Chem Exam #1 (thermochem & thermodynamics)

what is enthalpy

a measure of a system’s total energy, and we can only measure changes in enthalpy

what is a state (or state function)

a position

what is a path (or path function)

connects two states

what is the first law of thermodynamics

energy is never created nor destroyed

what is an exothermic process

energy is released by the system (and absorbed by the surroundings)

what is an endothermic process

energy is absorbed by the systen (and released by the surroundings)

What do endo- and exothermic reactions feel like on your skin?

endothermic is cooling your skin because your skin is the surroundings and loses heat. Exothermic is warming it because your skin absorbs the released energy

what is heat and what direction does it move

Heat is energy that is transferred from a hotter object to a cooler one, always moving from areas of higher temperature to lower temperature.

When do substances undergo phase changes?

when they gain/lose heat at specific constant temperatures

What happens to the temperature of substances during phase changes? Between phase changes?

The temperature of a substance remains constant during a phase change. Between phase changes, the temperature increases or decreases depending on heat transfer.

Are substances undergoing phase changes exchanging energy? Where does this energy go/come from if the temperature is not changing?

substances undergoing phase changes are exchanging energy in the form of heat. This energy is absorbed or released from the environment, allowing the phase change to occur without a change in temperature.

what is calorimetry

technique used to measure q for chemical and physical processes

what is Hess’s Law?

∆H for a process is equal to the sum of ∆H for each step of the process

How are bond energies connected ∆H_rxn? What happens to the total potential energy of a system when bonds break or form? How does that relate to ∆H?

Bond energies represent the energy required to break bonds; thus, the change in total potential energy during a reaction corresponds to the difference in bond energies between reactants and products, affecting ∆Hrxn.

what is spontaneity

refers to possibility of a process occurring without external intervention.

it also implies directionality: processes that are spontaneous in one direction are non-spontaneous in the reverse direction

what makes a process spontaneous

it means it can happen under a given set of conditions

what makes a process non-spontaneous

it cannot happen under a given set of conditions

what is entropy

measures the dispersal of matter or energy in a system (disorder/randomness)

when does entropy of a system increase

• Temperature increases

• Gases form from liquids or solids

• Pure substances are mixed to make a solution

• The number of gas molecules increases in a chemical reaction

• Breaking down of complex molecules into simpler molecules

what is Gibb’s free energy

measures the maximum reversible work obtainable from a closed system, helps predict the spontaneity of processes.

what is the 2nd law of thermodynamics

the entropy of the universe increases for any spontaneous process

∆Suniv > 0, ∆Suniv = ∆Ssystem + ∆Ssurroundings

What are the general rules for deciding if products are higher or lower in entropy than reactants?

Products have higher entropy if they are in a gaseous state, consist of more molecules compared to reactants, or are mixtures or solutions rather than pure substances. Generally, increased temperature and complexity also lead to higher entropy.

How do the signs of ∆H and ∆S determine the spontaneity of a chemical reaction?

The signs of ∆H (enthalpy change) and ∆S (entropy change) together help predict spontaneity: a reaction is spontaneous if ∆G is negative, which can occur when ∆H is negative (exothermic) or ∆S is positive (increase in disorder).

what is an exergonic reaction

net release of free energy; spontaneous

∆G is negative

what is an endergonic reaction

net absorption of free energy; non-spontaneous

∆G is positive

what is reaction coupling

Using the energy released from an exergonic reaction to drive an endergonic reaction, allowing non-spontaneous processes to occur.

what are the fundamental properties of (ideal) gasses

compressible, expandable, occupy containers, exert pressure on anything they come in contact with

what is kinetic molecular theory

1. gas molecules are tiny compared to the volume they occupy (far apart & is mostly empty space)

2. gas molecules move constantly and randomly

3. average kinetic energy of gas molecules depends on the temperature of the gas

4. gas molecules continually collide with each other & the walls of the container

5. there are no attractive forces between the molecules

What does the kinetic molecular theory say about the distribution of molecular speeds within a gas?

gas particles are in constant random motion with a wide range of speeds (Boltzmann Distribution). This distribution is asymmetric, showing a peak (most probable speed) with a long tail representing fewer, very fast molecules

Why might the kinetic molecular theory be a useful lens to use when thinking about matter and chemical reactions?

It provides insight into how gas behaviors influence reaction rates, pressure, and temperature, helping to predict and understand molecular interactions during reactions.

what is the ideal gas constant

A physical constant that relates the pressure, volume, temperature, and amount of an ideal gas, commonly denoted as R.

what is the combined gas law

P1V1/T1 = P2V2/T2

what is the ideal gas law?

PV = nRT