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Thermochemistry

Thermochemistry. Unit 7. thermochemistry. the study of heat changes in chemical reactions and physical changes. SURROUNDINGS. SYSTEM. system —specific part being analyzed surroundings —everything outside the system (usually the immediate area) universe = system + surroundings. closed.

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Thermochemistry

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  1. Thermochemistry Unit 7

  2. thermochemistry the study of heat changes in chemical reactions and physical changes

  3. SURROUNDINGS SYSTEM system—specific part being analyzed surroundings—everything outside the system (usually the immediate area) universe = system + surroundings closed isolated open energy nothing **Exchange: mass & energy

  4. Energyis the capacity to do work • Thermal energy is the energy associated with the random motion of atoms and molecules • Chemical energy is the energy stored within the bonds of chemical substances • Nuclear energy is the energy stored within the collection of neutrons and protons in the atom • Electrical energy is the energy associated with the flow of electrons • Potential energy is the energy available by virtue of an object’s position

  5. Temperature = Thermal Energy 900C 400C Energy Changes in Chemical Reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of the thermal energy. greater thermal energy

  6. Units for Measuring Heat Flow • calorie - the amount of heat needed to raise the temperature of 1 g of H2O by 1 oC. -Used except when referring to food • Calorie (capital C) refers to the energy in food

  7. Units for Measuring Heat Flow 1 Calorie = 1 kilocalorie = 1000 cal SI unit of heat and energy = Joule 4.184 J = 1 cal 1 J = 0.2390 cal

  8. Energy Transformations • Heat – (variable is q) – energy that transfers from one object to another • only changes can be detected! • flows from warmer to cooler object

  9. 2H2 (g) + O2 (g) 2H2O (l) + energy Energy is written on the product (right) side of the equation q and ΔH are negative (-) Exothermic - process that gives off heat- energy goes from system to surroundings. • Δ is a Greek letter – means change in

  10. exothermic The products are lower in energy than the reactants Thus, energy is released. ΔH = -395 kJ • The negative sign does not mean negative energy, but instead that energy is lost.

  11. energy + 2HgO (s) 2Hg (l) + O2 (g) energy is written on the reactant (left) side of the equation q and ΔH are positive (+) Endothermic - process in which heat has to be supplied – energy goes from surroundings to system

  12. endothermic The products are higher in energy than the reactants Thus, energy is absorbed. ΔH = +176 kJ • The positive sign means energy is absorbed

  13. Changes of state • When a substance changes state the temperature remains constant during the actual phase change.- this is because the heat energy is being used up by the phase change process.

  14. The specific heat (C) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. Specific heat formula q = m X Dt X C q = heat m = mass ( in grams) Dt = tfinal – tinitial C = specific heat q = DH

  15. How much heat is given off when an 869 g iron bar cools from 940C to 50C? C of Fe = 0.444 J/g •0C m = 869 g Dt = tfinal – tinitial = 50C – 940C = - 890C q = mCDt = 869 g x 0.444 J/g •0C x –890C = -34,000 J 6.4

  16. The specific heat of water is high due to hydrogen bonding

  17. Calorimetry the measurement of the heat into or out of a systemheat released = the heat absorbed • enthalpy changes are measured with a calorimeter No heat enters or leaves!

  18. The Law of Conservation ofEnergy • in any chemical or physical process, energy is neither created nor destroyed. - All the energy is accounted for as work, stored energy, or heat.

  19. thermo chemical equations thermo chemical equations— equations that show heat changes enthalpy (H)—heat content of a substance Cannot measure directly but we CAN measure • change in enthalpy = ΔH; • heat change for a process; usually measured in kJ (kilojoules)

  20. H2O (l) H2O (g) H2O (s) H2O (l) DH = 6.01 kJ DH = 44.0 kJ Heat of reaction • The physical states of all reactants and products must be specified Equation – = given mol x DH 1 # mol

  21. How much heat is evolved when 266 g of white phosphorus (P4) burn in air? P4(s) + 5O2(g) P4O10(s)DH = -3013 kJ x 3013 kJ 1 mol P4 x 1 mol P4 123.9 g P4 Thermo chemical Equations 266 g P4 = 6470 kJ

  22. ΔHf = HEAT OF FORMATION = heat absorbed or released to make 1 mol of a compound from its elements • ΔHsoln = Heat of Solution = heat change caused by dissolving of one mole of substance

  23. x 826 kJ 1 mol Fe2O3 x 1 mol Fe2O3 160 g Fe2O3 DHf for the formation of rust (Fe2O3) is –826 kJ/mol. How much energy is involved in the formation of 5 grams of rust 5.0 g Fe2O3 1 = 25.9 kJ

  24. x 445.1 kJ 1 mol NaOH x 1 mol NaOH 40.0 g NaOH When 1.0 g of solid NaOH (DHsoln = – 445.1 kJ/mol) dissolves in 10 L of water, how much heat is released? 1.0 g NaOH 1 = 11.1 kJ

  25. Molar Heat of Combustion = ΔHcomb = heat released in combustion of 1 mol of substance Molar Heat of Fusion (DHfus.) = the heat absorbed by 1 mol of a substance in melting from a solid to a liquid Molar Heat of Solidification (DHsolid.) = the heat lost when 1 mol of liquid solidifies (or freezes) to a solid

  26. Molar Heat of Vaporization (DHvap.) = the amount of heat necessary to vaporize 1 mol of a given liquid. Molar Heat of Condensation (DHcond.) = amount of heat released when 1 mol of vapor condenses to a liquid

  27. Standard heat of reaction • Hess’s law of heat summation states that if you add two or more thermochemical equations you can add the heats of reaction to give the final heat of reaction

  28. Standard Heats of Formation • The standard heat of formation (ΔHf0) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound Standard heat of formation for elements = 0

  29. Standard Heat of Reaction DH = H (products) – H (reactants) • Multiply the standard heat of formation by the number of moles for each reactant and product. • Add the reactants together • Add the products together • Subtract the sum of the reactants from the sum of the products

  30. Calculate DH for the following reaction. C2H4(g) + H2(g) C2H6(g) DH for C2H4(g) = 52.5 kJ/mol; DH for H2(g) = 0 kJ/mol; (free element) • H for C2H6(g) = –84.7 kJ/mol - 84.7 - 52.5 = - 137.2 kJ

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