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Determining the specific heat of lead – Example 7-2 illustratedFIGURE 7-3

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7

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Heat Capacity• The quantity of heat required to change the

temperature of a system by one degree.

– Molar heat capacity. q = n T• System is one mole of substance.

– Specific heat capacity, c.• System is one gram of substance

– Heat capacity• (Mass of system) x specific heat.

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7

q = mcT

q = CT

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Law of conservation of energy

• In interactions between a system and its surroundings the total energy remains constant— energy is neither created nor destroyed.

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7

qsystem + qsurroundings = 0

qsystem = -qsurroundings

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7-2 Heat•Thermal Energy–Kinetic energy associated with random molecular motion.

–In general proportional to temperature.

–An intensive property.

•Heat and Work–q and w.–Energy changes.

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7-3 Heats of Reaction and Calorimetry

• Chemical energy. – Contributes to the internal energy of a system.

• Heat of reaction, qrxn.

– The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature. In practice, a small temperature difference may be permitted/required.

General Chemistry: Chapter 7

Calorimetry – Unfortunate Name?• Energy changes associated with both physical

and chemical processes are measured with calorimeters. The q values obtained are either constant pressure measurements (qP for a constant pressure calorimeter – isobaric change) or constant volume measurements (qV for a constant volume calorimeter – isochoric change). qP and qV values can differ appreciably if the process involves gases.

Coffee Cups • For some chemical and physical processes,

when high accuracy is not required, a simple coffee cup can be used to measure qP values. These constant pressure measurements are of interest for many processes occurring on the Earth’s surface. Why? Chemists like to report so-called “heats of reaction” on a per mole basis. We introduce enthalpy, H:

• ∆H = qP and ∆ = qP/n

“Coffee Cup” Calorimeters

• A coffee cup normally has a negligible heat capacity compared to the system being studied. A coffee cup, or a more accurate device, affords us a constant pressure measurement of heat transfer or a ΔH value for a physical or chemical change. Such a calorimeter is suited for studies where the reactants do not need to be confined and great accuracy is not required.

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The “Coffee-Cup” Calorimeter

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7

FIGURE 7-6 •A Styrofoam “coffee-cup” calorimeter

A simple calorimeter.

Well insulated and therefore isolated.

Measure temperature change.

qrxn = -qcal

Coffee Cup Reactions• Reactions occurring when two aqueous solutions

are mixed are easily studied using a coffee cup calorimeter. For dilute aqueous solutions we can often assume that the solution density and specific heat capacity are very similar to the values for pure water. Possible reactions:

• HNO3(aq) + KOH(aq) → KCl(aq) + H2O(aq)

• AgNO3(aq) + HCl(aq) → AgCl(s) + HNO3(aq)

• MgO(s) +2 HCl(aq) → MgCl2(aq) + H2O(l)

• H2O(s,0oC) + H2O(l,”hot”) → H2O(l,”moderate T”)

Coffee Cup & Limiting Reagents• Example: Determination of the molar enthalpy

of neutralization of strong acid and strong base (completely ionized acid and base). In an experiment 125 mL of 0.400 mol L∙ -1 HCl(aq) were mixed with 175 mL of 0.280 mol L∙ -1 NaOH(aq). The temperature of the solution formed after rapid mixing increased from 20.9 oC to 24.0 oC. Find the molar enthalpy of neutralization and write the total and net ionic equations for the reaction. Limiting reagent?

Bomb Calorimeters – Big Budget!• Bomb calorimeters are big and expensive

devices. They have large heat capacities and often contain water - which serves to keep the temperature rise resulting from highly exothermic reactions at a moderate level. Bomb calorimeters are suited for studying reactions where the starting materials might be volatile and, without a sealed calorimeter, both starting materials and heat might escape from the calorimeter.

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Bomb Calorimetry

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FIGURE 7-5•A bomb calorimeter assembly

qrxn = -qcal

qcal = q bomb + q water + q wires +…

Define the heat capacity of the calorimeter: qcal = miciT = CcalTall i

heat

Typical Bomb Calorimeter Reactions• Bomb calorimeters are especially suited to the

study of combustion reactions. In such studies an excess of oxygen is normally employed. Why? Typical reactions:

• Mg(s) + ½ O2(g) → MgO(s)

• C9H20(l) + 19 O2(g) → 9 CO2(g) + 10 H2O(l)

• CH3CH2COOH(l) + 5/2 O2(g) →3 CO2(g) + 3 H2O(l)

Bomb Calorimeter Example• The combustion of a rocket fuel,

methylhydrazine (CH6N2(l)), is described by• CH6N2(l) + 5/2 O2(g) →N2(g) + CO2(g) + 3 H2O(l)

• In a calorimeter having a heat capacity of 8.004 kJ.K-1 the combustion of 2.018 g of methyl hydrazine caused the temperature of the calorimeter to rise from 24.001 oC to 31.124 oC. Determine a value for ∆Combustion of methyl hydrazine. (Elton John wants to know?)