Lesson 4 - Calorimetry and Hess's Law

Question 1

Calorimeter

Answer 1

A device used to measure the heat released or absorbed by a physical/chemical process occurring within it

Question 2

Bomb calorimeter

Answer 2

Allows reactions to occur at a constant volume

Question 3

Constant-pressure calorimeter

Answer 3

Open to the atmosphere and maintain a constant pressure (room pressure)

Question 4

Styrofoam cup calorimeter

Answer 4

Typically used as the insulated vessel since styrofoam prevents heat from leaving

Question 5

Calorimetry

Answer 5

The study of changes associated with physical/chemical processes using a calorimeter

Question 6

Calorie

Answer 6

The original unit for heat transfer (used water as the standard)

Question 7

First law of thermodynamics in terms of calorimetry

Answer 7

The energy of the universe is constant.
This ensures that any energy absorbed/released by the system must be released/absorbed by the surroundings.

Question 8

Second law of thermodynamics in terms of calorimetry

Answer 8

The enrgy always moves from a higher to lower temperature until both objects reach the same temperature.
This ensures that the temperature of the surroundings will represent the thermal equilibrium of the entire calorimeter.

Question 9

Characteristics of a simple calorimeter

Answer 9

• stacked with 2 vessels, like styrofoam cups
• relatively accurate and cheap
• polystyrene is a good insulator
• liquid/solid products remain in the cup but gases can escape through the holes, providing constant pressure

Question 10

Assumptions in calorimetry

Answer 10

1. Assume that the system is isolated (even though gases can escape)
2. Assume that the energy exchanged with the cups, stirring rod is small enough to be ignored (insignificant)
3. Assume that if a substance is dissolved in the water, the solution is dilute enough that it retains the properties of water (density, heat capacity)
4. Assume the process takes place under constant pressure.

Question 11

Qw=

Answer 11

Qw=mwcwΔTw = mwcw(T2,w-T1,w)

Question 12

Qr=

Answer 12

Qr=-Qw

Question 13

Qr=

Answer 13

Qr=ΔHr

Question 14

Why can we calculate large numbers of enthalpies of reactions from small number of measurements?

Answer 14

• ΔH is an extensive property
• ΔH changes when a reaction is reversed
• ΔH values of reactions can be summed with the ΔH values of other reactions to represent net reacrions

Question 15

Hess’s Law

Answer 15

Hess’s Law states that the enthalpy change of a physical/chemical change depends only on the initial and final conditions of the process (state function)

Question 16

How to calculate enthalpy change with Hess’s Law

Answer 16

The enthalpy change of a multi-step process is the sum of enthalpy changes of each individual step