Thermochemistry Flashcards

1
Q

Define open, closed and isolated systems

A

OPEN - material and energy exchange (ex open beaker)
CLOSED - only energy exchange (ex. closed erlenmeyer flask)
ISOLATED - neither material nor energy exchange (ex. thermos)

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2
Q

What is the first law of thermodynamics? Equations for this law?

A
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
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3
Q

Define energy

A

the capacity tp do work (w) or supply heat (q)

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4
Q

Does thermal energy = temperature?

A

NO

E thermal is the type of E kinetic associated with random molecular. It is not T.

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5
Q

What is an exothermic reaction? Different types of this reaction?

A

ΔH < 0
reaction RELEASES HEAT
Rapid combustion - produces heat/+light in the form of glow/flames
Slow combustion - low temperatures, produces CO2 and H2O

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6
Q

What makes a fuel more efficient/better?

A

A higher heat of combustion

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7
Q

What is an endothermic reaction?

A

ΔH > 0
reaction REQUIRES heat
ex. photosynthesis

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8
Q

if system does w on surroundings, what is the value of w?

A

NEGATIVE because work is being done by system on surroundings. System loses energy.
We focus on the system, not the surroundings

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9
Q

what is a state function?

A

values that DON’T depend on path taken to get there.

Ex. ΔU and ΔH

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10
Q

what is a path function?

A

values that DO depend on path taken to get there.

Ex. q and w

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11
Q

Energy changes in a system can happen through ___ or ___

A

work or heat

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12
Q

What is q?

A

HEAT
the quantity of energy exchanged between a system and its surroundings as a result of temperature difference.
q system = q rxn

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13
Q

What happens when q rxn is neg, pos or =0?

A

Negative –> EXOTHERMIC
Positive –> ENDOTHERMIC
= 0 –> ADIABATIC (no heat produced/required)

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14
Q

What is w? Equations for w?

A

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

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15
Q

What happens when w is neg, pos or =0?

A

Negative –> gas expands/work done on surroundings
Positive –> gas contracts/work done on system
= 0 –> no work done

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16
Q

Define specific heat capacity (c)

A

Quantity of heat needed to change the temperature of

1 GRAM of PURE SUBSTANCE by 1°C

17
Q

Define molar heat capacity (cm)

A

Quantity of heat needed to change the temperature of

1 MOLE of PURE SUBSTANCE by 1°C

18
Q

Define heat capacity (C)

A

Quantity of heat needed to change the temperature of

a SYSTEM of by 1°C

19
Q

Equation for q = heat capacity

A

c: q = m x c x ΔT
cm: q = n x cm x ΔT
C: q = C x ΔT

20
Q

what is bomb calorimetry used for? Equations?

A
  • 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

21
Q

what is coffee calorimetry used for? Equations

A
  • 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
22
Q

relationship between ΔU and ΔH

A

ΔH = ΔU + Δn(gasses)RT

23
Q

Definition of enthalpy

A

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

24
Q

Does qp=qv?

A

YES unless a reaction results in a change in the # moles of gas

25
What happens to temperature when a substance is changing states?
Temperature remains CONSTANT because thermal energy is being transferred
26
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)
27
Each phase of a substance has a ____ heat capacity
UNIQUE
28
q calculation for phase change
q = ΔH x n
29
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 ```
30
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
31
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
32
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 ```
33
Define Hess's Law
H of a reaction can be calculated from multiple chemical processes who's H's are known
34
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