Thermochemistry


 

Chapter 5  Thermochemistry

 

Section 5.1 Heat and Energy Changes

 


·        Thermochemistry is the study of the energy changes that accompany any change in matter, either chemical, physical or nuclear

·        Energy that is released in a chemical reaction or a physical change is released as thermal energy, a form of kinetic energy that results from the motion of the molecules

·        It is important to distinguish between the chemical system and the system’s surroundings when studying the transfer of energy

·        A system is represented by a chemical equation. For example the burning of ethyne is:

 

            2C2H2(g)  +  5O2(g)  →  4CO2(g)  +  2H2O(g)  + energy

 

This represents a thermochemical equation.

 

·        The surroundings include that absorbs the energy released.

·        Energy is released or absorbed in the form of heat, q.

·        Heat moves between the system and its surroundings and the measurement of the temperature of the surrounding is used to classify the reaction as exothermic or endothermic.

 

 

 

                                                            The diagram represents an exothermic change when energy is

                                                            released from the system, causing an increase in the                       

 

                                                              temperature of the surroundings.

                                                           In an endothermic change energy is absorbed by the system

                                                           accompanied by a decrease in the temperature of the

                                                           surroundings.

 

 

 

·        The chemical potential energy in a system is converted to heat energy, which is transferred to the surroundings and is used to increase the thermal energy of the molecules of air. Since the molecules of the surroundings have greater K.E. the temperature of the surroundings increases measurably.

·        Chemical systems may be further classified as open systems or closed systems. Open systems allow for both matter and energy to move into and out of the system. A closed system prevents movement of matter.

 

Homework

 

Pg. 300 Nelson “Chemistry 12” do questions #2, 3, & 5

 

 

 

 

 

Calorimetry

 

·        To measure the heat flow, in a chemical change a technique called calorimetry is used. It depends on careful measurements of masses and temperature changes.

 

·        An apparatus that measures heat flow is called a calorimeter

 

·        Three factors are required to calculate the quantity of energy, Q, transferred. These are: mass(m), temperature change (ΔT), and type of substance.

 

·        Therefore q = mcΔT  where c is the specific heat capacity of a substance. Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 gram of a substance by 1 0C or 1 K. The specific heat capacity of water is 4.18 J/g· oC

 

·        Specific heat capacities vary from substance to substance. It is a characteristic physical property of a substance.

 

Substance Specific Heat Capacity
(J.kg-1.oC-1)
Specific Heat Capacity

(J.g-1.oC-1)

water 4180 4.18
ice 2100 2.10
steam 2010 2.01
ethanol 2400 2.40
methanol 2918 2.92
copper 390 0.390
aluminum 900 0.900
iron 444 0.444
glass 840 0.840
mercury 140 0.140
wood 1700 1.70
lead 130 0.130

 

 

Activity: Energy in food

 

Pg 297 describes the activity done as a demo. in class. From this activity, using calorimetric principles, the energy transferred to a specified volume of water can be calculated.

Principles that apply: q(surrounding) = -q(system)

Then, if this assumption is true, the energy liberated by 1 mol of peanut oil can be calculated.

 

 

Sample calculations: Quantity of Heat Calculations

 

If 1.00 L of water is heated from 20.0 oC to boiling to make a pot of coffee, how much heat flows into the water?

 

vol. of H2O = 1.00 L

initial temperature = 20.0 oC

final temperature = 100.0 oC

 

find q, quantity of heat

 

1.      calculate mass of water

      since       D = m/v

      therefore m = Dv

 

      m = 1.00g/mL x 1000 mL

          = 1000 g

 

2.      calculate ΔT

      ΔT = Tf - Ti

            = 100.0 oC – 20.0 oC

            = 80.0 oC

 

3.      use the formula q  = mcΔT

                                    = 1000 g  x  4.18 J.g-1.oC-1  x  80.0 oC

                                    = 334 400 J

                                    = 334 kJ

334 kJ of heat flows into water.

 

 

H.W. pg. 302 do 8 to 13

 

Go the following website:

           http://www.chm.davidson.edu/java/calorimetry/calorimetry.html

 

This web site provides an applet that simulates a calorimetric experiment. Use it to collect data to

do calorimetry problems.

 

You must find the specific heat capacity of water. Use the heater for this.

 

You must find the heat of solution of each solute. Set the liquid to water in each case; you can vary the volume of water in the calorimeter. You can also change the mass of the solute used. Experiment!

 

Try to find the heat capacity of the calorimeter.

Submit your answers on Thursday, Feb 12.

Heat Transfer and Enthalpy Changes

 

·        It is difficult to measure the energy of a chemical system.

 

·        Chemical systems contain different forms of energy , both kinetic and potential.

       Kinetic energy includes:

1.      electron motion in an atom

2.      the vibrations of atoms connected by chemical bonds

3.      the rotation of molecules

4.      the translation of molecules; the motion of the molecule through space.

 

Potential energy includes:

1.      the forces of attraction between molecules

2.      chemical bond energy present within the molecule

3.      energy associated with electrons

4.      forces holding nuclear particles together

 

·        The total of these energies is known as the molecular heat content or molecular enthalpy.

 

·        The heat content of 6.02 x 1023 molecules gives the molar heat content or the molar enthalpy

 

·        The symbol H represents the molar enthalpy.

 

·        The enthalpy of a system is very difficult to measure, for this reason the change in enthalpy or ΔH is determined.

 

·        Enthalpy change is related to energy absorbed or released to the surroundings when a system changes from reactants to products.

 

·      Enthalpy change of a system equals the quantity of heat that flows from the system to its surroundings, or from the surroundings to the system.

 

 

                            high potential energy                                     high kinetic energy              In this example of an

                                                                                                                             exothermic change, the

                                                                                                                                                      the change in potential energy

Energy                                                                                                                  of the system, ΔH, equals the

                                                                                                                             change in kinetic energy of the

                                                                                                                             surroundings, q.

 

 

 

                            

                            low kinetic energy                                             low potential energy

 

 

 

                                                       Reaction progress

 

·      This assumption applies as long as there is no production of a gas.

 

·      This is consistent with the law of conservation of energy, that states: energy can be converted from one form to another, or transferred from one substance to another, but the total energy of the system and its surroundings remains constant.

                 ΔHsystem  = +/- |qsurroundings|

 

·     Energy changes in chemical systems are measured at standard conditions of temperature and

     pressure, such as SATP, before and after the reaction.

 

·     Under these conditions , the enthalpy change is due to the change in the potential energy of the

     system because the kinetic energies of the system’s molecules stay constant.

 

·     Enthalpy changes can be observed during phase changes , chemical changes, or nuclear reactions.  See table 2 pg. 304 for examples of different enthalpy changes.

 

·     The magnitude of the enthalpy change varies from reaction to reaction. See figure 7 pg 304 for specific values.

     In general the enthalpy change of a physical is low, in the order of magnitude of 100 to 102 

     kJ/mol, chemical reactions are in the order of magnitude of  102 to 104 kJ/mol, and nuclear

     reactions are typically in the order of 106 to 1012 kJ/mol

 

Homework pg. 304  do #14, 16

                   pg. 356 do # 2, 4, 5

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