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Entry Task: May 22 nd 23 rd Block 1

Entry Task: May 22 nd 23 rd Block 1. Question: What is temperature change of 50.0 grams of water absorbs 35kJ of energy. (specific heat 4.18 J/ gC ) You have 5 minutes!. Agenda:. Discuss Ch. 16.sec 2 and worksheet In-class (small notes) on Sec. 3 HW:

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Entry Task: May 22 nd 23 rd Block 1

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  1. Entry Task: May 22nd 23rd Block 1 Question: What is temperature change of 50.0 grams of water absorbs 35kJ of energy. (specific heat 4.18 J/gC) You have 5 minutes!

  2. Agenda: Discuss Ch. 16.sec 2 and worksheet In-class (small notes) on Sec. 3 HW: Look at the specific heats of metal Lab

  3. Endothermic reactions absorb energy (heat) from the surroundings for the reaction to occur. What are endothermic reactions?

  4. Exothermic reactions release energy (heat) to the surroundings when the reaction occurs. What are exothermic reactions?

  5. A calorimeter is an insulated device used for measuring the amount to heat absorbed or released during a chemical or physical process. Define a calorimeter

  6. A known mass of water is placed in an insulated chamber. • The chamber contains the reaction- any heat absorbed in the reaction will drop the temperature of the water. • Any heat released in the reaction will raise the temperature of the water. How does it work?

  7. q= cmΔT • 9750 J = (4.184 J/g˚C)(335g)(X) • 9750 (4.184)(335) 1401.64 6.96 ˚C is the temperature difference 65.5 - 6.96 = 58.5 ˚C If 335g of water at 65.5˚C loses 9750 J of heat, what is the final temperature of the water?

  8. q= cmΔT • 5650 J = (4.184 J/g˚C)(Xg)(26.6˚C) • 5650 (4.184)(26.6) 111.29 = 50.8 g The temperature of a sample of water increases from 20.0˚C to 46.6˚C as it absorbs 5650 J of heat. What is the mass of the sample?

  9. Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes Define thermochemistry

  10. Chaos Most energy Taking in energy (endothermic) Releases energy (exothermic) Taking in energy (endothermic) Releases energy (exothermic) Taking in energy (endothermic) Remember this? Order Less energy Less order More energy Releases energy (exothermic)

  11. The system is the reaction • The surroundings is the everything other than the reaction. • The universe is the system AND surroundings. Surroundings UNIVERSE System What is the difference between the system, surroundings and universe?

  12. Enthalpy is the heat content in a system at constant pressure. • *its almost impossible to know the absolute amount of energy held in a substance. • Enthalpy (heat) of reaction is the change of energy- one substance to another. • ΔHrxn = Hfinal – Hinitial What is the difference between enthalpy and the enthalpy (heat) of reaction?

  13. The sign of the enthalpy reactions For an exothermic reaction, heat energy is _____________. When the Hreactants is subtracted from the ________Hproduct, a ________ value for ΔHrxn is obtained. Enthalpy changes for exothermic reactions are always_______________. released smaller negative negative

  14. Provide the enthalpy change reaction for Iron/oxygen reaction 4Fe (s) + O2 (g)  2Fe2O3 (s) ΔH= -1625 kJ Notice the phase change (s) + (g)  (s) Gas to solid loses energy- exothermic Disorder to order = lose energy

  15. The sign of the enthalpy reactions For an endothermic reaction, heat energy is _____________. When the Hreactants is subtracted from the ________Hproduct, a ________ value for ΔHrxn is obtained. Enthalpy changes for endothermic reactions are always_______________. absorbed larger positive positive

  16. Provide the enthalpy change reaction for cold pack with NH4NO3 NH4NO3(s)  NH4+ (aq) + NO3- (aq) ΔH= 27 kJ Notice the phase change (s)  (aq) + (aq) Solid to liquid gains energy- endothermic Order to disorder gains energy

  17. Identify the reaction as endothermic or exothermic-PAY ATTENTION TO PHASE CHANGES Endothermic C2H5OH (l)  C2H5OH(g) NH3(g)  NH3(l) Br2(l)  Br2(s) NaCl (s)  NaCl (l) C5H12 (g) + 8O2 (g)  5CO2 (g) + 6H2O(l) Exothermic Exothermic Endothermic Exothermic

  18. B C A F D E G

  19. 8. A scientist is studying the solution in the flask. What is the system? 9. What are the surroundings? 10. What is the universe? The solution The Flask and everything around the flask. Solution + surroundings

  20. Thermochemical equations Are balanced chemical equations that include physical states of all reactants and products AND the energy change ∆H. NH4NO3(s)  NH4+ (aq) + NO3- (aq) ΔH= 27 kJ

  21. Types of enthalpy ∆H(comb) ∆H(vap) ∆H(fus) Change in heat with combustion Change in heat with vaporization Change in heat with fusion

  22. Thermochemical equations for changes in state Consider: H2O(l)  H2O(g) ΔH(vap)= 40.7 kJ What is happening? Its vaporizing. What ∆H would you use? Would it be + or -? Positive- endothermic

  23. Thermochemical equations for changes in state Consider: H2O(s)  H2O(l) ΔH(fus)= 6.01 kJ What is happening? Its melting. What ∆H would you use? Would it be + or -? Positive- endothermic

  24. Pay attention to the signs ∆Hvap= -∆Hcond ∆Hfus= -∆Hsolid So -∆Hcond for water is H2O(g)  H2O(l) ΔH(cond)= - 40.7 kJ So -∆Hsolid for water is H2O(l)  H2O(s) ΔH(fus)= - 6.01 kJ Makes sense- both are exothermic -∆H

  25. Calculating energy released in a reaction How much heat is released when 54.0 g of glucose (C6H12O6) is burned? C6H12O6 (s) + 6O2(g) 6CO2(g) + 6H2O(l)∆H comb= -2808 kJ ∆H comb= -2808 kJ is per mole of glucose Convert gram to mole 1 mole glucose 54.0g glucose = 0.3 mol 180 g glucose = 0.3 mol X -2808 ∆H comb =842 kJ

  26. You Try!! 1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point. Which ∆H would you use? ∆H (fus) because its melting. So 3.22 its is. ∆H(fus) be positive or negative? ∆H (fus) be positive. ∆H (fus)= 3.22 per mole.

  27. You Try!! 1. Calculate the heat required to melt 25.7 g of solid methanol at its melting point. ∆Hfus= 3.22 kJ is per mole of methanol Convert gram to mole 1 mole methanol 25.7g methanol = 0.803 mol 32 g methanol = 0.803 mol X 3.22 ∆H fus=2.58 kJ

  28. You Try!! 2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? Which ∆H would you use? ∆H (vap) because its condensing. So 23.3 its is. ∆H(vap) be positive or negative? ∆H (vap) be negative. ∆H (vap)= -23.3 per mole.

  29. You Try!! 2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? ∆Hvap= -23.3 kJ is per mole of ammonia Convert gram to mole 1 mole ammonia 275g ammonia = 16.2 mol 17 g ammonia = 16.2 mol X -23.3 ∆H vap=377 kJ

  30. You Try!! 3. What mass of methane must be burned in order to liberate 12880kJ of heat? Methane (CH4) ∆Hcomb= -891 kJ/mol 12880 ÷ -891 =14.5 moles of methane 16 g methane 14.5molmethane = 232 g 1 mole methane

  31. You Try!! 2. How much heat is evolved when 275g of ammonia gas condenses to a liquid at its boiling point? ∆Hvap= -23.3 kJ is per mole of ammonia Convert gram to mole 1 mole ammonia 275g ammonia = 16.2 mol 17 g ammonia = 16.2 mol X -23.3 ∆H vap=377 kJ

  32. TonightFinish backside w.sANDPre-lab Questions

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