exam 3 Flashcards
(36 cards)
potential energy vs kinetic energy and thermal energy
potential energy: energy due to position or composition
- holding a ball above ground
- energy held in the bonds of compounds (have capacity to break or release a lot of energy)
kinetic energy: energy due to motion of an object
thermal energy: kinetic energy associated with motion of atoms/molecules
energy & heat definition
+ law of conservation of energy
energy: capacity to produce heat or do work
heat: transfer of energy between 2 objects due to the temperature difference between them (ice melting in hand because heat from hand going into ice and melting it)
law of conservation of energy: energy can be converted from one form to another but never created or destroyed
exothermic vs endothermic reactions
exothermic reactions: release energy (heat flows out of system)
endothermic reactions: absorb energy (heat flows into system)
how many joules in 1 cal?
1 calorie = 4.184 J
1 Calorie = 1000 cal (or 1 kcal)
- there’s a difference between the lower and capital letter C !!
calorie (cal): amount of energy required to raise temperature of 1g of water by 1° C
joule (J): amount of energy used when a force of 1 newton moves an object 1 meter
heat capacity
+ formula and units
heat capacity (C): amount of heat (q) needed to raise the temperature of a substance by 1° C
- units of J/°C or J/K
C = q/∆T
specific heat capacity vs. molar heat capacity + units
specific heat capacity: amount of energy required to raise the temp of 1 gram of substance by 1°C
- units of J/°C or J/K⋅g
molar heat capacity: amount of energy required to raise the temp of 1 mol of substance by 1°C
- units of J/°C or J/K⋅mol
equation for heat transfer
q = mC∆T
q = heat transferred
c = specific heat capacity
m = mass
∆T = change in temp
enthalpy + equation
enthalpy: total amount of internal energy in a system
- change in enthalpy (∆H) of a system equals the flow of heat into or out of a system
∆H reaction = H products - H reactants
what does ∆H° mean
used to indicate specific conditions:
1 atm
1 M
25 °C
what is a state function
variable that only depends on the state of a system
ex:
- potential energy
- temperature
- enthalpy
3 characteristics of ∆H for reactions
- if ∆H is negative, heat is released (exothermic)
- if ∆H is positive, heat is absorbed (endothermic)
- if a reaction is reversed, ∆H is reversed
- magnitude of ∆H is directly proportional to the amount of reactant and products in a reaction (if all of the moles get multiplied by 3, have to multiply the ∆H by 3 too)
∆H equation (moles & energy)
∆H = amount of energy released/amount of moles
∆H = q/n
standard enthalpy of combustion
∆Hcº
enthalpy change when 1 mole of substance combusts (under standard conditions - 1 atm, 25ºC)
standard enthalpy of formation and formula
**standard enthalpy of formation (∆Hfº) = enthalpy change from the formation of 1 mole of compound from its elements (specifically from its element that exist in nature)
formula = ∑∆Hº(products) - ∑∆Hº (reactants)
Hess’s Law
goal: get the equations to look like the given one by rearranging so they cancel out on both sides
- reverse reactions as needed (switch the sign of ∆H)
- multiply reactions by coefficients to give the correct numbers of reactants and products (∆H value is multiplied by the same integer)
what are STP values?
pressure = 1 atm
temperature = 0 C
how to calculate ΔH° using bond energies
calculated by bonds broken (the reactants) minus bonds formed (the products)
only works for gases
when do you use q=mCΔT and when do you use q=cΔT?
mcΔT is for when you have specific heat capacity and CΔT is for when you have just regular heat capacity
when a metal is being added to water, what equation do you usually use and what do you need to be careful of?
you use -mcΔT = mCΔT where metal is losing heat if the water temp is increasing
be careful with the signs here
what temperature unit is used for gases ALWAYS?
Kelvin !!! have to convert from C
relationship between P and T
direct (if they’re on opposite numerator and denominator type from PV=nRT then they’re directly related)
1 mole of ideal gas is how much L at STP
22.4 L
equation for ideal gas law with density
P (molar mass M) = dRT
mole fraction and partial pressure
they’re equivalent and you can use them interchangeably for some reason
