What are some non- examples of endothermic process?

Answer 1

You are given the following chemical rection

N

2

(

g

)

O

2

(

g

)

→

2

N

O

(

g

)

Provide an explanaition for why this reaction is endothermic (both conceptual, and mathematical);

Is this reaction spontaneous at 298 K?

If not, at what temperature does it become spontaneous?

Data given:

Δ

H

∘

f

=

90.4

kJ/mol

for

N

O

and

Δ

S

reaction

=

24.7

J/K

Let's start with the math to get it out of the way. A reaction is said to be endothermic if its change in enthalpy,

Δ

H

reaction

, is positive. We can calculate this change in enthalpy from what the data provides us.

Δ

H

reaction

=

2 moles NO

⋅

90.4

k

J

m

o

l

−

(

1 mole

N

2

)

⋅

0

k

J

m

o

l

−

(

1 mole

O

2

)

⋅

0

k

J

m

o

l

Δ

H

reaction

=

2 moles NO

⋅

90.4

k

J

m

o

l

=

180.8

k

J

The trick here was to be aware of the fact that the enthalpy of formation (

Δ

H

∘

f

) for elements is zero.

Since

Δ

H

reaction

>

0

, the reaction is indeed endothermic.

Conceptually, this reaction is endothermic despite the fact that a bond (between

N

and

O

) is formed; this happens because the

N

2

molecule has its two atoms bonded together by a very strong triple bond, which means that more energy must be put into breaking this bond than is released when the

N

O

molecule is formed.

Now, in order for a reaction to be spontaneous, the sign of

Δ

G

• the Gibbs free energy - must be negative at the given temperature.

We can therefore determine this reaction's spontaneity by using

Δ

G

reaction

=

Δ

H

reaction

−

T

⋅

Δ

S

reaction

Δ

G

reaction

=

180.8

⋅

10

3

J

−

298

K

⋅

24.71

⋅

J

K

=

173.4

k

J

The reaction is not spontaneous at this temperature. We can determine at what temperature the reaction starts to be spontaneous by setting

Δ

G

reaction

=

0

.

0

=

Δ

H

reaction

−

T

⋅

Δ

S

reaction

T

=

Δ

H

reaction

Δ

S

reaction

=

180.8

⋅

10

3

J

24.71

J

K

=

7317

K

This reaction becomes spontaneous at

7317

K

.

As a conclusion, questions about endothermic or exothermic processes revolve around

Δ

H

,

Δ

S

, and

Δ

G

• if a reaction's spontaneity is in question.