T6_ehthalpy

8/13/2019 T6_ehthalpy

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Collect

2 Styrofoam cups & a cup lid, Digital thermometer

Stop watch (from TA)

Prepare

400 mL beaker

50/100 mL graduated cylinder* Student on duty must bring hot

water in a kettle from water fountain

T6-The Enthalpy of Reactions


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Objective & Skills

Objective

Determine the heat capacity of calorimeter and Heat of neutralization

Heat of solution

Heat of redox reaction

Use Hess law (the enthalpy of reactions are additive) to

calculate the heat of formation (Hf) for MgO

Skills

Use of digital thermometer

Setup a simple calorimeter

Operation of graduated cylinder


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H: enthalpy change,

or heat of reaction at constant

pressure

Exothermic reaction, H < 0

Endothermic reaction, H > 0

The expression of H

,

Numerical value

Unit: kcal/mol, kJ/mol

Notice the significant digits

Ex: -14 kcal/mol

H+(aq) + OH-(aq)

H2O(l)

H < 0

H

NH4+

(aq) + NO3-(aq)

NH4NO3(s) + H2O(l)

H > 0

H

Introduction:

Enthalpy of Reaction


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Introduction: Calculating H

Setup a simple calorimeter as an

isothermal system (qsystem= 0)

Hrxn = - (qsoln qcalorimeter) qcalorimeter: C T

C: the heat capacity of calorimeter (cal/oC)

qsoln = m s T

qsoln: change in heat of solution (cal)

m: mass (g)

s: specific heat of solution (1.0 cal/g-oC)

T: change in water temperature (oC)

Two Styrofoam cups

stacked together400 mL beaker

Thermometer and probe


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Procedure 1: Determine the

Heat Capacity of Calorimeter (1)

Measure 50 mL cold waterinto calorimeter

Wait 3~5 min till equilibrium

Record temperature

Place graduated cylinder on

the inner side of the lab

bench to avoid breaking it


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Prepare warm water (10~15 oC

warmer than cold water)

Mix hot and cold water in

beaker thoroughly

Obtain warm water with temp.

ca. 35~40 oC

Measure 50 mL warm water with

graduated cylinder

Place the thermoprobe in the center of

soln and take the equilibrium temp.

The probe should not touch the walls ofcylinder


Heat Capacity of Calorimeter (2) Take temp. of warm water


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Pour the prepared warm

water into calorimeter

Place the lid on

Put the thermoprobe in cup

Shake and mix the soln

Record the equilibrium temp.

(the highest temp. forexothermic rxn.)


Heat Capacity of Calorimeter (3)

Calculation:

q1 (heat released by warm water)

= q2 (heat absorbed by cold water)+ q3 (heat absorbed by calorimeter)


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Neutralization of HCl and

NaOH

Measure 50.0 mL of 1.0

M HCl into calorimeter;

record the equilibrium

temp. Measure another 50.0 mL

of 1.0 M NaOH with

graduated cylinder;

record equilibrium temp.

Place the lid on

Put the thermoprobe

in cup

Shake and mix thesoln

Record the

equilibrium temp. (the

highest temp. forexothermic rxn.)

Pour NaOH(aq)

into calorimeter


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Use 50.0 mL of 1.0 MCH3COOH to replace

the HCl(aq) Repeat the above

procedures Determine the molar

heat of neutralizationof acetic acid and

Neutralization of CH3COOH and

NaOH


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Procedure 4: Molar Heat of

Solution of Ammonium Nitrate Measure 50.0 mL of H2O into

calorimeter

Take the equilibrium temp. Weigh ca. 4 g NH4NO3(s) and

record the accurate wt.

Add NH4NO3(s) into

calorimeter

Shake and mix the soln

Take the equilibrium temp.

(the lowest temp. for

endothermic rxn.)

Solid reactants must be

dissolved completely


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Procedure 4: Molar Heat of

Formation of Magnesium Oxide Heat of reaction of Mg and HCl(aq)

Measure ca. 0.2 g of Mg

React with 100.0 mL of 1.0 M HCl in

calorimeter

Take the equilibrium temp.

Heat of reaction of MgO and HCl(aq)

Measure ca. 0.7 g MgO

React with 100.0 mL of 1.0 M HCl in

calorimeter

Mg(s) + 2H+

(aq) Mg2+

(aq) + H2(g) H1

MgO(s) + 2H+

(aq) Mg2+

(aq) + H2O(l) H2

H2(g) + O2(g) H2O(l) Hfo

(H2O)= -68.4 kcal/mol

Mg(s) + O2(g)

MgO(s)

Hfo

(MgO)Hfo(MgO)= H1 H2 + Hfo(H2O)


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Notices

Determine the heat capacity of calorimeter

The temperature difference of hot and cold water shouldbe about 10 oC

The student on duty should carry the hot water from waterfountain in a kettle

Mix hot and cold water thoroughly in a beaker, thenmeasure with graduated cylinder and take temperature

The tip of the thermoprobe should be in the center of thesolution to avoid touching the walls of the container(causing the temperature to be lower)

After measuring the warm water temperature with

thermoprobe, rinse probe with tap water to let it return toroom temperature, then place back into calorimeter

When mix warm water with cold water in calorimeter,equilibrium temperature is achieved with speed,

temperature should be read immediately


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Wash and wipe dry the Styrofoam cups after each trial

Determining the equilibrium temperature:

Exothermic reaction: Stop recording temperature when thetemperature rises to the highest point and starts to decrease

Endothermic reaction: Stop recording when thetemperature of solution decreases to the lowest point andstarts to increase

Solid reactants as NH4NO3(s), Mg(s), MgO(s) need to be mixedthoroughly for complete reaction to avoid experimental errors

Notices


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Reminders

Mg and MgO are not easily dissolved in acid. After the

reaction, observe there are any remaining solids causing

inaccuracy (include the observation in report) Recycle

Waste liquids: the salt solution resulting fromneutralization can be discarded in sink

Styrofoam cups and lids should be handed back in forreuse

Broken lids and cups can be discarded

Bring calculators to class, scientific calculators are

recommended Show all the calculations in your report:

Notice about:/, significant figures, and units (kcal/molor kJ/mol)

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