Thermochemistry - Ch 16 Academic Chemistry

Summary: The study of energy and it’s transformations.

Objectives: • Understand energy and its forms, including kinetic, potential, chemical, and thermal energies.• Understand the law of conservation of energy and the processes of heat transfer.• Use thermochemical equations to calculate energy changes that occur in chemical reactions and classify reactions as

exothermic or endothermic.• Perform calculations involving heat, mass, temperature change, and specific heat.• Use calorimetry to calculate the heat of a chemical process.

Vocabulary: thermochemistry, exothermic, endothermic, collision theory, activation energy, activated complex, enthalpy, heat of reaction, specific heat capacity, calorimeter, sublimation, deposition, vaporization, evaporation.

CHECKLIST: How to be successful in OTHS Academic Chemistry Pay attention and take notes in class Ask questions in class on material that is not clear Work every page in the practice packet for the unit Check answers to the practice packet online at http://othschem.weebly.com/ Come to tutorials with any chemistry teacher Do the online homework and check solutions once they become available Always do the practice test for every test & ask good questions on review day Keep up with the calendar for the class/be aware of approaching quizzes, tests, & other deadlines Use videos posted on website as a quick and convenient tutorial Read the chapter in the book

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Name___________________________ Per. _____

Memorize:

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Name: Date: Period:

Over the Hill: Investigations of Energy Change in Reactions A chemical reaction is a chemical rearrangement. Before atoms can be rearranged, bonds must be broken. Ultimately, new bonds must be formed. Bond making and bond breaking involve energy. Today you will be introduced to the energy of activation and heat of a reaction. We will look at how these energies are interpreted from a potential energy graph of the reaction pathway. This is a graph that shows how energy changes as a reaction proceeds.

Activation energy (Ea) is a measure of the amount of energy required to reach the highest point on the reaction pathway graph.

The heat of reaction is the energy difference between the energy of the reactants and the products. This value can be either positive or negative depending on if the reaction is exothermic or endothermic. Will exo or endothermic reactions have negative heat of reaction values?

Answer the following questions:

1. The diagram below shows one way to think about a chemical reaction. You can think of the top of thehill as the transition state, an intermediate step in the reaction. This can also be called the activatedcomplex. Some bonds of the reactants are broken to reach the transition state. Then, atoms cometogether to form the bonds in the products.

a. Bond breaking requires energy. How is bond breaking related to the energy of activation, Ea?Explain your thinking.

b. Use the law of conservation of energy to explain why bond making releases energy.

c. The energy diagram shown represents a combustion reaction. Why is a spark required to getthe reaction started?

d. Is the reaction pathway shown here exo or endothermic? How can you tell?

e. Heat must be supplied to reverse the reaction. Do you need more/less/equal amounts of heatfor the reverse reaction to start? Explain.

2. The diagrams at the top of the next page show the changes in energy for two different reactions.a. What is the energy of activation for Reaction 1?b. What is the energy of activation for Reaction 2?c. Which reaction requires more energy to get it started?d. What is the heat of reaction for Reaction 1?e. What is the heat of reaction for Reaction 2?f. Which reaction releases more heat?

3. The two energy diagrams below show the formation of HCl and HBr.

a. Is Reaction 1 exothermic or endothermic?b. Is Reaction 2 exothermic or endothermic?c. Which bond(s) need to be broken to get the reaction started?d. Which bond(s) are formed when this reaction is successful?e. For which reaction is the energy required for bond breaking greater than the energy

released for bond making?

Making Sense Question: Explain the energy of activation and the heat of reaction in terms of bond breaking and bond making.

Check-in Use the energy diagram to answer the following questions: • Which arrow represents the activation energy?• Which arrow represents the heat of reaction?• For the reaction described by this energy diagram,is the energy required to break bonds greater than the energy released upon forming bonds? Explain. • Is this process exo or endothermic?

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Homework Name: ___________________________ Complete the following for homework: Refer to the appropriate diagrams to answer the questions:

1. Examine the above diagrams. What is similar about Reactions 1 and 2?

2. How are Reactions 1 and 2 different from each other?

3. Draw arrows to show the energy of activation for each reaction. Label them.4. Draw arrows to show the heat of reaction for each reaction. Label them.5. Which reaction will take more energy to get started?

Explain.6. Examine the diagram on the right. What does each

arrow represent?

Below is the energy diagram for the reverse reaction of Reaction 2, at the top of the page.

7. Which arrow represents how much energy would be released from the reaction in the form ofheat?

8. Which arrow represents the activation energy?

9. Is the activation energy large or small? What does this mean for the reaction?

10. Which arrow represents the heat of reaction? Is it a positive or a negative number? How do youknow?

11. Which reaction is more likely to occur, Reaction 2 (top of page) or its reverse reaction? Explain.

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Name:_____________________________________ Thermochemistry Worksheet

1. Which of the following processes are exothermic?

A] H2 + Cl2 2HCl + 44 kcalB] H2O (l) + 10.5 kcal H2O (g)C] N2 (g) + 3H2 (g) 2NH3 (g) ∆H= – 22 kcal D] 2NH3 (g) + 22 kcal N2(g) + 3 H2O (g)

2. Indicate whether ∆H is positive or negative:

A] N2 + O2 + 43.25 kcal 2NO ∆H is_______ B] 2 C2H6 + 7O2 4CO3 + 6H2O + 683.5 kcal ∆H is_______ C] the reactants contain more enthalpy than the products ∆H is_______ D] The products contain more enthalpy than the reactants ∆H is_______ E] the surroundings gain heat as a reaction occurs ∆H is_______ F] the surroundings lose heat as a reaction occurs ∆H is_______

3. Rewrite the following equations by expressing the energy change as a term inthe equation:

A] H2O (g) H2O (l) ∆H= – 10.76 kcal

B] 4 Al + 3 O2 2 Al2O3 ∆H= – 803.8 kcal

C] 2 H2SO4 2 SO2 + 2 H2O + O2 ∆H= +130.6 kcal

4. Given : CaO (s) + 2HCl (g) CaCl2 (s) + H2O (g)

Calculate ∆H using the heats of formation in your notes:

Is this reaction exothermic or endothermic? ________________________

Rewrite the equation with the heat term on the correct side:

Is the heat being absorbed or released in this reaction?_________________

5. When carbon (in the form of solid graphite, C) reacts with chlorine gas to form carbon tetrachloride (a liquid),135 kJ of heat are given off. Write a balanced thermochemical equation for this process.

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For the reactions in # 6-14,

(a) use standard heats of formation (ΔHf˚) in your notes to calculate the change in enthalpy (ΔH), also known as heat of reaction. (b) Then classify each reaction as exothermic or endothermic.

6. 2 CO(g) 2 C(s) + O2(g)

7. 2 H2O2 (l) 2 H2O(l) + O2 (g)

8. 4 NO (g) + 6 H2O (g) 4 NH3 (g) + 5 O2 (g)

9. HCl(g) + NaOH(s) NaCl(s) + H2O(l)

10. 2 H2S(g) + 3 O2 (g) 2 H2O(g) + 2 SO2 (g)

11. 2 C2H6(g) + 7 O2(g) 4 CO2(g) + 6 H2O(l)

12. 2 C2H2(g) + 5 O2(g) 4 CO2(g) + 2 H2O(l)

13. CaCO3 (s) CaO(s) + CO2 (g)

14. The following is known as the thermite reaction: 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(s)

Find the heat of reaction (ΔH˚) for the thermite reaction.

The thermite reaction is highly exothermic. Does your answer support this piece of information?

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Name_______________________________ Per. ___

Practice Problems: Thermal Energy (Heat) Calculations

PART 1: HEAT TRANSFER q=mC∆T q = heat energy m = mass C = specific heat ∆T = change in temperature (final - initial)

Sample A How much thermal energy (in Joules) is needed to raise the temperature of 50.0 grams of water from 14oC to 83 oC? (14 000 J)

Sample B How much thermal energy (in Joules) must be added to 50.0 kg of aluminum at –5 oC to raise its temperature to 125 oC? (5 870 000 J)

Sample C A copper wire has a mass of 165 grams. An electric current runs through the wire for a short time and its temperature rises from 21 oC to 39 oC. What quantity of thermal energy has the copper absorbed? (1140 J)

Sample D A 500. gram block of metal absorbs 5016 Joules of thermal energy when its temperature changes from 20 oC to 30 oC. Calculate the specific heat of the metal. (1.00 J/g . oC)

1. How much thermal energy is absorbed by 250 grams of water when it is heated from 10oC to 85oC? (78 000 J)

2. How much thermal energy is absorbed by 60. grams of copper when it is heated from 20oC to 80oC?(1400 J)

3. A 38 kg block of metal is heated from –26oC to 180oC. It absorbs 1,957,000 J of thermal energy duringthe heating. What is the specific heat of this metal? (0.25 J/g .oC)

4. 5.00 kg of ice cubes are moved from the freezer of a refrigerator into a deep freeze. Therefrigerator’s freezing compartment is kept at – 4 oC. The deep freeze is kept at –17 oC. How muchthermal energy does the deep freeze’s cooling system remove from the ice cubes? (-134 000 J)

5. A 200. gram glass at room temperature, 20 oC, is plunged into hot dishwater at 80 oC. If the temperatureof the glass reaches that of the dishwater, how much thermal energy does the glass absorb? (7970 J)

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Name__________________ P d _ Specific Heat Worksheet

Q=(m)(c)(∆T); Units for specific heat = __J___, ∆T= Temp final – Temp initial g ∙ °C

1. What is the specific heat of a substance that absorbs 2.5 x 103 joules of heat when a sample of 1.0 x 104 g of the substance increases in temperature from 10.0°C to 70.0°C? (4.2 x 10-3 J/g.oC)

2. How many grams of water would require 2.20 x 104 joules of heat to raise its temperature from 34.0°C to 100.0°C? (79.7 g)

3. If 200. grams of water is heated from 24.0°C to 100.0°C to make a cup of tea, how much heat was added? (63 500 J)

4. A block of aluminum weighing 140. g is cooled from 98.4°C to 62.2°C with the release of 1080 joules of heat. From this data, calculate the specific heat of aluminum. (0.213 J/g . oC)

5. A cube of gold weighing 192.4 g is heated from 30.0°C to a higher temperature, with the absorption of 226 joules of heat. The specific heat of gold is 0.030 J/g∙°C. What was the final temperature of the gold? (69.2oC)

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