Calorimetry
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Transcript of Calorimetry
CALORIMETRYPhysics 12
Objectives Define specific heat capacity. Solve problems involving specific heat
capacities. Explain the difference between solid, liquid,
and gaseous phases. Explain in terms of molecular behavior why
temperature does not change during a phase change.
Define latent heat. Solve problems involving latent heats.
Activities Worksheet Lab demonstration: Determining the
specific heat capacity of a substance Lab demonstration: Determining the
latent heat of a substance Design lab: Design a homemade
calorimeter and test it
Specific heat capacity If heat flows into an object, its
temperature rises.
What factors might affect the magnitude of the temperature change?
Specific heat capacity The amount of energy Q required to change
the temperature of a given material is proportional to the mass m of the material and to the temperature change ΔT, shown by the simplistic expression:
Q = mc ΔT
where c is the characteristic of the material called its specific heat capacity
Specific heat capacity A high specific heat capacity means that
more energy is required to achieve the same temperature change, i.e. it is more “difficult” to raise the temperature of that material.
If a material is a good heat conductor (e.g. metals) would you expect it to have a high or low specific heat capacity?
Specific heat capacitySpecific heat capacities of specific substances
Substance Specific heat capacity, c / Jkg-
1°C-1
aluminum 900
copper 390
iron or steel 450
lead 130
wood 1700
water (ice) 2100
water (liquid) 4186
water (steam) 2010
human body (average)
3470
Specific heat capacityQ / J m / kg c / Jkg-1C-1 ΔT / °C or
°K? increased constant constant? constant constant increased
constant increased constant ?increased constant constant ?
? constant increased constant
Specific heat capacity
(a) How much heat input is needed to raise the temperature of an empty 20-kg vat made of iron from 10C to 90C?
(b) What if the vat is filled with 20 kg of water?
Specific heat capacity
(a) 720 kJ(b) 7400 kJ
Specific heat capacityYou accidentally let an empty iron frying pan get
very hot on the stove (approx. 200C). What happens when you dunk it into a few inches of cool water in the bottom of the sink?
Will the final temperature be midway between the initial temperatures of the water and pan? Will the water start boiling?
(Assume the mass of the water is roughly the same as the mass of the pan.)
Calorimetry In discussing heat and thermodynamics,
we shall often refer to systems.
What is the difference between open, closed, and isolated systems?
Calorimetry Open system
Mass and energy may leave and enter Closed system
Energy may leave and enter but mass may not Isolated system
Neither mass nor energy may leave or enter
Calorimetry We will often make the assumption that
the systems we are dealing with are isolated.
Why is this necessary?
Calorimetry In an isolated system, heat lost by one part of
the system is equal to the heat gained by another part:
heat lost = heat gainedor
energy out of one part = energy into another part
Calorimetry
If 200 cm3 of tea at 95C is poured into a 150-g glass cup initially at 25C, what will be the common final temperature T of the tea and cup when thermal equilibrium is reached, assuming no heat flows to the surroundings?
Calorimetry
T = 86C
Would this be the case in the “real world”?
Calorimetry The exchange of energy (as shown in the
previous example) is the basis for the technique known as calorimetry.
Calorimetry is the quantitative measurement of heat exchange.
A calorimeter is used.
CalorimetryA simple water calorimeter
Calorimetry
An engineer wishes to determine the specific heat of a new metal alloy. A 0.150-kg sample of the alloy is heated to 540C. It is then quickly placed in 400 g of water at 10.0C, which is contained in a 200-g aluminum calorimeter cup. The final temperature of the system is 30.5C. Calculate the specific heat of the alloy.
Calorimetry
c = 500 Jkg-1C-1
Calorimetry In order to determine the specific heat of a
particular substance, the following expression is used:
Qlost = Qgained
m1c1ΔT1 = m2c2 ΔT2
where the two substances share a final temperature
Thus, all other quantities except one must be measured or known.
Phase change Recall that matter most commonly exists
in three states: solid, liquid, and gas.
What are the differences between these three states (or phases) in terms of molecular structure and motion?
Phase changeComparison of the three common phases of matter (on Earth)
Shape Volume Particle motion
solid
liquid
gas
Phase change
Comparison of the three common phases of matter (on Earth)
Shape Volume Particle motion
solid definite definite vibrational
liquid indefinite definitevibrationalrotational
translational
gas indefinite indefinitesame as liquid
but quicker
Phase change When a material changes phase from
solid to liquid or liquid to gas, a certain amount of energy is involved in this change of phase.
Phase changeTemperature as a function of heat added to 10.0 g of ice
Phase change The heat required to change a substance from
solid to liquid is called latent heat of fusion.
The heat required to change a substance from liquid to gas is called the latent heat of vaporization.
Values for latent heats will vary depending on the substance.
Phase change
Latent heats
Substance Heat of fusion /kJkg-1
Heat of vaporization /
kJkg-1
oxygen 14 210
ethyl alcohol 104 850
water 333 2260
iron 289 6340
Phase change What factors might affect the amount of
energy needed to change the phase of a substance?
Phase change The heat involved in a change of phase Q
depends not only on the latent heat but also on the total mass of the substance, i.e.
Q = mLwhere m is the mass of the substance and L is the latent
heat
Phase change
How much energy does a freezer have to remove from 1.5 kg of water at 20C to make ice at –12C?
Phase change
6.6 x 105 J
Phase change
At a reception, a 0.50-kg chunk of ice at –10C is placed in 3.0 kg of tea at 20C. At what temperature and in what phase will the final mixture be?
The tea can be considered as water. Ignore any heat flow to the surroundings, including the container.
Phase change
T = 5C
Phase change
The specific heat of liquid mercury is 140 Jkg-1C-1. When 1.0 kg of solid mercury at its melting point of –39C is placed in a 0.50-kg aluminum calorimeter filled with 1.2 kg of water at 20.0C, the final temperature of the combination is found to be 16.5C. What is the heat of fusion of mercury in Jkg-1?
Phase change
L = 11 x 103 Jkg-1
Objectives Define specific heat capacity. Solve problems involving specific heat
capacities. Explain the difference between solid, liquid,
and gaseous phases. Explain in terms of molecular behavior why
temperature does not change during a phase change.
Define latent heat. Solve problems involving latent heats.
Measuring specific heat
mass of metalinitial temperature of metalmass of calorimetermass of calorimeter + waterinitial temperature of waterfinal temperature
Data collection
Measuring specific heat
Data processing Include propagation of uncertainty
mass of waterΔT of waterΔT of metal
Measuring specific heat
Data processing Include propagation of uncertainty
Q = mcΔTQ (lost) = Q (gained)
mass of waterΔT of waterΔT of metal
Measuring specific heat Homework
Write a conclusion and evaluation of the lab activity
Design a homemade calorimeter using the materials available. Try to minimize the amount of energy lost to the surroundings.
You will use this calorimeter for the next lab activity. The calorimeter with the smallest percent discrepancy gets bonus points.
Measuring latent heat of fusion
Data collection
mass of cup 1mass of cup 1 + watermass of cup 2mass of cup 2 + iceinitial temperature of iceinitial temperature of waterfinal temperature
Measuring latent heat of fusion
Data processing
mass of waterT of waterΔ
mass of iceT of iceΔ
Measuring latent heat of fusion
HomeworkComplete your data processing.Make sure to include a sample calculation that shows the propagation of uncertainty.Write a conclusion for your data, is the known value for the latent heat of fusion of water within your range of uncertainty?