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Purpose of the Experiment

Thermochemistry

(Heat of Reaction)

Determine the heat of neutralization for

the reaction of various acids and bases.

Determine the heat of fusion of ice.

Thermodynamic Definition of Enthalpy (H):

H = E + PV

E = energy of the system

P = pressure of the system

V = volume of the system

Definition of Enthalpy

What is the Heat of Reaction?

Recall, by definition a change in energy equals heat transferred (q) plus work (w):

E = q + w

Consider a process carried out at constant pressure.  At constant pressure, work involves only a change in volume.  We can then substitute -PV  for w.

E = qp - PV

Then if we want to solve for the heat transferred, qp, at constant pressure, we simply rearrange the equation.

qp = E + PV

At Constant Pressure

Recall our original definition of enthalpy:

H = E + PV

Then for a change in enthalpy:

H = E + (PV)

If we set P constant, then:

H = E + P V

Since

qp = E + PV

Then

H = qp

The change in enthalpy,  H, is then equal to the heat transferred at constant pressure, qp

Enthalpy = Heat Transferred

In a chemical reaction

H = H products   H reactants

If H >0, then qp >0

The reaction is Endothermic.

Heat goes from the surroundings into the system.

If H <0, then qp <0

The reaction is Exothermic.

Heat goes from the system into the surroundings.

An example of an exothermic reaction:

Heat Capacity, C

��C�� is an extensive property; so a large object has a larger heat capacity than a small object made of the same material.

Using the Equation:

Looking at the figures on the left, it can be seen that the temperature change is constant, but the heat absorbed by the larger object is greater.

This results in a larger heat capacity for the larger object because more heat is absorbed.

• Specific heat capacity: The energy (joules) required to          raise the temperature of 1 gram of substance by 1C

Unit:  J g-1K-1  or  J g-1 1C-1

• Molar heat capacity: The energy (joules) required to raise the temperature of 1 mol of substance by 1C

Unit:  J mol-1 K-1 or  J mol-1 1C-1

Substance

Specific Heat, Cs

(cal/gram��C)

(J/kg ��C)

Pure water

1.00

4,186*

Wet mud

0.60

2,512

Ice (0 ��C)

0.50

2,093

Sandy clay

0.33

1,381

Dry air (sea level)

0.24

1,005

Quartz sand

0.19

295

Granite

0.19

294

1 calorie = 4.186 joules

*The high heat capacity of water makes it ideal for storing heat

in solar heating systems.

Neutralization

HClaq + NaOHaq NaClaq + H2O

The reaction between an acid and a base

which results in a salt plus water.

Another example, cyanic acid and a hydroxide ion.  If we use KOH, what salt will form?

For example, hydrochloric  acid and sodium hydroxide:

acid  +  base       salt     +  water

Heat of Neutralization

Energy released by reaction = Energy absorbed by solution

Cs = q / [(mass) (Tfinal-Tinitial)]

Net ionic equation for neutralization:

H+(aq) + OH-(aq) H2O(l)

Specific heat capacity, Cs, is defined as the quantity of heat transferred, q, divided by the mass of the substance times the change in temperature.  A value of Cs is specific to the given substance.

q = (mass) Cs (Tfinal-Tinitial

This can then be rearranged to solve for the heat transferred.

Enthalpy of Fusion (Melting)

Enthalpy of Fusion is defined as the heat that is absorbed when the melting occurs at constant pressure.  If the substance freezes, the reaction is reversed, and an equal amount of heat is given off to the surroundings; i.e.,     ��Hfreez = - ��Hfus

Melting (fusion) is an endothermic process

solid

liquid

Calorimetry

Science of measuring heat

based on observing the

temperature change when a

body absorbs or loses

energy as heat.

A calorimeter can be created by doing something as simple as inserting one Styrofoam cup inside another.

A Calorimeter may be used to determine the Heat Capacity, Cs, of a material by measuring the temperature change when a known mass of the material at a higher temperature is placed in a known mass of water, usually at room temperature, and the system is allowed to reach a final intermediate temperature.

Heat lost by hot object = Heat gained by cold water

(mass)material Cs material (Tfinal-Tinitial)material = -(mass)water Cs water (Tfinal-Tinitial)water

Note:  The heat capacity is related to the atomic mass and the intermolecular forces in the material.

Calorimetry

is a mnemonic – an easy way to remember mCsT

A Calorimeter may be used in a similar manner to determine the enthalpy change associated with other processes, such as:

• Chemical reactions* (bond energies)
• Phase changes* (intermolecular forces)
• Mixing (intermolecular forces)
• Solvation (intermolecular forces)

Calorimetry

*These are the processes you will be learning today.

Have you ever wondered about how they determine the calories in food?  They use a Bomb Calorimeter. It can be used to determine the caloric value of food and of fuels, by burning them in excess oxygen and measuring the amount of heat evolved.  A basic combustion reaction:

An example of an exothermic reaction

from the S&T mining dept:

CxHy  +  O2(excess) --> x CO2  +  y/2 H2O  +  heat

Bomb Calorimeter

The Computer Display Setup for Today��s Experiments

Parameters:

Temperature:  10-50 oC

Time: 0-1000 seconds

(Check:  Probe should display 15-25 oC resting on lab bench

and should read higher when warmed by hand.)

If probe displays less than 15 oC, notify your TA.

Temperature change is important.  Exact time is not important.

Temperature will drift toward ambient before and after reaction

Transition will be faster if NaOH is added rapidly and well stirred.

(That is, you will have a more nearly vertical temp. rise.)

HCl and NaOH mixed,

reaction begins

Reaction is completed,

heat released, begin

slow cooling to ambient

The Heat of Neutralization Experiments

Mixture not stirred fast enough Resulting line is not vertical.

IMPORTANT: a.) Use only 1 ice cube; b.) the entire cube must melt.

The Heat of Fusion Experiment

Melting complete,

begin slow warming

Mixture not stirred fast enough Resulting line is not vertical.

Checkout

1 - Calorimeter (Thermos)

1 - Styrofoam Cup

– When all experiments are completed, rinse with water and

return all 3 items to the stockroom.

Reagents in Lab

(Record concentration from carboys onto datasheets.

Note:  Concentrations may vary from one class to the next.)

_____M HCl  (strong acid)

_____M CH3CO2H (weak acid)

_____M NaOH (strong base)

Important:

Use distilled water from carboys*,

NOT from the tap.

(*Distilled water from the tap is normally not at room temp.)

Hazards

HCl, strong acid, corrosive

CH3CO2H, weak acid, corrosive

NaOH, strong base, corrosive

Waste

Liquid waste labeled ��Heat of Neutralization��

Due:  Thermochemistry pp 91-102 in the lab packet

and a calculations page

and Dimensional Analysis #4-5 in the first book*

4.1 & 4.2:  a, c, f, & h only; 5.1 5.4:  a & c only

pp 103-118 in the lab packet

For April 7-9

*Lost your 1st book? Go to http://web.mst.edu/~tbone/Subjects/TBone/Chem2.html

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