heat , Calorimetry specific heat, heat capacity

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Name:……………………………………………
Grade: ……………………………………..
Subject: Chemistry
Date: …………………………………………
Section 2: Heat
 Some chemical reactions absorb heat whereas others release heat.
 The Calorie information that appears on packaged food is obtained from
combustion reactions carried out in calorimeters.
 Calorimetry
 Calorimeteris an insulated device used for measuring the amount of heat
absorbedor releasedduring a chemical or physical process.
 A known mass of water is placed in an insulated chamber to absorb
the energy released from the reacting system or to provide the energy
absorbed by the system.
 The data to be collected is the change in temperature of this mass of water.
 Figure 5 shows the kind of calorimeter, called a bomb calorimeter,
that is used by food chemists.

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 Determining specificheat
 Satisfactory results can be obtained in your calorimetry experiments using the much
simpler foam-cup calorimeter.
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 Assuming no heat is lost to the surroundings, the heat gained by the water is
equal to the heat lost by the metal.
 This quantity of heat can be calculated
q = c × m × ΔT
 First, calculate the heat gained by the water
 Specific heat of water, 4.184 J/(g·°C).
 q water = 4.184 J/(g·°C) × 125 g × (29.30°C - 25.60°C)
 q water = 4.184 J/(g·°C) × 125 g × 3.70°C
 q water = 1940 J

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 The heat gained by the water, 1940 J, equals the heat lost by the metal,
q metal , so you can write this equation.
 q metal = q water
 q metal = −1940 J
 c metal × m × ΔT = -1940 J
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 EXAMPLE Problem 3

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 Prctice Problems

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 Chemical Energyand the Universe
 Every chemical reaction and change of physical state either releases or
absorbs heat.
 Thermochemistry
Is the study of heat changes that accompanychemicalreactions and phase
changes.
 In thermochemistry, the system is :
The specific part of the universe that contains the reactionor process you wish
to study.
 Everything in the universe other than the system is considered the surroundings.
 Therefore, the universeis defined as:
The system plus the surroundings.
 Universe = system + surroundings
 During exothermicreactions
 Heat produced bythe reaction flows from the system to the surroundings.
 In an endothermic reaction or process ,
 The flow of heat is reversed. Heat flows from the surroundings to the system
 Enthalpyand enthalpychanges
 it is impossible to know the total energy content of a substance.
 Chemists are usually more interested in changes in energy during reactions than in
the absolute amounts of energy contained in the reactants and products.
 For many reactions, the amount of energy lost or gained can be measured
conveniently in a calorimeter at constant pressure

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 Enthalpy(H)
Is the heat content of a system at constant pressure.
 Although you cannot measure the actual energy or enthalpy of a substance, you can
measure the change in enthalpy, which is the heat absorbed or releasedin a
chemical reaction.
 Enthalpy(heat) of reaction (Δ H rxn ).
 The change in enthalpy for a reactionis calledthe enthalpy (heat) of reaction
(Δ H rxn )
 Δ H rxn = H final - H initial
 Δ H rxn = H products - H reactants
 The sign of the enthalpyof reaction
 Exothermic reaction lose heat.
 Therefore,
 H products < H reactants .
 Enthalpy changes for exothermic reactions are always negative.
 Example heat-pack:
 4Fe(s)+ 3 O 2 (g) → 2F e 2 O 3 (s) Δ H rxn = -1625 kJ
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 For this endothermic process,
H products > H reactants
 Therefore,
 A positive value for Δ H rxn is obtained.
 Example cold -pack :
 N H 4 NO 3 (s) → N H 4 + (aq) + N O 3 - (aq) Δ H rxn = 27 k
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 The enthalpy change, ΔH, is equal to q p , the heatgained or lost in a reaction
or process carriedout at constant pressure.
 q = Δ H rxn .

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SECTION2 REVIEW