CHE112 midterm 1 flashcards

list all the state functions and path functions that are relevant to the course

path: only q and w

state: the rest (U,p,T,V)

Isobaric

constant pressure

Isochoric

constant volume

Isothermal

constant temp

Adiabatic

no heat transfer

Conversion between constant pressure and constant volume heat capacities

1. MAKE SURE they are both molar heat capacities. If not, convert (divide by the number of moles)

2. cp = cv + R (constant pressure heat capacity is ALWAYS greater)

Converting gauge pressure to absolute pressure

Gauge pressure is the SMALL one. Gauge pressure PLUS 1atm is absolute pressure

Work done by a gas if volume doesn’t change

Constant volume - work must be 0, U=q

When are w and w each positive?

positive work - work done ON the system (compression of a gas is positive), positive heat transfer - heat transfered INTO the system

Two types of heat

Latent - resulting phase change, no resulting temperature change

Sensible - temperature increases

Pressure formula

Delta P = (density)(gravity)(delta height), so if the height changes by a certain amount the temperature changes proportionally. Going higher up away from the centre of the earth DECREASES pressure. Higher = LESS pressure

Finding enthalpy of reaction from combustion and/or formation enthalpies

Always just use Hess’s law: write out the full reactions, make sure your moles are right, and then Hess it out

Things to remember when doing Hess’s law

1. There’s a lot to keep track of. You probably messed up. Be completely sure everything cancels properly

2. Make sure your enthalpies have the correct signs. Combustion is always exothermic (negative), formation of any stable oxide or of water is also always exothermic, and for phase changes you know it.

What is a calorie in SI units

4.184 Joules

When will spontaneous compression or expansion of a gas STOP?

When the pressure of the gas is equal to the external pressure. The mass of anything pushing down on the gas should be added to the atmospheric pressure to get the external pressure

Pressure in SI units (Pa)

Pressure = Force / Area = N / m²

Force = mass x acceleration = Kg/m*s^(-2)

Pressure = (kg) / (m * s²)

Pascal’s principle

Pressure remains constant throughout a fluid, so if you have a much greater area you will also get a much greater force

Partial pressures (Dalton)

Total pressure of a mix of gasses = the sum of all the partial pressures

Partial pressure of a gas = its mole fraction * the total pressure of all the gasses

The mole fraction of a gas = (the number of moles of that gas) / (the total number of moles of all the gasses including that gas)

Van der Waal’s equation: what do a and b represent

a - particles have intermolecular forces

b - particles have volume

What is the definition of reversible work (mathematically)

work is equal to (-1) times the integral of P(ext) from (Vf - Vi) with respect to V (work is the area under the pressure-volume curve between Vi and Vf)

What criteria must ALL be met in order to use this formula: w=-nRTln(Vf/Vi)

1. Must be an ideal gas

2. Must be an isothermal process (constant temperature)

3. MUST be a reversible process (NOT spontaneous)

What criteria must ALL be met in order to use this formula: w=-P(ext) (delta V)

1. Must be an irreversible process

2. External pressure must be constant

Reversible vs irreversible process (not formulas, conceptually)

Reversible: system is in equillibrium with its surroundings throughout, which means that Pext is always approximately equal to Pgas at every step

Irreversible: Often sudden/spontaneous expansion or compression against constant external pressure. Pext is NOT equal to Pgas throughout the process. Usually for an irreversible process external pressure IS constant though, so we can just use -Pext(delta V) to find work. Pext doesn’t HAVE to be constant for a process to be irreversible, though. It just has to be not always equal to Pgas.

Is irreversible work or reversible work less for the same change

Irreversible work (sudden change) is ALWAYS LESS work than reversible work