, and H – F. Remember, hydrogen bonding is not a true “bond”; it has 5% of the strength of a covalent bond.