TOPIC BWORK, CALORIMETRY, AND CONSERVATION OF ENERGY

• Work: • Energy can also be transferred via work.

• In chemistry, we think of work in terms of gases and their expansion and contraction.

• Consider a gas inside a piston.

• As the gas expands:• particles collide with the piston• energy is transferred from the gas to the piston

• Work is done by the gas on the piston• expressed as w = PΔV [ Work = F d ]

• As the gas expands:• - it does work on the piston• - energy is transferred to the piston• - the piston moves.

• Because of conservation of energy, • any energy lost by one system (in this case the gas),• must be gained in equal magnitude by the other system (in this case the

piston)

• In this work scenario, energy flows from one system to the other.

• Heating and cooling curves:• Indicate graphically changes from one phase to another

as energy is added or removed

• Only the temperature is changed and heating and cooling curves result.

• Starting with a solid below its melting point the following effects can be observed.

• 1. The temperature of the solid increases at a constant rate until it begins to melt.

• 2. When melting begins, the temperature is constant until the solid has all turned to liquid.

• • 3. The temperature of the liquid increases at a constant rate until it

begins to boil.• • 4. When boiling begins, the temperature is constant until the liquid

has all turned to gas.• • 5. The temperature of the gas increases at a constant rate.

• In summary:• Energy is either being used to change the temperature

but not the phase,

• Or it is being used to change the phase and not the temperature.

• A plateau represents a stage when two phases are in equilibrium with one another and the phase change is occurring.

• Calorimetry :

• Calorimetry is a technique used to measure energy changes in a chemical system.

• In chemistry, calorimetry measures energy change in a chemical reaction

• Typically water is used to measure heat released by the reaction (+ΔT of water)

• Or heat being absorbed by the reaction (-ΔT)

• The heat capacity of the water must be known.

• The chemical reaction takes place, and energy is transferred between the chemical reaction and the water.

• If we know the specific heat capacity of the water (a.k.a. the heat bath), then we can calculate the amount of energy that has been lost or gained by the water

• q = m c ΔT

• If the temperature of the water increases – chemical reaction released energy (exothermic).

• If the temperature of the water goes down - chemical reaction absorbed energy (endothermic).

• We assume that the energy lost or gained by the chemical reaction is equal to the energy gained or lost by the heat bath.

• Heat Lost = Heat Gained

• Practice:• (1)• Propane is commonly used in gas grills. A sample of propane with a

mass of 44.0 g, is completely burned in oxygen and in the process it releases 2002 kJ of energy. A transfer of energy from the reaction to the water takes place.

• Assuming the specific heat capacity of water to be 4.200 J g-1 K-1, and that in 100 % of the energy generated is transferred to the wate,r answer the questions that follow.

• (a) In which direction does the energy flow? • (b) Calculate the change in temperature of 20.00 kg of water. • (c) Is the combustion (burning) of propane an exothermic or an

endothermic process? Explain your answer.

• (2) Consider the following:

• (a) How much energy is released, when 1.00 mole (18.0 g) of water at 20.0 oC is frozen, and then cooled to a temperature of -2.00 oC?

• • (b) Considering ONLY the energy released in the phase change

above, what would be the temperature rise observed in a heat bath made of gold, that has a specific heat capacity of 0.129 J/gK and a mass of 0.502 kg?

Specific Heat of Ice 2.05 J/gK

Specific Heat of Water 4.18 J/gK

Specific Heat of Steam 2.08 J/gK

Molar Heat of Fusion for H2O 6.02 J/mol

Molar Heat of Vaporization for H2O 40.7 kJ/mol

Melting Point of Ice 0.00oC

Boiling Point of Water 373 K