System & Surroundings
What is the equation for a change in energy?
change in energy = q + w
q is heat
w is work
What is heat capacity?
amount of heat required to raise an entire mass by one degree Celsius
11.0 grams of NH4Cl were combined with 100.0 grams of water in a coffee-cup calorimeter, causing the water temperature to decrease by 7.40 °C. How much heat energy was required to dissolve the sample of NH4Cl?
What is our first step?
Write down what you have.
11.0 grams of NH4Cl, 100.0 gram of water = 111.0 g of solution or msol
change in Tsol: -7.40 °C
23.0 g of ethanol are placed inside a bomb calorimeter with a known heat capacity of 28.72 kJ/°C. When the ethanol ignites, the temperature of the calorimeter rises from 24.09 °C to 45.74 °C. How much heat did the ethanol release?
What is our first step in solving this problem?
Write down what you have.
m of ethanol = 23.0 g
C = 28.72 kJ/°C
T1= 45.74 °C
T2 = 24.09 °C
What is reaction enthalpy (Hrxn)?
describes energy changes in a reaction; also called heat of reaction
When a reaction is exothermic, is q positive or negative?
When a reaction is endothermic, is q positive or negative?
exothermic: -q
endothermic: +q
What is specific heat?
the amount of heat required to raise 1 gram of a material by 1 degree Celsius
11.0 grams of NH4Cl were combined with 100.0 grams of water in a coffee-cup calorimeter, causing the water temperature to decrease by 7.40 °C. How much heat energy was required to dissolve the sample of NH4Cl?
What is our 2nd step in solving this problem?
dilute aqueous solution, so we can use the specific heat of water.... Cs of solution = 4.184 J/g x °C
in total, we have Cs,sol, msol, change in Tsol
23.0 g of ethanol are placed inside a bomb calorimeter with a known heat capacity of 28.72 kJ/°C. When the ethanol ignites, the temperature of the calorimeter rises from 24.09 °C to 45.74 °C. How much heat did the ethanol release?
What is our second step in solving this problem?
Calculate change in T
T2-T1 = 45.74°C - 24.09°C = 21.65°C
C2H6O (l) + 3 O2 (g) --> 2 CO2 (g) + 3 H2O (l)
ΔHrxn = – 1368 kJ
What is the enthalpy change when 7.0 moles of O2 react?
What is the first step to solving this problem?
Write down what's given
7.0 mol O2
ΔHrxn = – 1368 kJ
Is work done by the system positive or negative?
Is work done on the system positive or negative?
work done by the system: -w
work done on the system: +w
C = q/change in T
where C is heat capacity, q is heat, and T is temperature
11.0 grams of NH4Cl were combined with 100.0 grams of water in a coffee-cup calorimeter, causing the water temperature to decrease by 7.40 °C. How much heat energy was required to dissolve the sample of NH4Cl?
What is our 3rd step in solving this problem?
Determine which equation to use.
qsol = msol x Cs,sol x change in Tsol
23.0 g of ethanol are placed inside a bomb calorimeter with a known heat capacity of 28.72 kJ/°C. When the ethanol ignites, the temperature of the calorimeter rises from 24.09 °C to 45.74 °C. How much heat did the ethanol release?
What is our third step in solving this problem?
Decide which equation to use.
qcal = C x change in T
C2H6O (l) + 3 O2 (g) --> 2 CO2 (g) + 3 H2O (l)
ΔHrxn = – 1368 kJ
What is the enthalpy change when 7.0 moles of O2 react?
What is the second step to solving this problem?
Recognize that enthalpy is affected by stoichiometry, so if we start with 7.0 moles of O2, we are going to be using some kind of stoichiometry
First Law of Thermodynamics or Law of Conservation of Mass
total energy is conserved by system and surroundings
change in energy of system = - change in energy of surroundings
specific heat equation
Cs = q/(m x change in T)
where Cs is specific heat, q is heat, m is mass, and T is temperature
11.0 grams of NH4Cl were combined with 100.0 grams of water in a coffee-cup calorimeter, causing the water temperature to decrease by 7.40 °C. How much heat energy was required to dissolve the sample of NH4Cl?
What is our 4th step in solving this problem?
Plug numbers into the equation and solve
qsol = (111.0 g)(4.184 J/g x°C)(-7.40 °C)
qsol = -3436.7376 J
23.0 g of ethanol are placed inside a bomb calorimeter with a known heat capacity of 28.72 kJ/°C. When the ethanol ignites, the temperature of the calorimeter rises from 24.09 °C to 45.74 °C. How much heat did the ethanol release?
What is our fourth step in solving this problem?
Plug in values to equation and solve
qsol = (28.72 kJ/°C)(21.65°C)
qsol = 621.788 kJ
C2H6O (l) + 3 O2 (g) --> 2 CO2 (g) + 3 H2O (l)
ΔHrxn = – 1368 kJ
What is the enthalpy change when 7.0 moles of O2 react?
What is the third step to solving this problem?
Set up the stoichiometry.
7.0 mol O2 x _____ / 3 mol O2
System v. Surroundings
System: substances or reactions being studied
Surroundings: the rest of the universe
Which type of calorimetry uses the relationship of heat capacity?
Which type of calorimetry uses the relationship of specific heat?
Constant volume calorimetry or "bomb calorimetry" uses the relationship of heat capacity
Constant pressure calorimetry or "coffee cup calorimetry" uses the relationship of specific heat
11.0 grams of NH4Cl were combined with 100.0 grams of water in a coffee-cup calorimeter, causing the water temperature to decrease by 7.40 °C. How much heat energy was required to dissolve the sample of NH4Cl?
What is our final step in solving this problem?
Adjust significant figures.
qsol = -3436.7376 J
qsol = -3.44 x 103 J
23.0 g of ethanol are placed inside a bomb calorimeter with a known heat capacity of 28.72 kJ/°C. When the ethanol ignites, the temperature of the calorimeter rises from 24.09 °C to 45.74 °C. How much heat did the ethanol release?
What is our final step in solving this problem?
Adjust significant figures.
qsol = 621.788 kJ
qsol = 621.8 kJ
C2H6O (l) + 3 O2 (g) --> 2 CO2 (g) + 3 H2O (l)
ΔHrxn = – 1368 kJ
What is the enthalpy change when 7.0 moles of O2 react?
What is the final step to solving this problem?
Finish set up and solve. Remember significant figures!
7.0 mol O2 x -1368 kJ / 3 mol O2
= -3192 kJ = -3.2 x 103 kJ