Unit 6: Thermochemistry

enthalpy

enthalpy

the heat exchanged at constant pressure

system

the part of the world that is being studied

surroundings

everything else around the system

universe

the system and the surroundings together

heat (q)

a form of energy that flows between two samples of matter because of their differences in temperature

work

the energy needed to move something against a force, equal to -PΔV

endothermic

heat is absorbed by the system from the surroundings; includes melting, boiling, vaporization, sublimation; breaking bonds; “takes to break”

exothermic

heat is released from the system to the surroundings; includes freezing, condensation, deposition; making bonds; “frees to form”

solvent or solute overcomes IMFs

endothermic process

solute and solvent recombine

exothermic process

solution temperature change

based on the temperature change of the surroundings

1st law of thermodynamics

energy cannot be created or destroyed, energy is released to/taken from the surroundings

activation energy

used to break bonds and/or IMFs

catalysts

added to reactions to increase the reaction rate, only affects activation energy, not enthalpy

temperature

measure of the average kinetic energy of the particles

kinetic energy formula

KE=1/2mv²

heat transfer

the transfer of energy from hotter particles to colder particles upon collision

thermal equilibrium

two substances result in the same temperature after heat transfer

energetically favorable reactions

exothermic reactions

lower potential energy

more stable

specific heat capacity (c)

amount of energy needed to raise the temperature of one gram of a substance by 1 degree C or 1 K

calorimeter

measures the amount of heat transferred during a reaction, a device that stops the transfer of heat between the inside of the coalorimeter and the surroundings

calorimetry

the measurement of the quantify of heat exchanged

heat calculation formula

q_lost=-q_gained

solid

low potential energy, requires energy to become liquid

liquid

medium potential energy, requires energy to become gas, releases energy to become solid

gas

high potential energy, releases energy to become liquid

enthalpy of fusion

involves going from solid to liquid and vice-versa

entahlpy of vaporization

involves going from liquid to has and vice-versa

constant-pressure calorimeter

made of two styrofoam cups, outer cup insulates, two solutions are mixed inside, heat produced/absorbed is measured with thermometer

constant-volume bomb calorimeter

filled with oxygen gas before being placed in bucket, sample is ignited electrically and heat can be determined by the temp increase of the surrounding water

given ΔH values

always based on the molar coefficient for the reaction as written

bond energy

the enthalpy changed required to break or form a given bond in one mole of gaseous molecules

ΔH°

signifies that it is standard state, which is 1 mol, 1 atm, and typically 298K (25 °C)

0 kJ/mol

standard enthallpy of element in its most stable form

ΔH°_f

energy change that occurs with the formation of 1 mole of a substance

ΔH°_rxn

energy change that occurs for a reaction

Hess’s Law

shows the energy changes associated with a combination of steps that leads to a overall reaction

Hess’s Law 3 Properties

1. heat is directly proportional to the amount of products and reactants in a reaction

2. if the reaction is reversed, the sign of the heat is also reversed, but the amount of heat is the same

3. in going from a particular set of reactants to a particular set of products, the change in enthalpy is wlasy the same, regardless of the route taken