Chemistry

Name: _____________________________

CALORIMETRY Calorimeter:  A device in which the heat associated with a specific process is measured. 

There are two basic types of calorimeters: 1. Constant-pressure calorimeter: also called _____________ calorimeters 2. Constant-volume calorimeter: also known as _____________ calorimeters

Constant-Pressure Calorimeter 

Used to measure the heat absorbed or released by a process or reaction taking place at constant pressure.



Examples: The heat of acid-base neutralizations or the heat associated with dissolving a salt in water can be measured.

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A constant-pressure calorimeter can be constructed using styrofoam cups.  This is called a coffee-cup calorimeter.

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A coffee-cup calorimeter is considered to be an _____________ system.



If an exothermic process occurs, the heat released will be absorbed by the water in which the reaction took place and by the calorimeter.

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If an endothermic process takes place, heat will be absorbed from the water and from the calorimeter. 1

Chemistry 

Name: _____________________________

Recall from the First Law of Thermodynamics that: qsystem = -qsurroundings



For a process in a simple calorimeter, the system is the chemical reaction, and the surroundings are the water and the calorimeter, so: qreaction = - (qcalorimeter+ qH2O)

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For simple calorimeters, qcalorimeter is often so _____________ that it can be omitted leaving us with: qreaction = - qH2O



Heat absorbed or released from the H2O is calculated from q = mc∆T.

Examples: 1. 10.0 g of metal X at 280.0 oC is dropped into 200.0mL of H2O at 20.0 oC in a coffee cup calorimeter. Metal X and H2O reach thermal equilibrium at 25.0 oC. Calculate the specific heat capacity of X.

It may help to visualize the equilibrium temperature as follows:

Temperature (oC)

Temperature vs. Time

300 250 200 150 100 50 0

(mcT)X (mcT)H2O

0

10

20

30

Time (arbitrary)

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Chemistry 2.

Name: _____________________________

4.00 g of CaCl2 (s) is dissolved in 100 g of water in a simple calorimeter. Before CaCl2 was added, the water’s temperature was 22.0 oC. When all the CaCl2 had dissolved, the water was at a final temperature of 24.2 oC. What is the heat of solution for this process? qsystem = -qsurroundings qCaCl2 = -qH2O qCaCl2 = -(mc∆T)H2O Note: a) For a process where the reactants _____________, the mass of the surroundings is the total mass of the solution. That is: mtotal= mCaCl2 + mH2O = 4.00 g + 100 g = 104 g b)

3.

We will assume that the solution is dilute enough so that we can use the specific heat capacity of H2O.

In the previous example, dissolving 4.00 g of CaCl2(s) released 0.96 kJ of energy. What is the molar enthalpy (∆H) of solution for CaCl2(s)?

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Chemistry 4.

Name: _____________________________

I. Consider the following neutralization reaction. HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) When 50.0 mL of 1.00 mol/L HCl(aq) and 50.0 mL of 1.00 mol/L NaOH(aq) are mixed in a coffee cup calorimeter the temperature of the resulting solution increased from 21.0 oC to 27.5 oC. What is the heat of this reaction, measured in kJ per mole of HCl(aq)? qsystem = -qsurroundings qreaction = -qH2O qreaction = -(mc∆T)H2O Note: a) Assume that density, dsolution = dH2O (1g = 1mL) b) Assume that specific heat capacity, csolution = cH2O c) Total mass, msurroundings = mNaOH + mHCl d) Assume that all heat released is absorbed by H2O and not by the styrofoam cup or surrounding air.

II. What is the molar heat of neutralization for the base?

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Chemistry

Name: _____________________________

Constant-Volume Calorimeter 

Constant-volume calorimeters are also called _____________ calorimeters.



Used to measure the heat released by a combustion reaction taking place at constant volume.

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A bomb calorimeter consists of several basic components:

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A substance is placed in a steel container (the bomb!)

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The bomb is then filled with high pressure oxygen.

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The bomb is surrounded by water, and the entire system is _____________ from the external environment.

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The substance in the bomb is ignited electrically.

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The resulting change in _____________ of the water and the bomb is measured at the end of the reaction.

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A bomb calorimeter is considered to be an isolated system.

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Chemistry

Name: _____________________________



Applying the First Law of Thermodynamics gives us: qsystem = -qsurroundings



For a process in a bomb calorimeter, the system is the chemical reaction, and the surroundings are the water and the calorimeter, so: qreaction = - (qcalorimeter+ qH2O)

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Bomb calorimeters are normally _____________ for a constant mass of water, with the heat capacity supplied by the manufacturer. This leaves us with the calculation: qreaction = -(qcalorimeter)

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The heat absorbed by the calorimeter is calculated from q= C∆T.

Example A 1.00 g sample of sucrose (C12H22O11) is burned in a bomb calorimeter which has a heat capacity of 7.00kJ/oC. The temperature increases from 25.0oC to 27.3oC. How much heat is released from this combustion reaction? What is the molar enthalpy of combustion (∆Hcomb)? qsystem = -qsurroundings qreaction = -qcalorimeter

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