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### Lesson 32 THERMOCHEMISTRY

Objectives:

-The student will explain state change.

- The student will explain the difference between heat and temp.

- The student will explain what specific heat capacity is.

- The student will perform calculations with specific heat capacity.

- The student will explain 2 common ways to measure heat and energy changes in substances.

- The student will apply delta H values to stoichiometric calculations

I. Heat and Temperature are different

a. Heat is a measure of the total amount of energy transferred from an object of high temperature to one of low temperature.

b. Temperatureis a measure of the average kinetic energy of random motion of particles in a substance.

c. A change in temperature is usually accompanied by the addition or removal of energy in the form of heat.

d. The more particles that are present, the more heat must be transferred in order to change the temperature of the substance.

e. This study of the changes of heat that accompany chemical reactions or phase changes is called Thermochemistry.

II. Specific heat capacity relates temperature changes to heat changes.

a. Specific Heat Capacity – the amount of heat energy required to raise the temperature of one gram of that substance by one degree Celsius.

b. The symbol is Cp.

c. Each pure substance has a characteristic specific heat capacity. It must be measured – it cannot be determined from a formula.

d. Substances with low specific heat capacities require little energy to change their temperature.

e. Substances with high specific heat capacities require a great deal of energy to change their temperature.

f. We can calculate a change in heat, knowing the mass of the substance, the specific heat capacity of that substance, and the temperature change in the substance that occurred.

i. H = cp x m x t

ii. t = (Change in temperature) =Temperaturefinal - Temperatureinitial

iii. H = Change in energy or heat, energy units (or Heat units) are in Joules.

g. See examples in class.

III. Measuring changes in heat energy in the lab.

a. We can use a calorimeter to measure changes in heat energy.

b. Two main types

i. “Coffee-cup calorimeter” – this is used to measure changes in temperature which occur during a reaction, usually occurring in water.

ii. “Bomb calorimeter” – this is used when a certain amount of substance is to be oxidized (burned) rapidly, and the heat given off measured.

IV. Stoichiometry and heat calculations

a. We discussed endothermic and exothermic reactions previously.

i. Enthalpy (H) is the heat content of a system at a constant pressure.

ii. The change in enthalpy for a chemical reaction is called the Enthalpy of Reaction (Hrxn)

iii. Hrxn = Hproducts - Hreactants

iv. If Hrxn is negative, the reaction is exothermic (gives off energy)

v. If Hrxn is positive, the reaction is endothermic (absorbs energy)

vi. Example: 4Fe + 3O2 2Fe2O3 Hrxn = -1625 kJ (exothermic reaction)

vii. Example: NH4NO3 NH4+ + NO3- Hrxn = 27 kJ (endothermic reaction)

viii. These enthalpy values can also be written as a part of the equation. If they occur in the reactant side, it is considered endothermic since energy is being added to the beginning of the reaction.

ix. If it is written as one of the products, it is considered exothermic since energy is produced.

- Examples:

4Fe + 3O2 2Fe2O3 + 1625 kJ

NH4NO3 + 27 kJ NH4+ + NO3-

b. The H value, which can be calculated from experiments, can be used as one of the components of the mole ratio in stoichiometry problems.

c. Example:

i. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -891 kJ

ii. Read: “One mole of CH4 plus 2 moles of O2 yields 1 mole of CO2 and 2 moles of H2O, and releases 891 kJ of energy.”

iii. Ratio: 1:2:1:2:-891 kJ

iv. If you burned 2 moles of CH4, you would get 2X as much energy.

Questions:

1. Compare and contrast the terms “heat” and “temperature”.

2. Explain the meaning of the term “specific heat capacity”.

3. Explain the term “enthalpy”.

4. Explain two ways in which changes in a substance’s heat can be measured.

Thermochemistry Problems:

1. How much heat is lost when a solid aluminum ingot with a mass of 4110 g cools from 660.0oC to 25oC? Al Cp = .9025 J/goC

2. Suppose a piece of iron with a mass of 21.5 grams at a temperature of 100.0oC is dropped into an insulated container of water. The mass of the water is 132 grams and its temperature before adding the the iron is 20.0oC. What will be the final temperature of the system. Fe Cp = .4494 J/goC

3. How much heat is required to raise the temperature of 854 g H2O from 23.5oC to 85.0oC?

4. Phosphorus trichloride, PCL3, is a compound used in the manufacture of pesticides and gasoline additives. How much heat is required to raise the temperature of 96.7 g PCl3 from 31.70oC to 69.20oC? the specific heat of PCl3is 0.874 J/goC.

5. Carbon Tetrachloride, CCl4, was a very popular organic solvent until it was found to be toxic. How much heat is required to raise the temperature of 10.35 g CCl4 from 32.1oC to 56.4oC? CCl4 Cp = .8565 J/goC

6. If a piece of aluminum with mass 3.90 g and a temperature of 99.3oC is dropped into 10.0 cm3 of water at 22.6oC, what will be the final temperature of the system? (Recall the density of water is 1.00 g/cm3.)

7. The color for many ceramic glazes comes from cadmium compounds. If a piece of cadmium with mass 65.6 g and a temperature of 100.0oC is dropped into 25.0 cm3 of water at 23.0oC, what will be the final temperature of the system. Cd Cp= .2311 J/goC

8. A piece of an unknown metal with mass 23.8 g is heated to 100.0oC and dropped into 50.0 cm3 of water at 24.0oC. The final temperature of the system is 32.5oC. What is the specific heat of the metal?

9. How many joules are required to heat 692 g of nickel from 22oC to 318oC? Ni Cp = .4442 J/goC

10. How many joules are required to heat 18.2 g of tin from 14.7oC to 47.7oC? Sn Cp = .2274 J/goC

11. Dysprosium was discovered in 1886. Its freezing point is 1400o C and its boiling point is 2600oC. If its specific heat is 0.1733 J/goC, how many joules are required to heat 10.0 g from its freezing point to its boiling point? Dy Cp = .1733 J/gCo

12. Copper has a specific heat of 0.384 52 J/goC. A 105-g sample is exposed to 15.2 kJ in an insulated container. How many degrees will the temperature of the copper sample increase? Cu Cp = .385 J/goC

13. Glass, which is mostly SiO2, is not a good insulator. How much energy does a 1400-g pane of glass lose as it cools from a room temperature of 25oC to an outside temperature of 5.0oC? Si Cp = .7121 J/gCo

14. A swimming pool, 10.0 m by 4.00 m, is filled to a depth of 2.50 m with water at a temperature of 20.5oC. How much energy is required to raise the temperature of the water to a more comfortable 30.0oC?

15. Fatty tissue is 15% water and 85% fat. When fat is completely broken down to carbon dioxide and water, each gram releases 9.0 kilocalories of energy. How many kilocalories are released by the loss of 2.2 kilograms of fatty tissue? (Assume that the fat is completely broken down.)

16. What would the final temperature of the system be if 21.2 grams of iron is added is added to 50.0 ml of water in a closed system. The water is 25oC and the metal is 140oC before they are added together. Cp of water= 4.184 Cp of Iron = .4494 J/goC.

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