Chapter 6 Flashcards
thermochemistry
describes the heat energy changes occurring during chemical reactions and physical transformations
potential and kinetic energy analogy
ball on ledge has potential energy (not kinetic)
chemical energy
a form of potential energy associated with positions of electrons and nuclei in a system
chemical energy can be found in what?
atoms
molecules
ionic compounds
condensed phases
system
the area you are studying
surroundings
everything else outside the system
what are the 3 types of systems?
open system
closed system
isolated system
what is an example of an open, closed and isolated system?
open system: open pot of boiling water
closed system: closed pot of water (no volume change)
isolated system: a thermos bottle with the lid screwed on tight
what is the law of conservation of energy/ the first law of thermodynamics?
energy can neither be created nor destroyed; rater, it can only be transformed or transferred from one object to another
what is the equation for internal energy?
ΔU = U final - U initial
what does Δ mean?
change in
what is the equation for change in internal energy of the universe?
ΔU = ΔU system + ΔU surr
surr= surroundings
according to the law of conservation of the energy what does ΔU universe equal?
0
what does ΔU sys equal?
ΔU sys = -ΔU surr or. -ΔU sys = ΔU surr
always has opposite signs because one is losing and one is gaining
energy lost by the system exactly equals energy gained by the surroundings
you pay tuition for university, the university receives money, what are the signs for each?
pay tuition (-)
university gains $ (+)
-ΔU sys = ΔU surr
heat
written as q
the flow of energy caused by a temperature difference (from hot object to cold)
work
written as w
the energy required to move something against a force
a force acting through a distance
ex. when you push a chair its work and therefore, would be (-)
internal energy change of the system is the sum of the heat transferred and the work done, how is this equation written?
ΔU = q + w
absolute temperature (T)
is different for the celsius and kelvin scales
0°C = 273.15 K
difference in temperature
ΔT is teh same for the celsius and kelvin scales
an increase of 10°C = an increase of 10 K
115.0 J of heat and 77.0 kJ of work, what is internal energy in KJ?
-76.9kJ
*slide 21 lecture 2-3
how do you calculate temperature?
ΔT = T final - T initial
what does q depend on?
the identity of material (ability to absorb heat)
heat capacity
written as C
quantity of heat required to change the temperature of the system by 1°C or 1K
(depends on mass of the heated object)
what is one equation q= ?
q = C x ΔT
specific heat capacity?
written as Cs
the amount of heat required to raise the temperature of 1 gram of the substance by 1°C or 1 K
what can K also be written as?
J g^-1 °C^-1
what is another equation q = ?
q = m x Cs x ΔT
molar heat capacity
written as Cn
per 1 mol (J mol^-1 °C^-1)
what is another equation q = ?
q = n x Cn x ΔT
what is the heat capacity of water?
4.184
calculate the energy required to increase the temperature of 2.0kg of water from 20.0°C to 100.0°C
Cs H20 = 4.184 J g^-1 °C^-1
6.7 x 10^5 J
*slide 26 lecture 2-3
what does heat lost by one substance exactly equal?
the heat gained by the other
qsys = -qsurr
what does the heat of a system = ?
qsys = -qsurr
what does the heat of a system = ?
-qmetal = qwater. or qmetal = -qwater
what is the overall equation for thermal energy transfer?
-𝓂metal x Cmetal x ΔTmetal = 𝓂H2O x CH20 x ΔTH20
or
𝓂metal x Cmetal x ΔTmetal = -𝓂H2O x CH20 x ΔTH20
a 65.0 g piece of iron at 525.0°C is put into 635.0 grams of water at 15.0°C. what is the final temperature of the water and the iron?
Cs H20= 4.184 J g^1 °C^-1
Cs Fe= 0.449 J g^1 °C^-1
answer is 20.76 or 20.5°C
*slide 29 and 30
can gases do work through expansion or compression against a constant external pressure?
yes!
work done by gases is also sometimes called what?
pressure volume or PV work
what does w = ?
w = -P x ΔV
what is ΔV?
ΔV = V final - V initial
what is P?
pressure with units of bar
100J = 1 barL
if 2.0L of gas at a pressure of 1.0 bar expand to a volume of 3.0L, determine w in units of J
-1.0 x 10^2 J
*slide 38 lecture 4
what is the ideal gas law?
PV = nRT
what is R?
ideal gas constant
R= 0.08314 L bar mol^-1 L^-1
or
R= 8.314 J K^-1 mol^-1
for work done by the expansion or contraction of gases what formula do we use?
w = -Δn gas RT
what is a combustion reaction?
substance combines with oxygen
creates heat
the complete combustion of candle wax (C31H64) in oxygen produces CO2 and liquid water. calculate the work associated with this combustion reaction at 298K in kJ mol^-1
39.64 KJ/mol
*slide 11 lecture 4
for chemical reaction the change in internal energy notation?
ΔrU
what are the units for the change in internal energy
kJ/mol
what is the final equation for ΔrU
ΔrU = U products - U reactants
state function
value depends only on the current state of the system and does not depend on how the system arrived at that state
how do you identify state function?
determine whether the path taken to reach the function affects the value
*slide 15 lecture 4
is internal energy a state function?
yes!
what is ΔU dependent on?
U final and U initial
ΔU = U final - U initial
ΔrU = how many kJ/mol
-295 kJ/mol (negative)
or
293 kJ/mol (positive)
why might ΔrU be negative?
if (-) value, the energy of the reactants is higher because products -reactants
ex. (50-60) = -10
why might ΔrU be positive?
when (+) products are larger than reactants
what if we force all of the energy associated with chemical reaction to manifest itself as heat rather than work? what equation will we get?
ΔU = q - PΔV
at a constant volume (closed system), what does ΔV and PΔV equal?
ΔV = 0
PΔV = 0
constant volume calorimeter
is sealed air-tight, not allowing the system to expand or contract
what is the equation to find the heat of a calorimeter? when there is no work done
qcal = Ccal x ΔT
what sign is qcal always going to be?
positive
what is qr =
qr = ΔU
ΔU is an extensive value in this equation
how do you convert the equation qr = ΔU to an intensive value?
divide by the number of moles of reactant
qr/ nr = ΔrU
calculate the internal energy change ΔrU, for the combustion of 29.3 g of vitamin C (C6H8O6, molar mass = 176.124 g mol^-1) if the combustion inside a bomb calorimeter, Ccal = 8.31 kJ °C^-1, causes a temperature change from 21.5°C to 68.3°C.
-2.34 x 10^3 kJ/mol
*slide 15 lecture 5-6
in an open system under conditions of constant pressure what can the energy evolve as?
heat and work
what is enthalpy?
parameter that relates the energy flow into or out of a system at constant pressure
what equation do we use to find enthalpy?
ΔH = ΔU + PΔV
ΔH = (q + w) + PΔV = (q + w) - w
most chemical processes in our world occur at constant ________
pressure
so enthalpy is a direct measure of the heat exchange
ΔH = qp
the change in enthalpy for a system is defined as the heat gained or lost by the system at…
constant pressure
what is an example of endothermic reaction?
cold pack on hot skin
absorbs energy from the surroundings
what is an example of exothermic reaction?
lighting a candle
it releases energy to the surroundings
in chemical reaction some bonds are broken, new ones are formed, nuclei and electrons reorganize to form the product with lower potential energy, is this endothermic or exothermic?
exothermic
higher potential energy is endothermic
chemical potential energy
the source of energy given off in an exothermic chemical reaction
what happens when weaker bonds break?
stronger bonds form
from an exothermic molecular view, what reorganizes?
electrons
molecules
what does potential energy convert to in exothermic reactions?
thermal energy
reaction gives off thermal energy
when gas isnt involved in a reaction, what happens to work?
no work is done
energy transfer for any chemical reaction
the enthalpy of reaction or heat of reaction
for a chemical reaction the change in enthalpy is what?
ΔrH
indicates the change in enthalpy per mol of reaction
how do you calculate the mole ratio?
coefficient of desired quantity/ coefficient of given quantity
N2 (g) + 3H2 (g) –> 2NH3 (g)
ΔrH = -91.8 kJ/mol
how much energy is given off when 222.4 g of N2 reacts?
-729kJ
*slide 35 lecture 5-6
Where does the energy lost by the reactants (as they transform to products) go?
ΔrU = U products - U reactants
If we define the thermodynamic system as the reactants and products of the reaction, then energy flows out of the system and into the surroundings.
why can kinetic energy not be the source of the energy given off in an exothermic reaction?
kinetic energy cannot be the source of the energy given off in an exothermic reaction because, if the atoms and molecules that compose the system were to lose kinetic energy, their temperature would necessarily fall—the system would get colder
under normal circumstances how does chemical potential energy arise?
primarily from the electrostatic forces between the protons and electrons that compose the atoms and molecules within the system.
how do you find enthalpy of a solution (q soln)?
q soln = m soln * C s,soln * ΔT
Hess’s Law
the enthalpy change of the reaction carried out in multiple steps is equal to the sum of the enthalpy changes for each individual step
(enthalpy change is a state function does not depend on the path taken)
what are the three principal phases in the standard state?
gases, solids/ liquids, solutions
what is the standard state of a pure chemical substance?
25°C and 1 bar
how many M does a solution have?
solution has 1M of concentration
what is the difference between ΔrU and ΔrH?
As we saw earlier for and , the difference between and is that the latter is a molar quantity and has units of , meaning an amount of energy per mol of reaction. has units of kJ
what liquids are pure substances?
Br2 and Hg
what gases are pure substances?
all noble gases, halogens, and Cl2 H2, O2, N2
other than the substances previously listed, what state are all the other elements in?
solids
what are the 7 diatomic elements?
have no fear of ice cold beer
hydrogen
nitrogen
fluorine
oxygen
iodine
chlorine
bromine
what is standard reaction enthalpy?
enthalpy change for a reaction in which all reactants and products are in their standard states
