Chem100- Thermochem

7/28/2019 Chem100- Thermochem

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Thermochemistry study of heat changes in

chemical reactions

Specific heat (s) = cal or jouleg x C g x C

- amount of heat needed to raise the temperature of

one gram of substance by 1 C

ex. specific heat of NaCl = 0.204 cal

g x C

sp heat of H2O = 1.00 cal or 4.18

g x C g x C1 cal = 4.18 joules


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Heat capacity (C) - amount of heat needed to

raise the temperature of a given amount of

substance by 1 C

C = m x s

= g x cal = calg xC C


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4/12

Problem 1

If 8.5 g of metal, initially at 82 C is placed in 45.0 mL H2O at

20 C and the final temperature of metal and H2

O is 22C.

Calculate the specific heat (s) of the metal in cal/g x C

q metal = - q water

q metal = m x s x t = - q water = - (m x s x t )

Heat given off by the metal = heat absorbed by the H2O

45g x 1cal/ g x C x (22-20 C) = -90 calories absorbed by

the H2O

s metal = - 90 cal = 0.18 cal

(8.5g)x -60 C g x C


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Problem 2

How many calories would be required to raise

the temperature of 45 g NaCl from 25 C to 85C. The sp heat of NaCl is 0.204 cal/ g x C

q NaCl = m x s x t

45g x 0.204 cal x 60 C = 551 calories

g x C


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Problem 3

Calculate the no. of joules of heat required to raise the

temp. of 125 g of Ag from 100 C to 250 C. The sp heat of

Ag is 2.34 x 102 joule/ kg x K

q Ag = m x s x t

125 g x 1 kg x 2.34 x 102

j x 150K = 4.39 x 102

j1000 g kg x K


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Enthalphy (H) = heat change or change in heat

content in a chemical reaction

H rxn = Hf products - Hf reactants

Q (Heat flow) in a reaction at constant

pressure = H for the reaction(Hrxn)

Hf of a compound, is the Hrxn to form one mole of the

compound from the elements in the standard state.

* The heat content of a free element in the std.state is zero


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Prob. 4

2C2H6 (g) + 7O2(g) 4CO2 (g) + 6H2O + 750 kcal or

3135 kja.Is the reaction exothermic or endothermic?

Exothermic rxn. (heat is given off)

A + B C + D + kj. (heat)

A + B C + D

H = - kcal or kj

Endothermic rxn. (heat is absorbed)

A + B + heat C + D

or A + B C + D

H = kcal or kj


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2C2H6 (g) + 7O2(g) 4CO2 (g) + 6H2O + 750 kcal or

3135 kj

a. Is the reaction exothermic or endothermic?

b. What is the Hrxn ?

Hrxn = -750 kcal or -3135 kj

c. What is the Hrxn for the reverse reaction?

Hrxn =750 kcal or 3135kj

d. What is the heat flow (Q) if the total equation isdivided by 2 ?

(Q) = -750/2 = - 375kcal


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Problem 5

Calculate the Hrxn:


2 CH3OH (l) + 3O2(g) 2CO2 (g) + 4H2O (g)

Hf (kcal/mole) ( -57.0) (-94.1) (-57.8)


Hrxn = 2 (Hf CO2) + 4(Hf H2O) 2n(HfCH3OH)

= 2 n (-94.1 ) + 4n (-68.3) 2n (-57.0)

= - 347.4 kcal


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Problem 6

Calculate the amount of heat produced from the

combustion of 10.0g C2H6 (w/c burns in O2)

10.0g C2H6 x 1mole C2H6 x 750kcal

30g C2H6 2 mole C2H6

=125 kcal or 522kj


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Problem 7

Chem100- Thermochem

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