Purpose of the Experiment

Thermochemistry(Heat of Reaction)

Determine the heat of neutralization for the reaction of a strong acid and base;and for a weak acid with a strong base.

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 -PV 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 + PVThen

H = qp

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

Enthalpy = Heat Transferred

In a chemical reactionH = H products – H reactants

If H >0, then qp >0The reaction is Endothermic.

Heat goes from the surroundings into the system.

If H <0, then qp <0The reaction is Exothermic.Heat goes from the system

into the surroundings.

An example of an exothermic reaction: http://www.youtube.com/watch?v=rdCsbZf1_Ng

Heat Capacity, C

T

q

turein tempera increase

absorbedheat 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 1C-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 1C-1

m

CCs

n

CCm

SubstanceSpecific 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 basewhich 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 = Cs (mass) (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.,

ΔHfreez = - ΔHfus

Melting (fusion) is an endothermic process

solid liquid

)()( solidHliquidHH mmfus

CalorimetryScience of measuring heat based on observing the temperature change when abody 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 waterCs material (mass)material (Tfinal-Tinitial)material = Cs water (mass)water (Tfinal-Tinitial)water

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

Calorimetry

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 reactionfrom the S&T mining dept:

http://www.youtube.com/watch?v=CIGJPWAynDQ

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

Bomb Calorimeter

Time (seconds)0 100 200 300 400 500 600 700 800 900 1000

Tem

pera

ture

(o

C)

10

20

30

40

50

The Computer Display Setup for Today’s Experiments

Parameters:Temperature: 10-50 oCTime: 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.

Time (seconds)0 100 200 300 400 500 600 700 800 900 1000

Tem

pera

ture

(o

C)

10

20

30

40

50

Temperature change is important. Exact time is not important. Temperature will drift toward ambient before and after reactionTransition will be faster if NaOH is added rapidly and well stirred.

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

HCl (or acetic acid)and NaOH mixed, reaction begins

Reaction is completed, heat released, beginslow cooling to ambient

The Heat of Neutralization Experiments

Mixture not stirred fast enough – Resulting line is not vertical.

Time (seconds)0 100 200 300 400 500 600 700 800 900 1000

Tem

pera

ture

(o

C)

10

20

30

40

50

IMPORTANT: Use only 1 ice cube, the entire cube must melt.

The Heat of Fusion Experiment

Ice cube added

Melting complete,begin slow warming

Mixture not stirred fast enough – Resulting line is not vertical.

Checkout1 - Calorimeter (Thermos)

– Return it to the stockroom after experiment.

1 - styrofoam cup – Return it to the stockroom after experiment.

Reagents in Lab_____M HCl (record concentration)_____M CH3CO2H (record concentration)_____M NaOH (record concentration)

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 (neutralize acid spills with solid NaHCO3) NaOH, strong base, pH>14, corrosive

Waste Liquid waste labeled “Heat of Neutralization”

For March 19-22 *Thermochemistry pp 11, 13, 15, & 17

and a calculations page are due.

*Read over “Radiochemistry” pp 19-32 green book

Next Week (March 12-15) – No Class (Spring Recess)