11.2

Calorimetry

Calorimetry• The Law of the Conservation of Energy-– Energy is neither created nor destroyed in any physical or

Chemical Change• But to study this requires an isolated system – Three types of systems– Open– matter and energy are easily transferred– Closed– matter can not be transferred but energy is able to

transfer– Isolated– no matter or energy is able to be transferred

Calorimetry

• Calorimetry is the technological process of measuring energy changes using an isolated system called a calorimeter– Studying the energy loss or gain of a known

system of water as a system undergoes physical or chemical change

Calorimetry

• Ex – Ice at its freezing point is placed in water. The temperature of the water is measured when all the ice is melted.

• As the ice melts it removes energy from the water

Heat (energy) gained by the ice = Heat (energy) lost by the water

• Calorimetry can be used to study both physical changes (heat of melting, heat of freezing, etc) or the chemical changes (the energy change when one mole of ammonia is dissolved, the energy change when 2 grams of octane is burnt)

• We will use it only to study the energy changes in chemical changes

• So for energy chemical energy changes

energygained or lost energy gained or lost by the chemical system by the water in the

calorimeter

H Q

• In a simple calorimeter experiment performed involving a burning candle and can of water, the temperature of 100 mL of water is increased from 16.4 to 25.2 when a candle burned for several minutes.

• Calculate the experimental value for the energy given off by the candle.

C C

cH Q

cH mc t

100 4.19 25.2 16.4cJH g Cg C

3687.2cH g Jg C

C

33.68 10 3.68J kJ

• Notes on symbols Symbol Meaning

Total energy change of a system (calorimeter) Energy change of a system undergoing combustionEnergy change of a system undergoing the chemical reaction shownEnergy change of a system undergoing a simple decomposition reactionEnergy change of a system undergoing a simple formation reactionEnergy change of a one mole of a substance undergoing the chemical reaction shown

Q

cH

rH

sdH

f H

r mH

• Notes on symbols Symbol Meaning

Total energy change of a system (calorimeter) Energy change of a system undergoing combustionEnergy change of a system undergoing the chemical reaction shownEnergy change of a system undergoing a simple decomposition reactionEnergy change of a system undergoing a simple formation reactionEnergy change of a one mole of a substance undergoing the chemical reaction shown

Q

cH

rH

sdH

f H

r mH

• Example• When 50 mL of 1.0 mol/L hydrochloric acid is

neutralized by 75 mL of 1.0 mol/L sodium hydroxide in a polystyrene cup calorimeter, the temperature of solution changes from 20.2 to 25.6 . Determine the enthalpy change that occurs in the chemical system C C

cH Q

rH mc t 25.6 20.250 75 125

4.19

t C Cm g g g

Jcg C

125 *4.19 *(25.6 20.2)rJH g Cg C

2828.25rH J 2.83rH kJ

• Is this reaction a endothermic reaction or an exothermic reaction. Explain?

• What does this mean for the energy stored in the reactants compared to the energy stored in the products?

• Molar Enthalpy change of reaction• Defined as the energy change that takes place when

one mole of a substance takes place• We do this because scientist often want to compare

the energy to an equation balanced in moles• So to calculate the molar enthalpy

rr m r m

r

H kJH H molar enthalpy of reactionn mol

H changeof enthalpy kJ of reaction

n number of moles

• Reads as the enthalpy of reaction is equal to the chemical amount (moles) of substance in the reaction times the molar enthalpy of reaction for the substance

• The molar enthalpies are mostly found in text books or given in the question

r r mH n H

• Predict the change in enthalpy due to the combustion of 10.0g of propane. The molar enthalpy of combustion for propane is -2043.9 kJ/mol.

??

2043.9

??10.0??

c

c m

HkJHmol

nm gM

mnM

10.044.11 /

gg mol

0.226705...mol

r r mH n H

0.226705... * 2043.9rkJH molmol

463.3643...rH mol * kJmol

463rH kJ

• What is the energy released when 55.0g of water is formed from its elements?

• To find the molar enthalpy of formation locate the table on pg.5 Of the data booklet.

??

285.8

??55.018.02 /

c

c m

HkJHmol

nm gM g mol

mnM

55.018.02 /

gg mol

3.05216...mol

f f mH n H

3.05216... * 285.8fkJH molmol

872.3085461...rH mol * kJmol

872rH kJ

Calorimetry Problems

• Since the basic principle in Calorimetry is that the energy gained or lost by the water came from or left the system (The chemical reaction)

• So

rH Q

Energy gained/lost by the water

Energy gained/lost by the chemical system

r mn H mc t

• In a research laboratory, scientist perform an experiment to compare the combustion of ethanol to the combustion of octane.

• When 3.50g of ethanol is combusted in a sophisticated calorimeter the temperature of 3.63L of water rises from

• Determine the molar enthalpy of Combustion for the ethanol

19.88 26.18C to C

waterethanol

???????3.50

46.09

c mHnm g

gMmol

4.19

3.63

6.30

Jcg C

m kg

t C

cH Q

c mn H mc t 33.50 3.63 10 *4.19 *6.30

46.09c m

g JH g Cg g Cmol

0.07593..gg

95821.11c mH g

mol

* Jg C

* C

0.07593.. * 95821.11c mmol H J

95821.110.07593..c m

JHmol

1261827.132c mJHmol

1.26c mMJHmol

• Since the temperature of water increased, the reaction is exothermic. To indicate that we use a negative sign to show that the system is losing energy (the reactants have more energy than the reactants)

1.26c mMJHmol

• Determine the final temperature of the calorimeter from the last question containing 3.63L at 21.25 if 3.50 g of octane was burnt in its combustion chamber. The molar enthalpy of combustion of octane is

C

5470.1c mkJHmol

cH Q

c mn H mc t

Octane

5470.1

???3.50

114.26

c mkJHmol

nm g

gMmol

water

4.19

3.63

???

Jcg C

m kg

t C

3.50 *5470.1 3.63 *4.19114.26

g kJ Jkg tg mol g Cmol

167.559...ggmol

* kJmol

15.2097k g * J

g* t

C

167.559...

15.2097

kJ tkJC

11.01662185 C

25.25 11.01662185ft C C

36.27ft C

• Calculate the experimental molar enthalpy when 1.50g of ethanol undergoes combustion in a precise calorimeter and the temperature of the 1.00L of water in the calorimeter increases from

• What assumptions are made in this experiment?

• List the manipulated, controlled and responding variables in this experiment

22.3 31.3C to C

Ethanol Water 1.50

?????

??

c m

c

m g

HH

M

31.00 10??

(31.3 22.3 ) 9.0

m gc

t C C C

H Q

c mn H mc t

ec m w

mH m c t

M

wc m

e

m c tH

mM

31.00 10 4.19 (31.3 22.3 )

1.5046.08

c m

gmol

Jg C Cg C

H g

31.00 10

c m

g

H

4.19 Jg C

(31.3 C

22.3 C )

1.50 g

46.08 gmol

1158451c mJHmol

31.16 10c mkJHmol

• Is the combustion of ethanol an exothermic process or an endothermic process. Explain.

• Determine the experimental error of the experiment if the accepted value for the combustion of ethanol is -1366.8kJ/mol

expermental value -accepted value 100%accepted value

Exp Error

31.16 10 -1366.8100%

1366.8

kJ kJmol molExp Error kJ

mol

15.1%

Homework

• Pg. 494• #3, 4, 5, 6, 7• Pg. 519• #16, 17, 18