Thermochemistry

Question 1

Define open, closed and isolated systems

Answer 1

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

Question 2

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

Answer 2

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

Question 3

Define energy

Answer 3

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

Question 4

Does thermal energy = temperature?

Answer 4

NO

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

Question 5

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

Answer 5

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

Question 6

What makes a fuel more efficient/better?

Answer 6

A higher heat of combustion

Question 7

What is an endothermic reaction?

Answer 7

ΔH > 0
reaction REQUIRES heat
ex. photosynthesis

Question 8

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

Answer 8

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

Question 9

what is a state function?

Answer 9

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

Ex. ΔU and ΔH

Question 10

what is a path function?

Answer 10

values that DO depend on path taken to get there.

Ex. q and w

Question 11

Energy changes in a system can happen through ___ or ___

Answer 11

work or heat

Question 12

What is q?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

What is w? Equations for w?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

Define specific heat capacity (c)

Answer 16

Quantity of heat needed to change the temperature of

1 GRAM of PURE SUBSTANCE by 1°C

Question 17

Define molar heat capacity (cm)

Answer 17

Quantity of heat needed to change the temperature of

1 MOLE of PURE SUBSTANCE by 1°C

Question 18

Define heat capacity (C)

Answer 18

Quantity of heat needed to change the temperature of

a SYSTEM of by 1°C

Question 19

Equation for q = heat capacity

Answer 19

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

Question 20

what is bomb calorimetry used for? Equations?

Answer 20

• 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

Question 21

what is coffee calorimetry used for? Equations

Answer 21

• 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

Question 22

relationship between ΔU and ΔH

Answer 22

ΔH = ΔU + Δn(gasses)RT

Question 23

Definition of enthalpy

Answer 23

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

Question 24

Does qp=qv?

Answer 24

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

Question 25

What happens to temperature when a substance is changing states?

Answer 25

Temperature remains CONSTANT because thermal energy is being transferred

Question 26

Define molar enthalpy of fusion and vaporization

Answer 26

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)

Question 27

Each phase of a substance has a ____ heat capacity

Answer 27

UNIQUE

Question 28

q calculation for phase change

Answer 28

q = ΔH x n

Question 29

ΔHrxn approximated equation

Answer 29

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

Question 30

What is the standard state of an element?

Answer 30

• 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

Question 31

Define standard enthalpy of formation (ΔH°f)

Answer 31

ΔH that occurs in the formation of 1 mol of substance from its individual elements in their standard states

Question 32

ΔHrxn accurate equation

Answer 32

ΔH°f can be used to find ΔH

ΔH°rxn = ΣnΔH°f (products) - ΣnΔH°f (reactants)
n = stoichiometric coefficients from balanced equation

Question 33

Define Hess’s Law

Answer 33

H of a reaction can be calculated from multiple chemical processes who’s H’s are known

Question 34

Rules for manipulating thermochemistry equations (to use Hess’s Law)

Answer 34

• 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