exam 3 Flashcards
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)
