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Worksheet 2

1. Energy and Enthalpy.

A system can exchange energy with its surroundings either by transferring heat or by

doing work. Using q to represent transferred heat and w = - P ΔV, the total energy change

of a system, ΔU, can be represented as

ΔU = q + w = q - P ΔV

where q has a positive sign if the system gains heat and a negative sign if the system loses

it. Rearrange this equation to give the amount of heat transferred. Then, think of two

ways in which a chemical reaction might be carried out: a reaction might be carried out in

a closed container with a constant volume, or carried out in an open flask that keeps the

pressure constant and allows the volume of the system to change freely. Write out the

two equations that would describe these two scenarios in terms of heat transfer and/or PV

work. Lastly, the enthalpy (H) of a system is described by the quantity U + PV. What is

the equation that relates heat to enthalpy at constant pressure?

q = ΔU + P ΔV

q

= ΔU

At constant volume; ΔV = 0

v

q

= ΔU + P ΔV

At constant pressure

p

q

= ΔU + P ΔV = ΔH

p

2. Fill in the blanks.

When reactions are performed in an open container, such as a coffee-cup calorimeter, the

pressure (= atmospheric pressure) does not change and q

= ΔH; in a closed container,

p

such as a bomb calorimeter, the volume (= volume of the bomb) does not change and q

v

= ΔU. Remember that even in an open container, unless a gas is involved in the reaction,

volume changes are usually negligible and ΔH ≅ ΔU.

3. What are extensive and intensive properties and some examples of each?

Intensive properties are values that do not depend on the sample size, such as

temperature and melting point. Thus, a small ice cube might have the same

temperature as a massive iceberg. Extensive properties, like length and volume,

have values that do depend on the sample size. An ice cube is much smaller than an

iceberg.

4. Suppose the enthalpy change when carbon monoxide is burned with oxygen to carbon

dioxide is given by the reaction

(g) ➝ CO(g)

C(s,gr) + ½ O

ΔH = -110.5 kJ

2

at 25°C. What is the internal energy of this reaction?

ΔU = ΔH – PΔV

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