Highschool Thermo

Temperature

Measure of average kinetic energy of particles in a sample of matter

Joule

Amount of energy transferred as heat kg*m²/s²

Heat

Energy transferred between samples of matter due to temperature differences

How does Heat flow

From high temperature (energy) to low temperature (energy)

Specific heat

Energy required to raise the temperature of one gram of the substance by one degree (celsius or kelvin)

Energy lost or gained equation

q = c_p*m* (change in temperature)

Enthalpy Change

amount of energy absorbed by a system as heat during a process at constant pressure

Enthalpy equation

delta H = H_products - H_reactants

Enthalpy of reaction (delta H_reaction)

The quantity of energy transferred as heat during a chemical reaction

Negative for Exothermic Reactions

Positive for Endothermic Reactions

Thermochemical Equation

Normal Chemical Equation that includes the quantity of energy (Joules) as a term

Endothermic Reaction: energy is a reactant and absorbed from surroundings; delta H is positive

Exothermic Reaction: energy is a product and released to surroudings; delta H is negative

Thermochemical Equation Specifics

Each term in the equation represents moles of a substance

Physical state (s, l, g) matters include it when you write equations

If you multiply the equation by a factor make sure to multiply the change in enthalpy too

Enthalpy change usually isn’t influenced that much by temperature

Exothermic Reaction

System releases energy to the surroundings

Endothermic Reaction

System absorbs energy from surroundings

Molar Enthalpy of Formation

Enthalpy change that occurs when 1 mole of a compound is formed from its elements at STP (25 degress Celcius and 1 atm of pressure)

Enthalpies of formation are given at room temperature and standard pressure. So water is a liquid (does not freeze or boil at 25 degrees celcius)\

Elements in their standard state have an enthalpy of formation = 0

The most stable compounds…

Have a large negative enthalpy of formation. Anything w/ a positive enthalpy of formation is unstable

Hess’s Law Tips

Use it if they give you a bunch of thermochemical equations

Manipulate the equation so you can cross out things that appear on both sides until you get the final equation

If you flip the equation, flip the sign of the enthalpy

If you multiply the equation by a factor, multiply the enthalpy by the same factor

Add up all the enthalpies at the end and that is the total enthalpy change of the reaction (your answer)

Reactions depend on…

Both Enthalpy (delta H) & Entropy (delta S)

A spontaneous reaction can either go from

a high-energy state to a low-energy state (negative delta H)

a highly ordered state to a more “chaotic” state (positive delta S)

Entropy (S)

a measure of the degree of randomness of the particles in a system

Entropy and Phase Changes

Least Entropy S < L < G Most Entropy

Solids: Particles are fixed in position; low randomness low entropy

Liquids: Particles are close together but can move; medium randomness medium entropy

Gas: Particles are far apart and can move; high randomness high entropy

3rd Law of Thermodynamics

The entropy of a pure crystalline solid at 0 kelvin (absolute 0) is 0

Entropy change ( delta S )

S_products - S_reactants

Increase in Entropy + delta S

Decrease in Entropy - delta S

Forming Solutions & Entropy

Increase in Entropy, + delta S

ex. mixing gasses, dissolving a liquid in another liquid, and dissolving a solid in a liquid

Free Energy equation

delta G = delta H - T* delta S

Positive delta G = non-spontaneous reaction ( will not occur naturally)

Negative delta G = spontaneous reaction ( will occur naturally )

Natural processes proceed in the direction that lowers free energy

If delta H is negative & delta S is positive

delta G is always negative; reaction is spontaneous at the conditions provided and will proceed

If delta H is negative & delta S is negative

delta G is negative at lower temperatures; reaction is spontaneous at lower temperatures

If delta H is positive & delta S is positive

delta G is negative at higher temperatures; reaction is spontaneous at higher temperatures

If delta H is positive & delta S is negative

delta G is always positive; reaction is never spontaneous & will never proceed (in nature)