A calorimeter is used to measure the amount of heat absorbed or released during a chemical reaction.A calorimeter is used to measure the amount of heat

• A A calorimetercalorimeter is used to measure the is used to measure the amount of heat absorbed or released amount of heat absorbed or released during a chemical reaction.during a chemical reaction.

• In today’s experiment you will measure the In today’s experiment you will measure the heat of a reaction in a calorimeter that is heat of a reaction in a calorimeter that is made out of 2 Styrofoam coffee cups.made out of 2 Styrofoam coffee cups.

• The inner cup holds an aqueous reaction The inner cup holds an aqueous reaction mixture. The outer cup gives additional mixture. The outer cup gives additional thermal insulation from the surrounding thermal insulation from the surrounding environment. The thermometer is used to environment. The thermometer is used to measure the initial and final temperatures measure the initial and final temperatures of the reaction mixture.of the reaction mixture.

CH 104: HEATS OF REACTIONCH 104: HEATS OF REACTION

CALORIMETRYCALORIMETRY

• The The first law of thermodynamicsfirst law of thermodynamics says that energy, in all its forms, is says that energy, in all its forms, is conserved in all processes. Or the heat lost (qconserved in all processes. Or the heat lost (q lostlost) by a system equals ) by a system equals

the heat gained (qthe heat gained (qgainedgained) by the surroundings.) by the surroundings.

qqlostlost + q + qgainedgained = 0 = 0

• The The fundamental equation of calorimetryfundamental equation of calorimetry says the heat lost by a says the heat lost by a reaction (qreaction (qlostlost = q = qreactionreaction) is gained by the surrounding water and ) is gained by the surrounding water and

calorimeter (qcalorimeter (qgainedgained = q = qwaterwater + q + qcalorimetercalorimeter).).

qqreactionreaction + q + qwaterwater + q + qcalorimetercalorimeter = 0 = 0

• q is negative if q is negative if heat is lost.heat is lost.

• q is positive if q is positive if heat is gained.heat is gained.

• q is measured q is measured in joules (J).in joules (J).


• Again, the Again, the fundamental equation of calorimetryfundamental equation of calorimetry is isqqreactionreaction + q + qwaterwater + q + qcalorimetercalorimeter = 0 = 0

• 4.184 joules (J) are needed to heat 1 gram (g) of water 1 degree Kelvin 4.184 joules (J) are needed to heat 1 gram (g) of water 1 degree Kelvin (K). This is the (K). This is the specific heatspecific heat of water. Therefore, the heat gained by of water. Therefore, the heat gained by the water isthe water is

• And the heat gained by the calorimeter isAnd the heat gained by the calorimeter isqqcalorimetercalorimeter = (heat capacity of the calorimeter) x (T = (heat capacity of the calorimeter) x (Tfinalfinal – T – Tinitialinitial))

WhereWhere• The mass of water is in grams.The mass of water is in grams.• The heat capacity of each calorimeter is unique. Therefore, in today’s The heat capacity of each calorimeter is unique. Therefore, in today’s

experiment you will measure the heat capacity of your calorimeter.experiment you will measure the heat capacity of your calorimeter.• TTfinalfinal is the final temperature of the water in either Kelvin or Celsius. is the final temperature of the water in either Kelvin or Celsius.• TTinitialinitial is the initial temperature of the water in either Kelvin or Celsius. is the initial temperature of the water in either Kelvin or Celsius.• Why can these temperatures be measured in either Kelvin or Celsius?Why can these temperatures be measured in either Kelvin or Celsius?• A Kelvin degree is the same size as a Celsius degree. Therefore, the A Kelvin degree is the same size as a Celsius degree. Therefore, the

change in temperature is the same if it is measured in either Kelvin or change in temperature is the same if it is measured in either Kelvin or Celsius.Celsius.


• Again, the Again, the fundamental equation of calorimetryfundamental equation of calorimetry is is

qqreactionreaction + q + qwaterwater + q + qcalorimetercalorimeter = 0 = 0

• OrOr

qqreactionreaction = –(q = –(qwaterwater + q + qcalorimetercalorimeter))

• Then substitutingThen substituting

MEASURING THE HEAT CAPACITY OF YOUR CALORIMETERMEASURING THE HEAT CAPACITY OF YOUR CALORIMETER

• In today’s experiment you will measure the heat capacity of your In today’s experiment you will measure the heat capacity of your calorimeter.calorimeter.

• A student puts 75.0 g water in a calorimeter. The temperature of this A student puts 75.0 g water in a calorimeter. The temperature of this water and the calorimeter is 21.4° C. Then he adds 74.5 g of water at water and the calorimeter is 21.4° C. Then he adds 74.5 g of water at 58.0° C. The final temperature of this mixture is 37.6° C. The specific 58.0° C. The final temperature of this mixture is 37.6° C. The specific heat of water is 4.184 Jgheat of water is 4.184 Jg-1-1KK-1-1..

• How much heat was lost by the hot water?How much heat was lost by the hot water?

qqwaterwater = 4.184 Jg = 4.184 Jg-1-1KK-1-1 x 74.5 g x (37.6° C – 58.0° C) = –6.36x10 x 74.5 g x (37.6° C – 58.0° C) = –6.36x1033 J J

• How much heat was gained by the cold water?How much heat was gained by the cold water?

qqwaterwater = 4.184 Jg = 4.184 Jg-1-1KK-1-1 x 75.0 g x (37.6° C – 21.4° C) = 5.08x10 x 75.0 g x (37.6° C – 21.4° C) = 5.08x1033 J J

MEASURING THE HEAT CAPACITY OF YOUR CALORIMETERMEASURING THE HEAT CAPACITY OF YOUR CALORIMETER

• How much heat was gained by the calorimeter?How much heat was gained by the calorimeter?qqlostlost + q + qgainedgained = 0 = 0

qqlost by waterlost by water + q + qgained by watergained by water + q + qgained by calorimetergained by calorimeter = 0 = 0

qqgained by calorimetergained by calorimeter = –(q = –(qlost by waterlost by water + q + qgained by watergained by water))

qqgained by calorimetergained by calorimeter = –(–6.36x10 = –(–6.36x1033 J + 5.08x10 J + 5.08x1033 J) = 1.28x10 J) = 1.28x1033 J J

• What is the heat capacity of the calorimeter?What is the heat capacity of the calorimeter?Heat Capacity of the Calorimeter = Heat Capacity of the Calorimeter =

qqgained by calorimetergained by calorimeter / (T / (Tfinalfinal – T – Tinitialinitial) = ) =

1.28x101.28x1033 J / (37.6° C – 21.4° C) = 78.7 JK J / (37.6° C – 21.4° C) = 78.7 JK-1-1

Heat capacity MUST be positive. If the calculated heat Heat capacity MUST be positive. If the calculated heat capacity of your calorimeter is negative, it is wrong.capacity of your calorimeter is negative, it is wrong.

HEAT OF REACTIONHEAT OF REACTION

• For a solution, the For a solution, the fundamental equation of calorimetryfundamental equation of calorimetry is is

qqreactionreaction = –(q = –(qsolutionsolution + q + qcalorimetercalorimeter))

• OrOr

• qqreactionreaction is called the is called the heat of reactionheat of reaction. If q. If qreactionreaction is measured is measured

at constant pressure, like in our calorimeters which are at at constant pressure, like in our calorimeters which are at atmospheric pressure, then qatmospheric pressure, then qreactionreaction is also called the is also called the change change

in enthalpyin enthalpy (ΔH). (ΔH).

• The The change in enthalpy per mole reactionchange in enthalpy per mole reaction is sometimes is sometimes written as .written as .

MEASURING THE HEAT OF A REACTIONMEASURING THE HEAT OF A REACTION

• In today’s experiment you will measure the change in enthalpy ( ) for In today’s experiment you will measure the change in enthalpy ( ) for the following reaction.the following reaction.

MgMg(s)(s) + 2HCl + 2HCl(aq)(aq) → H → H2(g)2(g) + MgCl + MgCl2(aq)2(aq)

• A student puts 100. mL of 2.0 M HClA student puts 100. mL of 2.0 M HCl(aq)(aq) (an excess) in a calorimeter. (an excess) in a calorimeter.

The temperature of this aqueous solution and the calorimeter is 21.4° The temperature of this aqueous solution and the calorimeter is 21.4° C. Then he adds 0.252 g of MgC. Then he adds 0.252 g of Mg(s)(s). The final mass of this solution is 100. . The final mass of this solution is 100.

g. The final temperature of this solution is 31.1° C. The specific heat of g. The final temperature of this solution is 31.1° C. The specific heat of this solution is 3.62 Jgthis solution is 3.62 Jg-1-1KK-1-1. Assume the heat capacity of the . Assume the heat capacity of the calorimeter is 78.7 JKcalorimeter is 78.7 JK-1-1..

• How much heat was gained by the solution?How much heat was gained by the solution?

qqsolutionsolution = 3.62 Jg = 3.62 Jg-1-1KK-1-1 x 100. g x (31.1° C – 21.4° C) = 3.5x10 x 100. g x (31.1° C – 21.4° C) = 3.5x1033 J J






• How much heat was gained by the calorimeter?How much heat was gained by the calorimeter?

qqcalorimetercalorimeter = (heat capacity of the calorimeter) x (T = (heat capacity of the calorimeter) x (Tfinalfinal – T – Tinitialinitial))

qqcalorimetercalorimeter = 78.7 JK = 78.7 JK-1-1 x (31.1° C – 21.4° C) = 7.6x10 x (31.1° C – 21.4° C) = 7.6x1022 J J






• How much heat was evolved by the reaction?How much heat was evolved by the reaction?

qqreactionreaction = –(q = –(qsolutionsolution + q + qcalorimetercalorimeter))

qqreactionreaction = –(3.5x10 = –(3.5x1033 J + 7.6x10 J + 7.6x1022 J) = –4.3x10 J) = –4.3x1033 J J






• How many moles of MgHow many moles of Mg(s)(s) reacted? The atomic weight of Mg is 24.305 reacted? The atomic weight of Mg is 24.305

g/mole.g/mole.

0.252 g of Mg /24.305 g of Mg mole0.252 g of Mg /24.305 g of Mg mole-1-1 = 0.0104 moles of Mg = 0.0104 moles of Mg






• What is the heat of reaction in kJ/mole?What is the heat of reaction in kJ/mole?

SAFETYSAFETY

• Give at least 1 safety concern for the following procedures that will be Give at least 1 safety concern for the following procedures that will be used in today’s experiment.used in today’s experiment.

• Heating with a flame.Heating with a flame.• Injury from a burn or causing a fire. Be careful. Do not wear loose Injury from a burn or causing a fire. Be careful. Do not wear loose

clothing or long hair. Glass can shatter when heating; wear your clothing or long hair. Glass can shatter when heating; wear your goggles at all times.goggles at all times.

• Using 2.0 M HCl, and MgUsing 2.0 M HCl, and Mg(s)(s)..• These are irritants. Wear your goggles at all times. Immediately clean These are irritants. Wear your goggles at all times. Immediately clean

all spills. If you do get either of these in your eye, immediately flush all spills. If you do get either of these in your eye, immediately flush with water.with water.

• Generating HGenerating H2(g)2(g)..• Hydrogen gas is flammable. Do Hydrogen gas is flammable. Do NOTNOT generate H generate H2(g)2(g) until all the Bunsen until all the Bunsen

burners in the laboratory are extinguished. Wear your goggles at all burners in the laboratory are extinguished. Wear your goggles at all times.times.

• Your laboratory manual has an extensive list of safety procedures. Your laboratory manual has an extensive list of safety procedures. Read and understand this section.Read and understand this section.

• Ask your instructor if you ever have any questions about safety.Ask your instructor if you ever have any questions about safety.

SOURCESSOURCES

• Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Barnes, D.S., J.A. Chandler. 1982. Chemistry 111-112 Workbook and Laboratory Manual. Amherst, MA: University Workbook and Laboratory Manual. Amherst, MA: University of Massachusetts.of Massachusetts.

• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.Saddle River, NJ: Prentice Hall.

• Petrucci, R.H. 1985. General Chemistry Principles and Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Publishing Company.

Documents

A calorimeter is used to measure the amount of heat absorbed or released during a chemical reaction.A calorimeter is used to measure the amount of heat