Chemistry - A Molecular Science

Chapter 9 Reaction Energetics

Example 9.1 What is the energy change of a system that absorbs 500 J of heat and does 300 J of work?

The system absorbs 500 J (q = +500 J), so its energy increases by 500 J. However, doing 300 J of work (w = -300 J) requires

the expenditure of 300 J. Thus,

ΔE = q + w = 500 J -

300 J = +200 J. The system increases in energy by 200 J.

9.2

ENTHALPY

Reactions are frequently carried out in vessels open to the atmosphere, so gases that are produced are lost to the atmosphere, keepi

ng the pressure constant. The reactants are

initially at room temperature and the products are

eventually

at room temperature, so the

reaction can be viewed thermodynamically as being carried out at constant temperature.

†^

Conditions of constant pressure and temperat

ure are quite common, so our treatment of

thermodynamics is limited to processes carried

out under these conditions. The gases that

escape to the atmosphere carry some of the ener

gy change of the reaction with them. This

energy is lost, so chemists typically discuss th

e heat of reaction rather than the energy of

reaction. The heat of a reaction at constant pressure is so common that it is given a special name, the

enthalpy or heat of reaction

, Δ

H.

† Thermodynamics is concerned only

with the initial and final states.

Thus, a reaction can be treated at

constant T and P as long as the

initial and final conditions are t

he same, even if the conditions vary

during the reaction.

Products

at

higher potential

energy

Reactants

at

higher potential

energy

Reactants

at

lower potential

energy

Products

at

lower potential

energy

H > 0, endothermicD

H < 0, exothermicD

(a)

(b)

Energy

Energy

surroundings

Reactants

®

Products

Energy changes result because

the potential energies of the products and reactants are

different. The energy required for an endothermic

reaction is used to convert reactants into

products that are at higher potential energy (Figure 9.1a);

i.e

., endothermic reactions are

said to go “uphill” in enthalpy. The energy comes from the surroundings, which causes them to cool, so

the reaction container of an endothermic reaction, which is part of the

surroundings, cools as the reaction proceeds

. The heat given off in an exothermic reaction

is produced when the reactants are converted to products that are at lower potential energy (Figure 9.1b);

i.e

., exothermic reactions are said to go

“downhill” in enthalpy. The energy

released in an exothermic reaction goes into the surroundings and causes them to warm. Thus,

the reaction container of an exothermic

reaction warms as the reaction proceeds

.

STANDARD STATES

Figure 9.1 Sign of enthalpy change and energy flow

Thermodynamic properties, such as enthalpy, va

ry with the state and concentration of the

substance, so care must be taken to

make comparisons unde

r similar conditions.

Consequently, thermodynamic properties are ty

pically reported for processes in which the

reactants and products are all in their

standard states

, which are defined as follows:

The enthalpy of the system a) increases in an endothermic reaction as energy is transferred from the surrounding to the system, and b) decreases in an exothermic reaction as energy is transferred from the system to

the surroundings. Therefore, the

surroundings cool in (a) and warm in (b).

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