Download Thermochemistry Lesson 2

Thermochemical Equations & Calorimetry
At the end of this lesson you should be to
• define heat capacity and specific heat capacity
• Calculate heat flow for one substance
• Calculate heat flow between two substances
Heat capacity
• This can be calculated using the equation:
• Q (q) = C ΔT where C is the heat capacity
of the system.
• ΔT = change in temperature (T2-T1)
• Q = heat
• The heat capacity is the amount of heat
required to raise the temperature of the
system by 10C or 1 K.
• Units are J/ 0C or J/K
At the end of this lesson you should be to
• define heat capacity
Specific heat capacities – don’t write
Aluminum
Copper
Gold
Iron
Sodium
Mercury
Ice
Water
Steam
Methanol
Ethanol
0.900
0.387
0.128
0.444
1.23
0.138
2.01
4.18
2.01
2.918
2.454
Specific heats in
J/goC
at 101.3 kPa
and 25oC (except
for ice and steam).
At the end of this lesson you
should be to
• define heat capacity and
specific heat capacity
Calculations Involving Heat Flow
• Heat with only one substance
• How much heat, in kJ, is required to raise the
temperature of 120 g Al from 250C to 1000C ?
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(Specific Heat of Al is 0.902 J /g 0C) Ans. 8118 J
Q=?
At the end of this lesson you
should be to
m = 120 g
• define heat capacity and
0
T1= 25 C
specific heat capacity
∆T= 100 - 25 =75
• Calculate heat flow for one
T2= 100 0C
substance
C = 0.902 J /g 0C
Q = mc∆T = 120g x 0.902 J /g 0C x 75 0C
= 8118 J = 8.1 kJ
Worksheet - SPECIFIC HEAT CAPACITY AND HEAT
TRANSFER
Calculations Involving Heat Flow
• Heat involved with two substances
• A piece of iron (Fe) weighing 40 g is heated to 800C and
dropped into 100 g of water at 250C. (Specific heat of Fe is
0.446 J /g0C and Specific heat of H2O is 4.182 J /g0C)
What is the temperature when thermal equilibrium has
been reached ? (Ans = 27.50C)
• Heat lost by iron is equal to heat gained by the water.
• -Q iron = Q water
• -(miron x Ciron x ∆Tiron) = (mwater x Cwater x ∆Twater)
• Basic math skills, negative sign is important.
• Finish the question above then go to Worksheet –
CALORIMETRY section
At the end of this lesson you should be to
• define heat capacity and specific heat capacity
• Calculate heat flow for one substance
• If
Specific heat capacity
the system is a pure substance with a mass of “ m ” grams
• Q = m c ΔT where c is the specific heat of the substance.
• The specific heat is the amount of heat required to raise
one gram of the substance by 1oC or 1 K.
• Units are J/ g0C or J/ g K)
At the end of this lesson you should be to
• define heat capacity and specific heat capacity
Measuring heat transfer in a lab
• A calorimeter
measures the amount
of heat transferred
during a reaction.
• Coffee Cup
Calorimeter is open to
the atmosphere it is
called a constant
pressure calorimeter
• The heat capacity of
the coffee cup
calorimeter is assumed
to be zero
At the end of this lesson you should be to
• define calorimeter and calorimetry
Enthalpy change for a reaction
• There are five ways to determine or calculate
enthalpy change for a reaction;
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Experiment / calorimetry
Hess’ Law
Use data of standard enthalpy of formation (ΔHf)
Use data of standard enthalpy of combustion (ΔHc)
Use data of standard enthalpy of atomization (ΔHa)
or Energy bonding (D).
At the end of this lesson you should be to
• define calorimeter and calorimetry
• Know five (5) ways to determine enthalpy change for a reaction
Using calorimeter to determine enthalpy of reaction
• During an experiment the solution absorbs or releases heat 
• Qsol = mC∆T
• m = mass of solution
• C = specific heat capacity of water
• Qreaction = - Qsolution
• If reaction/system releases heat, so surroundings
(solution/water and calorimeter) absorb heat.
• Temperature increase  Solution absorbs heat therefore the reaction is
exothermic
• Temperature decrease  solution releases heat therefore the reaction is
endothermic.
• (ΔHrxn = Qrxn/n)
• Assumptions - For dilute acid or bases the density is close to water
therefore you can measure the volume and calculate the mass and use
the C for water in all the calculations.
At the end of this lesson you should be to
• define calorimeter and calorimetry
• Know five (5) ways to determine enthalpy change for a reaction
• Do calculations for enthaphy of reaction using calorimetry