chapter 6 Flashcards
what is kinetic energy?
associated
with motion
what is potential energy?
associated with the position
what is thermal energy?
associated with the
temperature
what is chemical energy?
associated with the position
of electrons and nuclei in
atoms and molecules
what is the law of conservation of energy? 3 rules?
Energy can be neither created nor destroyed
The energy of the universe is constant
- Energy can be transferred between objects
- Energy can be converted between different forms
- When energy is transferred, it appears as work or heat
how do we track change in energy?
System: The part of the universe we are examining
Surrounding: Everything with which the system can
exchange energy
what is the first law of thermodynamics?
The total energy of the universe is constant
what is internal energy?
Internal Energy (U) - sum of kinetic and potential energy of all particles of the system The internal energy of this system is all of the kinetic energy from the molecules moving, plus the potential energy in the bonds
what are state functions?
value that depends only on the state
of the system, not how it arrived at that state
• Since state functions only depend only on the state of
the function, the value of change in a state function is
always the difference between the final and initial values
• The state is specified by parameters such as
temperature, pressure, concentration, physical state
state functions formula
🔺rU= Ufinal - U initial 🔺rU= U products - Ufinal
what is an example of a state function?
altitude
change in altitude depends only on difference between initial and final values, not the path taken
what is the formula for the first law of thermodynamics?
law of conservation ΔUuniverse= ΔUsystem + ΔUsurroundings = 0
Therefore, a change in the internal energy of system must
be balanced by an equal and opposite change in the
energy of the surroundings
ΔUsystem=-ΔUsurroundings
what happens when initial internal is higher than final?
When the internal energy of the initial state is higher than the final state, energy is transferred to the surroundings Ufinal < Uinitial so ΔUsys<0
what happens when final internal is higher than initial?
When the internal energy of the final state is higher than the initial state, energy is absorbed from the surroundings Ufinal > Uinitial so ΔUsys>0
how is energy transferred between the system and surroundings?
• Energy is transferred between the system and its
surroundings through heat (q) and/or work (w)
ΔU = q + w
q is heat
what is energy transfer caused by?
üExchange of thermal energy between system and
surroundings
üCaused by a temperature difference between the
two
üHeat transfer occurs until thermal equilibrium is
reached
what is heat capacity? formula?
C, heat capacity - quantity of heat required to change
its temperature by 1℃, units of J/℃
• Is an extensive property - depends on the amount of
matter that is being heated
q = C x ΔT
what is the specific heat capacity?
• Cs, specific heat capacity - quantity of heat required to
change 1g of the substance by 1℃, units J/g℃
• can also be referred to as molar heat capacity, J/mol℃
• Intensive property
q = C x ΔT
q = m x Cs x ΔT
what is the specific heat capacity?
• Cs, specific heat capacity - quantity of heat required to
change 1g of the substance by 1℃, units J/g℃
• can also be referred to as molar heat capacity, J/mol℃
• Intensive property
q = C x ΔT
q = m x Cs x ΔT
how do we convert between heat capacities?
To convert between specific heat capacity and molar heat
capacity, we multiply or divide by the molar mass of the
substance
what can we calculate the energy transferred for?
We can calculate the energy transferred from one object
to another through heat/temperature transfer in an
isolated system
• Eg. A hot substance (metal), put into a beaker of water
at a lower temperature
• Water will absorb the heat from the metal until thermal
equilibrium is reached
-qmetal = qwater
-mmetal x Cs,metal x ΔTmetal = mwater x Cs,water x ΔTwater
describe work done on a system.
Work done on a system -Increasing the pressure of the surroundings, moves the piston in -Energy is transferred from the surroundings to the system as work done by the surroundings on the system ΔUsys is positive
describe work done by a system.
-The formation and expansion of the gas causes the piston cylinder to move -Energy transferred as work done by the system on the surroundings
how do we calculate work?
Just as we can calculate heat with an observed
temperature change, we can calculate the amount of
work associated with a volume change
• Work - force acting through a distance
• Work - force is caused by a volume change against an
external pressure
F = P x A
w = P x A x Δh
w = F x d ➞
➞ w = P x ΔV
➞ w = -PΔV
sign conventions for q (heat)
+ system gains thermal energy
- system loses thermal energy
sign conventions for w (work)
+ work done on a system
- work done by a system
sign conventions for ΔU (change in internal
energy
+ energy flows into the system
- energy flows out the system
how can work be calculated for PV work?
• If we only focus on P-V work, the work done by a
chemical reaction can be calculated when gases are
involved
w = -ΔnRT
w = -PΔV & PV = nRT
what does internal energy measure?
During a chemical reaction the internal energy (ΔU) is a
measure of all of the energy, heat and work
how do we calculate internal energy?
If we can calculate the change in temperature and
volume that occurs then we can calculate the ΔU
q = C x ΔT w = -PΔV = 0
• If the reaction takes place at constant volume, then all of
the energy change is associated with a reaction is
evolved as heat
what is bomb calorimetry? formula?
• Bomb calorimetry - known mass of a substance is
burned in a container with constant volume
qcal = -qr
what is enthalpy? formula? is it a state function?
When a reaction occurs open to the atmosphere, energy
can evolve as heat and work
• For many reactions we are more concerned with the
heat given off and not the work
• We define enthalpy as the sum of the internal energy
and the product of pressure and volume
ΔH = ΔU + PΔV
• Internal energy, pressure, and volume are all state
functions, so therefore, enthalpy is also a state function
relationship between ΔH and ΔU formulas
Most reactions involve very little P-V work ΔH = ΔU + PΔV ΔH = ΔU + ΔPV ΔH = ΔU + Δ(nRT) ΔH = ΔU + ΔngRT
relationship between H and U for liquid and solid phase reactions
For liquid and solid phase reactions volume changes only
a little ΔH ≃ ΔU
relationship between U and H for gas phases based on mols
When the moles of gas are the same for a gas-phase
reaction ΔH = ΔU
• When the moles of a gas phase reaction change, PΔV ≠ 0,
but q is usually much bigger than PΔV so ΔH ≃ ΔU
enthalpy formula
ΔH = ΔU + PΔV Remember: ΔU = q + w and PΔV = -w Therefore: ΔH = ΔU + PΔV ΔH = (qp + w) + PΔV = qp + w - w ΔH = qp The sign on ΔH tells you the flow of energy either to or from the system
where does energy come from/
If a reaction gives off heat as thermal energy does it come
from the thermal energy of the reactants? No
üThermal energy comes from the kinetic energy of the
system
üExothermic reactions get warmer, endothermic reactions
get colder
describe exothermic reactions
üIn exothermic reactions if the heat released to the
surroundings came from thermal energy of the reactants
the system would get colder!
üExcess thermal energy is generated from the potential
energy of the system - energy mostly from the
electrostatic forces between protons and electrons that
compose atoms and molecules
what does ΔrH represent? depends on?
Can represent enthalpy of reaction as ΔrH
• It is an extensive property, and therefore depends on the
amounts of reactants that react
• For non-stoichiometric amounts you need to adjust the
heat released/absorbed accordingly
what is coffee cup calorimetry?
• Coffee cup-calorimetry is open to the atmosphere
• A known mass of solid (system) is added to a known
mass of liquid (surroundings)
• Or two volumes of solutions are mixed together - where
the dissolved species reacting is the system and the
aqueous water is the surroundings
• Any change in temperature of the surroundings is
measured and we can relate the heat change of solution
to the heat change of the solid/reactants
qsoln = msoln x Cs,soln x ΔT = -qrxn
relationships with ΔrH
- When a chemical reaction is multiplied by a factor, ΔrH
is multiplied by the same factor: - If a chemical reaction can be expressed as a sum of
series of steps, then ΔrH for the overall reaction is the
sums of the steps
A + B ➞ C + D ΔrH
2A + 2B ➞ 2C + 2D 2ΔrH - When a chemical reaction is reversed, the sign of ΔrH is
reversed:
A + B ➞ C + D ΔrH
C + D ➞ A + B -ΔrH
what does Hess’ law state?
Hess’s Law states that if a chemical equation can be
expressed as the sum of a series of steps, then ∆rH for the
overall equation is the sum of the ∆rH for each step
why are there standard states?
Thermodynamic variables, such as ∆H, vary somewhat with
conditions
• Therefore, a set of specific conditions called standard
states were established
ΔHº - º sign indicates values were measured with
chemicals in standard states
3 points of standard states
- For a gas, the standard state is 1 bar and ideal behavior
- For a substance in aqueous solution, the standard state is 1M
concentration - For a pure substance (element or compound), the standard state
is the most stable form of the substance at 1 bar and the
temperature of interest (usually 25°C)
how do we determine ΔrH from standard enthalpies
of formation?
• Just as there are ΔrH for reactions, there is also an
enthalpy associated with the formation of individual
compounds, ΔfH
• Standard enthalpies of formation are derived from
formation equations
ü1 mole of a substance is formed from its elements
üFractional coefficients are often used with reactants to
obtain 1 mole of products
C(s) + 2H2(g) ➞ CH4(g) ΔHfº = -74.9 kJ
Na(s) + 1/2Cl2(g) → NaCl(s)
2C(s) + 3H2 + 1/2O2 → C2H5OH(l)
∆!!
°
= -411.1 kJ
∆!!
°
= -277.6 kJ
standard states
For an element in its standard state, =0
üThe standard state for molecular elements (e.g: Cl2, H2,
O2) is the molecular form, not for single atoms
• Most compounds have a negative ∆!
how do we Determining ∆H°
rxn from ∆H° f
• We can use ∆H° f values to determine the ∆H°rxn for any reaction • Imagine the reaction occurring in two steps: 1.reactants decompose into their elements 2.Each product forms from its elements Decomposition of reactants to elements is the reverse of the formation reaction, so the standard enthalpy change is -∆H
hess’ law
formula for Determining ∆H°
rxn from ∆H°f (Hess’ law)
ΔH° reaction = Σ n ΔHf°(products) − Σ n ΔHf°(reactants) Determining ∆H° rxn from ∆H° f Σ is the symbol for sum n is the number of moles of component (from balanced chemical reaction
what is bond energy?
• Bond energy is the amount of energy it takes to break
1 mol of bonds in the gas phase
• Bond energies are all positive, indicating you must put
energy into a bond to break it
• Larger bond energies reflect stronger chemical bonds
N-N triple bond 946 kJ/mol
O-O double bond 498 kJ/mol
C-H single bond 414 kJ/mol
how can bonds vary?
Not all bonds are created equally
• The bond energy of a C-H bond will vary depending on
what else is bonded to the C
üCH4 438 kJ/mol
üCHF3 446 kJ/mol
üCHCl3 401 kJ/mol
üCHBr3 402 kJ/mol
• Reported bond energies are averages of all of the
specific bond energies for a large number of compounds
how do we find delta H from bond energies?
For simple reactions we can estimate enthalpies using
the bond energies taking into account the number and
type of bonds broken and the number and type of bonds
formed
∆ rH = Σ (ΔH bonds broken) − Σ(ΔH bonds formed)
