Thermal Chemistry Flashcards
exothermic
releasing heat
- surroundings heat up
- negative heat charge
endothermic
absorbs heat
- surroundings cool down
- positive heat charge
flat areas
melting/boiling
rising areas
heating up H2O
heat
always added, related to temperature and phase change
q=mcp∆T
heating liquid
q in q=mcp∆T
thermal energy (joules)
m in q=mcp∆T
mass (grams)
cp in q=mcp∆T
specific heat
- a constant hat varies from substance to substance
- joules per grams-kelvin (J/g˚C)
- the amount of energy required to raise 1g of a substance by 1 degrees celsius
∆T in q=mcp∆T
change in temperature (kelvin or celsius)
specific heat for identifying unknowns
- putting an unknown object in water allows heat to flow between the unknown and the water
- thermal equilibrium is reached when temperature becomes the same
(Tf) same for both water & object
- amount of heat gained/lost by the water is equal to the amount of heat lost/gained by unknown
calorimetry
measurement of heat changes for physical and chemical processes
calorimeter
measure the amount of heat absorbed or released
Q=n*∆H fus
n = number of particles in moles
∆H fus = heat of fusion -> solid to liquid, positive, heat in
Q=n*∆H vap
n = number of particles in moles
∆H vap= heat of vaporation -> liquid to gas, positive, heat in
∆H values are given in either
Joules per mol (J/mol) or kilojoules per mole (kilo/Joules)
when a substance changes phase
there Is a heat value of the change in state for each substance
when you go up the heating curve (solid -> liquid -> gas)
endothermic, heat is positive
when you go down the heating curve (gas -> liquid -> solid)
exothermic, heat is negative
fus and solid
∆H fus = - ∆H solid
vap and cond
∆H vap = - ∆H cond
∆H fus
s->l
melting
-∆H solid
l->s
freezing
∆H vap
l->g
boiling
-∆H cond
g->l
condensation
flat part equation
q=mol*∆Hfus
or
q=mol*∆Hvap
CpH2O(l)
4.18 J/g˚C
energy
ability to do work (measured in Joules)
potential energy
stored energy
kinetic energy
energy of motion
heat vs temp
temperature is the measure of the average kinetic
- energy of random motions of particles in a substance (measured in degrees)
- heat is the measure of the total amount of energy (measured in joules or calories)
temperature change
T∆ = Tf - Ti
∆T sign
- positive if temp is increasing
- negative if temp is decreasing
heating curve states
solid only -> melting (solid/liquid) -> liquid only -> boiling (liquid/gas) -> gas only
temperature change
solid only, liquid only, gas only
phase change
melting/boiling
heat of fusion
is the energy needed to melt one mole
∆Hfus occurs at melting/freezing point
- temperature stays constant!!!!
Q=(∆Hfus)(#moles)
constant temperature (heat of fusion)
- solid -> liquid ∆Hfus is positive (heat in
- liquid -> solid ∆Hfus is negative (heat out)
heat of vaporization
is the energy needed to boil one mole
occurs at the boiling/condensing point
Q=(∆Hvap)(#moles)
constant temperature (heat of vaporization)
- liquid -> gas ∆vap is positive (heat in)
- gas -> liquid ∆vap is negative (heat out)
heat of reaction
(∆Hrxn) (or enthalpy): heat energy absorbed or released during a reaction
heat of formation
(∆Hf) heat energy absorbed or released, in the formation of 1 mole of a compound from its elements
heat of solution
(∆Hsol): heat energy absorbed or released when a substance dissolves
activation energy
Ea, energy needed to start a reaction
activated complex
the unstable arrangement of atoms that exists momentarily at the peak of the activation energy
energy diagram
- starts with reactants
- goes up & down in curve
- ends with products either higher or lower than before
- diff in reactants and products is ∆H
exothermic energy diagram
- reactants higher than products
- reactants are less stable than products (higher energy case)
- products are more stable than products (lower energy case)
∆H rxn = product - reactants = negative
endothermic energy diagram
- products higher than reactants
- reactants are more stable (lower energy state)
- products are less stable (higher energy state)
∆H rxn = product - reactants = positive
cataylsts
speed up reactions by lowering the activation energy
enthalpy
total energy content of a system
enthalpy change
∆H
the heat energy absorbed or released in an rxn
hess’s law
the total enthalpy change in a reaction
∆Hrxn = sum of Hfproducts - sum of ∆freactants
enthalpy changes
1) determine ∆Hrxn for the decomposition of H2O
2) make sure equation is balanced (account 4 moles of each reactant & products)
3) look up ∆Hf for reactants and products on chart (elements are 0!)
4) solve for ∆Hrxn
