intro to thermochemistry

systems

where a physical or chemical change occurs

surroundings

the rest of the universe outside the system

open system

both matter and energy can freely flow into the surroundings

closed system

energy can flow to surroundings but matter cannot

isolated system

• neither energy nor matter can flow to the surroundings

• cannot have perfect isolated system for a long period of time

• energy transfer will occur overtime

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what is thermochemistry?

• the study of the energy changes that accompany physical or chemical changes in matter

• changes that occur in matter: physical changes. chemical changes, nuclear changes (changing the nucleus)

• all accompanied by a change in energy that can be measured and quantified

thermal energy

• energy available from a substance as a result of the motion of its molecules

• stored in particles through translational, rotational, and vibrational motion

translational motion

move from place to place along a linear path

rotational movement

rotate about a bond axis through the center of mass; particles spin

vibrational movement

vibrate or oscillate back and forth

phase and movement type: gas

• free translation

• free rotation

• free vibration

• particles far apart and moving freely, mainly lots of fast translational motion

• weak intermolecular forces

phase and movement type: liquid

• restricted translation

• restricted rotation

• free vibration

• particles close together but may move past each other

• moderate amount of intermolecular forces

phase and movement type: solid

• absent translation

• very restricted rotation

• free vibration

• particles locked in place, can only vibrate

• very strong intermolecular forces

heat

• Q

• amount of energy transferred between substances

• measured in Joules (J)

• Q = mcΔT

temperature

• T

• the average kinetic energy of the molecules in a sample

• measured in C or K

relativity of temperature and heat

• feeling heat is truly feeling rate at which heat is conducted away or towards you

• cold: when surrounding particles hit you, you transfer some heat to them, you feel cold

• hot: faster moving particles collide with yours, heat is transferred to you, you feel hot

energy flow

• energy flows between substances because of their difference in temperature

• depends on ability of substance to conduct heat towards or away from itself

• NOT relative

why do metals feel cooler?

a network of delocalized electrons, always moving, transfer electricity and heat quickly

exothermic reactions

• exo = exit

• release of thermal energy

• heat flows from system to surroundings

• causes an increase in the temperature of the surroundings

• negative Q value, Q < 0

endothermic reaction

• endo = enter

• absorption of thermal energy

• heat flows from surroundings into system

• causing a decrease in temperature of surroundings

• positive Q value, Q > 0

relation of heat for system and surroundings

Q_system = -Q_surroundings

what is the internal energy of a system equal to?

• Internal energy is the total energy inside a system, coming from all the particles in it

• Internal energy = kinetic energy of particles + potential energy of all particles

kinetic energy

• stored in

• moving electrons in atoms

• vibration, rotation and translation of atoms and molecules

chemical potential energy

• stored in

• nuclear potential energy of protons and neutrons

• bond energy

• intra and intermolecular forces

explain this diagram

• total energy remains constant

• reactions usually require activation energy

• covalent bonds broken

• atoms rearrange to have electrons in most stable state

• form water molecules

specific heat capacity

• c

• the amount of energy required to raise the temperature of one gram of a substance one °C or one K

• dependent on the three ways its molecules, atoms or ions can store thermal energy

• more atoms, stringer bonds = higher heat capacity

heat formula

• Q = mcΔT

• the amount of heat transferred (Q) depends on the mass of the sample measured in grams (m), temperature change in C or K (ΔT), and specific heat capacity measured in J/gC or J/gK (c )

temperature readings explanation on a thermometer

• temperature is a measure of the average translational energy of the molecules striking the thermometer

• must have collision for transfer of energy

specific heat vs. heat capacity

• specific heat capacity: amount of heat required to raise the temperature of 1g of substance through 1 kelvin

• heat capacity: amount of the heat required to raise the temperature of the body through 1 K

• heat capacity is additionally dependent on how much of the substance is present

energy for change of state: heat of fusion

• heat of fusion, L_f: amount of energy required to change 1 gram of pure substance from solid to liquid at its melting point

• NOT a change in temperature

• Q = mL_f

energy for change of state: heat of vapourization

• the amount energy required to convert 1 gram of pure substance from liquid to gas at its boiling point

• NOT a change in temperature

• Q = mL_v