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Unit 13-Thermochemistry

Unit 13-Thermochemistry. AHS Chemistry 2012-2013. Work. Created. Destroyed.

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Unit 13-Thermochemistry

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  1. Unit 13-Thermochemistry AHS Chemistry 2012-2013

  2. Work Created Destroyed WHAT IS ENERGY?Energy is the ability of a physical system to perform______.The First Law of Thermodynamics states that energy cannot be _______ or ________. However, it can be ________ ___into another type of energy! This is also referred to as the ____________________________ Transformed Law of Conservation of Energy

  3. Types of Energy 1. Kinetic Energy-Energy of Motion *Remember, temperatureis a measure of average internal kinetic energy. So, the HIGHER the temperature, the MORE MOVEMENTthere is in the substance! Heat (Q) also known as Thermal Energy is a form of kinetic energy-transfer of kinetic energy from a substance at a HIGHERtemperature to a system at a LOWERtemperature. 2. Potential Energy-STOREDenergy. *In chemistry, potential energy is usually due to one of two things – either BONDING or INTERMOLECULAR FORCES. Chemical Energy-potential energy that is made available during a chemical reaction in which old bonds are broken and new bonds are formed. • Energy is RELEASED when bonds form. • Energy is REQUIRED to break bonds.

  4. **Label as KE, PE, TE, CE • Energy of motion. • Energy stored in chemical bonds. • Heat energy. • Temperature is a measure of average _____ energy • Stored energy, or energy of position. • Changes when a substance is heated or cooled. • Changes during a phase change.

  5. Transfer of Heat • CONDUCTIONis the transfer of heat between objects by direct contact with one another (meaning they have to touch). • CONVECTIONis the transfer of heat through a fluid (liquid or gas) caused by the circulation or movement of molecules. • RADIATIONis energy that is transmitted in the form of waves (electromagnetic radiation) and transformed into kinetic energy. Also, it can travel through a vacuum – like outer space!

  6. **Label as Conduction, convection or radiation • Must have direct contact • Travels as waves • Travels only through fluids • Matter must be present (2 answers) • Can travel through a vacuum • Heat from the sun. • Heat from hot water to cold water. • Heat from your hand to a piece of ice.

  7. Units of Heat Energy (Q) • Joule is the SI Unit of heat • A calorie is defined as the amount of energy needed to raise the • temperature of 1 gram of water by 1 °C. • It takes 4.184 joules of energy to do the same thing. 1 calorie (cal) = 4.184 joules (J) 1000 calories (cal) = 1 Calorie (Cal) = 1 kilocalorie (kcal)

  8. Types of Phase Changes Physical changes!!! Vaporization: liquid changing to vapor (evaporation) ENDOTHERMIC Condensation: vapor changing to liquid (formation of dew) EXOTHERMIC Melting: solid changing to liquid (ice to water) ENDOTHERMIC Freezing: liquid changing to solid (water to ice)EXOTHERMIC Sublimation: solid changing to vapor (dry ice) ENDOTHERMIC Deposition: vapor changing to solid (formation of frost from water vapor in the air) EXOTHERMIC

  9. PHASE EQUILIBRIUM:where one phase change is occurring at an EQUAL rate to the opposite phase change. Phase Diagram– shows what state of matter is present at different Temperatures and Pressures. The NORMAL BOILING POINTis the boiling point that occurs at a pressure of 1 atm (100°C for water). The NORMAL MELTING POINTis the melting point that occurs at a pressure of 1 atm (0°C for water). TRIPLE POINT:the point at which all three states of matter coexist together. Video of triple point: http://www.youtube.com/watch?v=BLRqpJN9zeA

  10. ** Phase diagram Answer the questions about the phase diagram for CO2 shown below. If a sample of CO2 has a temperature of – 100°C and a pressure of 100 atm, what state is that sample in? If you take the sample from part a and decrease the pressure to 0.01 atm, what phase change has been made? If you take the sample from part a and increase the temperature to – 20°C, what phase change has been made?

  11. Heating/Cooling curves DIAGONAL LINES = HEATING/COOLING OF ONE PHASE *Kinetic energy (velocity of the molecules) is changing HORIZONTAL LINES = PHASE CHANGES *Potential energy is changing (intermolecular bonds are strengthening or weakening) *We can determine the boiling point and melting/freezing point from the graph Thermal (heat) energy is a type of kinetic energy however, kinetic energy does NOT change during a phase change!

  12. **Heating/Cooling Curve Label the diagram with the following information: solid, liquid, gas, boiling, melting, freezing, condensation, temperature, time Do phase changes occur on the diagonal sections or the horizontal (flat) sections of the curve? What happens to the temperature of the molecules during a phase change? Describe how the kinetic energy of the molecules changes during a phase change. Describe how the potential energy of the molecules changes during a phase change.

  13. 18 Calculating Specific Heat The Greek letter Δ means “change in”

  14. Specific Heat capacity-Cp Specific heat capacity of water: is the amount of heat required to raise ONE GRAM of a substance ONE DEGREE CELCIUS. • intensive physical property Low Cp: able to change temperature relatively rapidly because they conduct heat very well. EX: most metals (cp of gold = 0.129 J/g•°C High Cp: take longer to heat up because they can absorb large quantities of heat. Because they do not conduct heat well, these substances make good insulators. EX: water has a cp= 4.184 J/g•°C

  15. CALCULATING ENTHALPY CHANGES OF A PHASE CHANGE MCATH (or Q) = mcp(T) ENTHALPY OF FUSION ΔH = (moles) x (Hfus) ENTHALPY OF VAPORIZATIONΔH = (moles) x (Hvap) Ex 1: Calculate the Enthalpy change that occurs when 10.0 g of steam at 150.oC cools to liquid water at 50.0 C

  16. Enthalpy (∆H or q) a certain amount of energy is required to get the reaction started is called the ACTIVATION ENERGY(Ea), The ACTIVATED COMPLEXis a short-lived structure that exists while old bonds are breaking and new bonds are forming. The ENTHALPY OF REACTION(ΔH) is the energy lost or gained by the system. It is calculated as the difference between the energies of the products and the reactants. A CATALYSTspeeds up a reaction by DECREASINGthe activation energy

  17. **Enthalpy calculations: 1. Calculate the amount of energy needed to vaporize 19.2 mol of liquid water. Hvap = 40.79 kJ/mo 2. Calculate the amount of energy RELEASED when 1044.2 g of water are cooled from 98.6°C to 32.3°C. Cp(liq water) = 4.184 J/g•°C 3. The temperature of a 95.4-g piece of copper increases from 25.0oC to 48.0oC when the copper absorbs 849 J of heat. What is the specific heat of copper 4. When 435 J of heat is added to 3.4 g of olive oil at 21oC, the temperature increases to 85oC. What is the specific heat of the olive oil?

  18. Reaction Energy Diagram • 1. Label the following on the energy diagram: * Activated Complex, Reactants (R), Products (P), Activated Complex (*), Ea, ΔH • 2. Calculate the activation energy. • 3. Calculate the enthalpy of the reaction. • 4. Draw a dashed line to show what would happen if a catalyst reduced the activation energy by 200 kJ/mol.

  19. ENTHALPY OF REACTION (ΔHrxn) : The Basics Is the reaction endothermic or exothermic? For exothermic reactions, heat is listed as a PRODUCT(ΔH value would be negative!) 2 H2 + O2 2 H2O + 483.6 kJ ΔH = -483.6 kJ For endothermic reactions, heat is listed as a REACTANT(ΔH value would be positive!) 483.6 kJ + 2 H2O  2 H2 + O2ΔH = +483.6 kJ

  20. The Basics cont’d When the enthalpy change for a reaction is given, it references the heat change for those exact stoichiometric amounts (always assumed to be the equation balanced with the lowest whole number ratio unless specified otherwise): 89.3 KJ + C(s) + 2S(s)  CS2(l) Ex 1: Calculate the amount of heat (in kJ) absorbed when 4.13 moles of carbon disulfide are formed. Ex2: Calculate the amount of heat (in kJ) absorbed when 16.83 g of sulfur is reacted with excess carbon to form carbon disulfide.

  21. **Identify the following reactions as exothermic or endothermic and write ΔH with the correct sign. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) + 2219.2 kJ 180.7 kJ + N2 (g) + O2 (g)  2 NO (g) **Calculate the amount of heat (in kJ) absorbed when 16.83 g of sulfur is reacted with excess carbon to form carbon disulfide (WE DID THIS ON THE LAST SLIDE  ) The production of iron and carbon dioxide from iron(III) oxide and carbon monoxide has a enthalpy value of ΔH = –26.3 kJ. How many kilojoules of heat are produced when 3.40 g of CO reacts with an excess of Fe2O3? (HINT: Place the energy as a reactant or product first!) Fe2O3(s) + 3CO(g)  2Fe(s) + 3CO2(g)

  22. Calorimetry Calorimetryis a technique used to measure the HEAT TRANSFERRED in a reaction or process. It is typically just an insulated container measured at either constant pressure or constant volume. Ex 1: A 0.303 gram sample of sucrose sugar (C12H22O11) was burned in a calorimeter containing 240. grams of water. During the process, the temperature of the water changed from 22.53°C to 27.55°C

  23. ** Calorimetry A 1.000 g sample of octane (C8H18) is burned in a bomb calorimeter containing 1200.0 grams of water at an initial temperature of 25.00°C. After the reaction, the final temperature of the water is 33.20°C. The specific heat of water is 4.184 J/g °C. Neglect the heat absorbed by the calorimeter. Calculate the heat released when the 1.000 g sample is combusted. mCalculate the heat of combustion of octane in kJ/mol.

  24. Calculating the Heat of Reaction (ΔHrxn) Steps to calculate ΔHrxn: • Multiply the ΔHf of each product and multiply it by its coefficient (coefficient = # moles). • Add the numbers in step #1 together. • Multiply the ΔHf of each reactant and multiply it by its coefficient (coefficient = # moles). • Add the numbers in step #3 together. • To find ΔHrxn, take Step 2 – Step 4. The unit is usually kJ. • Use standard heats of formation values given to calculation the enthalpy change • for the combustion of propane (C3H8). Is the reaction endothermic or exothermic? • C3H8 (g) + 5 O2 (g) 3 CO2(g) + 4 H2O(g)

  25. **heat of reaction • Use standard heats of formation values given to calculation the enthalpy change • for the formation of sucrose (C12H22O11) and oxygen in photosynthesis. • Is the reaction endothermic or exothermic? 12 CO2(g) + 11 H2O(g) C12H22O11 (s) + 12 O2 (g)

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