File: Hess’s Law, Calorimetery, Book Problems.doc
Name ____________________________________________
Period ______________
1) How many joules of heat are needed to change 50.0 grams of ice at -15.0 °C to steam at
120.0 °C? ( calculate heat to warm ice, melt ice, warm water, convert to steam, warm steam)
(look in notes for heat of fusion and heat of vaporization)
2) Calculate the number of joules given off when 32.0 grams of steam cools from 110.0 °C to
ice at -40.0 °C. (this one is as tough as #1)
Hess’s Law
3) Given the following equations and Ho values, determine the heat of reaction (kJ) at
298 K for the reaction:
2 F2(g) + 2 H2O(l) 4 HF(g) + O2(g)
H2(g) + F2(g)
2 HF(g) Ho/kJ = -542.2
2 H2(g) + O2(g)
2 H2O(l)
Ho/kJ = -571.6
4) Given the following equations and Ho values, determine the heat of reaction (kJ) at
298 K for the reaction:
6 Fe2O3(s)
O2(g) + 4 Fe3O4(s)
4 Fe(s) + 3 O2(g)
3 Fe(s) + 2 O2(g)
2 Fe2O3(s)
Fe3O4(s)
Ho/kJ = -823.41
Ho/kJ = -1120.48
File: Hess’s Law, Calorimetery, Book Problems.doc
o
5) Given the following equations and H values, determine the heat of reaction at 298 K
for the reaction: P4(s) + 10 Cl2(g) 4 PCl5(g)
P4(s) + 6 Cl2(g)
PCl3(l) + Cl2(g)
4 PCl3(l)
PCl5(g)
Ho/kJ = -1150
Ho/kJ = -111
6) Calculate Ho/kJ for the following reaction using the listed standard enthalpy of
reaction data:
2 N2(g) + 5 O2(g) 2 N2O5(s)
N2(g) + 3 O2(g) + H2(g)
2 HNO3(aq)
Ho/kJ = -414.0
N2O5(s) + H2O(l)
2 HNO3(aq)
Ho/kJ = -86.0
2 H2(g) + O2(g)
2 H2O(l)
Ho/kJ = -571.6
Problems from Book:
Read pages 523-528 and do these problems (16-19, 22 copied out of textbook):
16) Describe how you would calculate the amount of heat absorbed or released by a substance when its temperature
changes.
17) Explain why H for an exothermic reaction always has a negative value.
18) Why is the measured volume of water an essential part of a calorimeter?
19) Explain why you need to know the specific heat of a substance in order to calculate how much heat is gained or lost
by the substance as a result of temperature change.
22) Describe a procedure you could follow to determine the specific heat of a 45g piece of metal.
File: Hess’s Law, Calorimetery, Book Problems.doc
Name _____ANSWERS_______________________________________Period ______________
1) How many joules of heat are needed to change 50.0 grams of ice at -15.0 °C to steam at
120.0 °C? ( calculate heat to warm ice, melt ice, warm water, convert to steam, warm steam)
(look in notes for heat of fusion and heat of vaporization)
Qtotal = m CiceT + mHfus + mCwaterT + mHvap + mCsteamT
Qtotal = 50(.5)15 + 50(80) + 50(1)100 + 50(540) +50 (.4) 20
Qtotal = 375 + 4000 + 5000 + 27000 + 400 = 36 775 cal.
Or 36 775(4.184 J/cal.) = 153 866 J or 153.9 kJ
2) Calculate the number of joules given off when 32.0 grams of steam cools from 110.0 °C to
ice at -40.0 °C. (this one is a tough as #7)
Qtotal = m CsteamT + mHvap + mCwaterT + mHfus + mCiceT
Qtotal = 32(.4)10 + 32(540) + 32(1)100 + 32(80)+ 32(.5)40
Qtotal = 128 + 17 280 + 3200 + 2560 + 640 = 23 808 calories or 99 612 J or 99.6 kJ
Hess’s Law
3) Given the following equations and Ho values, determine the heat of reaction (kJ) at
298 K for the reaction:
2 F2(g) + 2 H2O(l) 4 HF(g) + O2(g)
2 H2(g) + 2 F2(g)
2 H2O(l)
2 F2(g) + 2 H2O(l)
4 HF(g)
2(-542.2) kJ
2 H2(g) + O2(g)
4 HF(g)+ O2(g)
=
-(-571.6)
-1084.4+571.6
The H is negative so the reaction is exothermic
H2(g) + F2(g)
2 HF(g) Ho/kJ = -542.2
2 H2(g) + O2(g)
2 H2O(l)
Ho/kJ = -571.6
kJ
= -512.8 kJ
File: Hess’s Law, Calorimetery, Book Problems.doc
4) Given the following equations and Ho values, determine the heat of reaction (kJ) at
298 K for the reaction:
6 Fe2O3(s)
O2(g) + 4 Fe3O4(s)
Triple and reverse the 1st equation;
Quintuple the 2nd equation:
6 Fe2O3(s)
12 Fe(s) + -9- O2(g)
12 Fe(s) + 8 O2(g)
Results in
6 Fe2O3(s)
4 Fe(s) + 3 O2(g)
3 Fe(s) + 2 O2(g)
4 Fe3O4(s)
4 Fe3O4(s) + 1
2 Fe2O3(s)
Fe3O4(s)
-3(-823.41)
O2(g)
4(-1120.48)
kJ =
kJ
2 470.23
= - 4 481.92
H = - 2 011.69 kJ
Ho/kJ = -823.41
Ho/kJ = -1120.48
5) Given the following equations and Ho values, determine the heat of reaction at 298 K
for the reaction: P4(s) + 10 Cl2(g) 4 PCl5(g)
Keep 1st equation the same
Quadruple the 2nd equation
P4(s) + 6 Cl2(g)
4 PCl3(l)
Results in:
P4(s) + 6 Cl2(g)
PCl3(l) + Cl2(g)
+ 4 Cl2(g)
P4(s) + (6+4) Cl2(g)
4 PCl3(l)
PCl5(g)
4 PCl3(l)
4 PCl5(g)
4 PCl5(g)
Ho/kJ = -1150
Ho/kJ = -111
Ho/kJ = -1150
Ho/kJ = 4(-111)
Ho = -1150 + 4(-111) = -1594 kJ
File: Hess’s Law, Calorimetery, Book Problems.doc
o
6) Calculate H /kJ for the following reaction using the listed standard enthalpy of
reaction data:
2 N2(g) + 5 O2(g) 2 N2O5(s)
Double 1st
Reverse 2nd and double it
Reverse 3rd equation
2 N2(g)
+ 6 O2(g) + 2 H2(g)
4 HNO3(aq)
2 H2O(l)
Results in: 2 N2(g)
+ 5 O2(g)
2 N2O5(s)
4 HNO3(aq)
Ho = 2(-414.0)
2 N2O5(s) + 2 H2O(l)
2 H2(g) + O2(g)
Ho = 2(-86.0)
Ho = -1(-571.6)
Ho =-828. -172.0 +571.6
N2(g) + 3 O2(g) + H2(g)
2 HNO3(aq)
Ho/kJ = -414.0
N2O5(s) + H2O(l)
2 HNO3(aq)
Ho/kJ = -86.0
2 H2(g) + O2(g)
2 H2O(l)
Ho/kJ = -571.6
= -428.4
File: Hess’s Law, Calorimetery, Book Problems.doc
Problems from Book:
Read pages 523-528 and do these problems (16-19, 22 copied out of textbook):
16) Describe how you would calculate the amount of heat absorbed or released by a substance when its temperature
changes.
I would use a calculation Q = mCT; where Q is the amount of heat; M is the mass of the material, C is the specific heat
of the material and T is the change in temperature of the material
(I would see what kind of change can be determined)
17) Explain why H for an exothermic reaction always has a negative value.
The energy of the products is less than the energy of the reactants in an exothermic reaction; the change will be a smaller
value minus a larger value)
18) Why is the measured volume of water an essential part of a calorimeter?
More water takes more energy to heat up
19) Explain why you need to know the specific heat of a substance in order to calculate how much heat is gained or lost
by the substance as a result of temperature change.
It is part of the equation used above
22) Describe a procedure you could follow to determine the specific heat of a 45g piece of metal.
Heat the metal to a known temperature (perhaps boiling pt of water) then have the hot material heat a known quantity of
water (perhaps 100grams) and measure how much hotter the water becomes and how much cooler the meat becomes as it
reaches equilibrium). I would calculate the heat gained by the water and use this quantity of heat to measure the specific
heat of the metal (same equation: Q = mCT)