Unit 5

Question 1

Heat capacity (C)

Answer 1

-the amount of heat required to raise the temperature of an object by 1 K, either at constant pressure (Cp) or constant volume (Cv)
- q=CΔT
-depends on the quantity of substance that is being heated

Question 2

Cv

Answer 2

-heat capacity at a constant volume
- qp = nCpΔT

Question 3

Cp

Answer 3

-heat capacity at a constant pressure
- qv = nCvΔT
-for any system, Cp is greater than Cv

Question 4

qp

Answer 4

• heat transferred at a constant pressure
• qv = nCvΔT

Question 5

qv

Answer 5

• heat transferred at a constant volume
• qv = nCvΔT

Question 6

Molar heat capacities Cp and Cv

Answer 6

-the system heat capacities Cp and Cv divided by the number of moles of substance in the system
-molar Cp/Cv is amount of heat required to increase the temp of 1 mol of substance by 1 K at constant pressure/volume

Question 7

Specific heat capacity

Answer 7

-the system heat capacity per gram of substance (at constant P or V)
- Q = mCΔT

Question 8

Heat capacity importance

Answer 8

-place two objects initially at different temperatures into contact, energy in the form is exchanged between them until they reach a common temperature
- amount of heat taken up by cooler object (q2) is equal to heat given up by hotter (-q1)

Question 9

Bomb calorimeter (constant volume)

Answer 9

-involves a small closed container (bomb) placed inside a calorimeter, which consists of a box with insulated walls and full of water
-sparker initiates combustion reaction within bomb between sample and O2, causing heat to either be taken up or given off by reaction
-ΔU = qv (heat of reaction under constant volume, w = 0 )

Question 10

heat capacity of bomb calorimeter (C)

Answer 10

-the amount of heat required to raise the temperature of the calorimete by 1C/K

Question 11

bomb calorimeter (constant pressure)

Answer 11

-qp = qv - w
(ΔU = q + w, qp = ΔU - w, ΔU= qv)

Question 12

w (work) in combustion reaction

Answer 12

-w = -Pext(ΔV)
- times 101.325 if originally in L/atm

Question 13

enthalpy (H)

Answer 13

-a state function for changes carried out at constant internal pressure
-extensive property, depends on amount of substance involved
-H = U + PV

Question 14

change in enthalpy (ΔH)

Answer 14

-heat of reaction under constant pressure
-ΔH = ΔU + Δ(PV)
-ΔH = qp

Question 15

heat required to raise temperature of n moles of substance

Answer 15

q = nCmΔT

Question 16

Cv of ideal monatomic gas

Answer 16

3/2R (8.314)

Question 17

Cp of ideal monatomic gas

Answer 17

5/2R (8.314)

Question 18

heating gas under constant V

Answer 18

-qv = nCvΔT
-ΔU = nCvΔT
or
-ΔU = n3/2RΔT

Question 19

heating gas under constant P

Answer 19

-qp = nCpΔT , qp = ΔH
-ΔH = nCpΔT
or
-ΔH = n5/2RΔT

Question 20

Cp and Cv relation

Answer 20

Cp = Cv + R

Question 21

Thermochemistry

Answer 21

the study of heat effects/transfers in chemical reactions

Question 22

reaction enthalpy

Answer 22

-the energy change in a chemical reaction carried out under constant pressure
-chemical reactions are usually studied at constant pressure
-qp = ΔH = ΔH(rx)
-if direction of chemical reaction is reversed, the enthalpy change reverses sign

Question 23

Exothermic reaction

Answer 23

-a reaction in which heat is given off so that ΔH is negative
-usually building bonds (synthesis)

Question 24

endothermic reaction

Answer 24

-a reaction in which heat is taken up, so that ΔH is positive
-usually breaking bonds (decomposition)

Question 25

Hess's law

Answer 25

-if two or more chemical equations are combined by addition or subtraction to give another equation, then adding or subtracting changes in enthalpies for these equations in the same way gives the enthalpy change associated with the resultant equation -ΔH = ΔU + Δ(PV) -Δ(PV) = Δ(nRT) = RTΔ(ng) -ng = (total moles of product gases - total moles of reactant gases)

Question 26

Standard state enthalpies

Answer 26

-for solids/liquids: 1 atm -for gases: 1 atm -for dissolved species: 1 M at 1 atm -elemetns in their standard states at 298 K produce zero enthalpy

Question 27

standard enthalpy ΔH°

Answer 27

-the enthalpy change for the reaction that produces products in their standard states from the reactants (also in their standard states) -for aA + bB → cC + dD: ΔH° = cΔH°f(C) + dΔH°f(D) → – aΔH°f(A) – bΔH°f(B)

Question 28

standard enthalpy of formation ΔH°f

Answer 28

-the enthalpy change for the reaction that produces one mole of a compound in its standard states from its element (also in their standard state) -ΔH°f is zero for an element already in its standard state because no further change is needed to bring it to standard state conditions

Question 29

Kirchoff's law

Answer 29

-describes how the enthalpy of a reaction changes with temperature -the change in enthalpy (ΔH) at a non-standard temperature can be calculated by considering the change in heat capacities (ΔCp) of reactants and products