Thermochemistry Flashcards
Define open, closed and isolated systems
OPEN - material and energy exchange (ex open beaker)
CLOSED - only energy exchange (ex. closed erlenmeyer flask)
ISOLATED - neither material nor energy exchange (ex. thermos)
What is the first law of thermodynamics? Equations for this law?
Law of conservation of energy - E can't be created or destroyed. (E total (U) = E potential + E kinetic) ΔUsystem = -ΔU surroundings ΔU=q+w q= heat, w = work
Define energy
the capacity tp do work (w) or supply heat (q)
Does thermal energy = temperature?
NO
E thermal is the type of E kinetic associated with random molecular. It is not T.
What is an exothermic reaction? Different types of this reaction?
ΔH < 0
reaction RELEASES HEAT
Rapid combustion - produces heat/+light in the form of glow/flames
Slow combustion - low temperatures, produces CO2 and H2O
What makes a fuel more efficient/better?
A higher heat of combustion
What is an endothermic reaction?
ΔH > 0
reaction REQUIRES heat
ex. photosynthesis
if system does w on surroundings, what is the value of w?
NEGATIVE because work is being done by system on surroundings. System loses energy.
We focus on the system, not the surroundings
what is a state function?
values that DON’T depend on path taken to get there.
Ex. ΔU and ΔH
what is a path function?
values that DO depend on path taken to get there.
Ex. q and w
Energy changes in a system can happen through ___ or ___
work or heat
What is q?
HEAT
the quantity of energy exchanged between a system and its surroundings as a result of temperature difference.
q system = q rxn
What happens when q rxn is neg, pos or =0?
Negative –> EXOTHERMIC
Positive –> ENDOTHERMIC
= 0 –> ADIABATIC (no heat produced/required)
What is w? Equations for w?
PRESSURE-VOLUME WORK aka WORK
- form of energy transferred between system and surroundings when a reaction results in a change of # moles of gas
- reaction needs to happen in a vessel where container can change volume
w = -Pexternal x ΔV
w = -ΔnRT
Kpa, L
What happens when w is neg, pos or =0?
Negative –> gas expands/work done on surroundings
Positive –> gas contracts/work done on system
= 0 –> no work done
Define specific heat capacity (c)
Quantity of heat needed to change the temperature of
1 GRAM of PURE SUBSTANCE by 1°C
Define molar heat capacity (cm)
Quantity of heat needed to change the temperature of
1 MOLE of PURE SUBSTANCE by 1°C
Define heat capacity (C)
Quantity of heat needed to change the temperature of
a SYSTEM of by 1°C
Equation for q = heat capacity
c: q = m x c x ΔT
cm: q = n x cm x ΔT
C: q = C x ΔT
what is bomb calorimetry used for? Equations?
- CONSTANT VOLUME experiment
- system isolated from surroundings
qcalorim = heat capacity of calorimeter x ΔT
qrxn = -qcalorim
- because E can only go/to from inside the system itself
ΔU = qv
what is coffee calorimetry used for? Equations
- CONSTANT PRESSURE experiment
- reaction takes place in a thermally isolated system. Open to air, keeping system at constant pressure.
ΔU = qp + w ΔU = qp - PΔV
relationship between ΔU and ΔH
ΔH = ΔU + Δn(gasses)RT
Definition of enthalpy
H = U + PV
- internal E of a system plus the product of its pressure and volume (if gas), which accounts for the E required of a gas to occupy whatever space it takes up
Does qp=qv?
YES unless a reaction results in a change in the # moles of gas
What happens to temperature when a substance is changing states?
Temperature remains CONSTANT because thermal energy is being transferred
Define molar enthalpy of fusion and vaporization
quantity of heat required to melt (fusion) or vaporize 1 mol of substance.
ΔH for when temperature increases POSITIVE
(fusion, vaporization)
ΔH for when temperature decreases NEGATIVE
(freezing, condensation)
Each phase of a substance has a ____ heat capacity
UNIQUE
q calculation for phase change
q = ΔH x n
ΔHrxn approximated equation
Average bond energies can be used to find aprx ΔH
ΔHrxn = ΔH(bond breaking) + ΔH(bond forming) ΔHrxn = ΣBE(reactants) - ΣBE(products) BE = bond energy
What is the standard state of an element?
- how an element exists at 1atm and 25°C
- HOFBrINCl are diatomic gasses
- Br and Hg are liquid
- all other elements are solid
- C = graphite
- S = S8
- P = P4
Define standard enthalpy of formation (ΔH°f)
ΔH that occurs in the formation of 1 mol of substance from its individual elements in their standard states
ΔHrxn accurate equation
ΔH°f can be used to find ΔH
ΔH°rxn = ΣnΔH°f (products) - ΣnΔH°f (reactants) n = stoichiometric coefficients from balanced equation
Define Hess’s Law
H of a reaction can be calculated from multiple chemical processes who’s H’s are known
Rules for manipulating thermochemistry equations (to use Hess’s Law)
- Revering a reaction = reverse sign ΔH
- Summing reactions = sum ΔH values
- Multiplying a reaction by a factor = multiple ΔH by same factor
- Include H of any phase change
