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Exploring Thermochemistry: Heat, Energy, and Reactions

Thermochemistry is the study of heat changes and energy in chemical reactions. Learn about types of energy, heat flow, specific heat capacity, and enthalpy. Find out how to measure heat exchange and calculate enthalpy changes in reactions with thermochemical equations.

douglasvasquez
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Exploring Thermochemistry: Heat, Energy, and Reactions

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  1. Thermochemistry Thermo = heat Chemistry = study of matter

  2. Thermochemistry • Study of heat changes and energy that accompany chemical reactions and phase changes

  3. Review of Energy • Capacity to do work or to create heat and or generate electricity • Types: • Chemical • Nuclear • Thermal • Radiant (light) • Electrical • Mechanical

  4. Law of conservation of Energy • Energy can be converted from one form to another but it cannot be created or destroyed

  5. Forms of energy: • Potential • Stored energy • Kinetic • Energy of motion

  6. Chemical potential energy • Energy stored within the structural units of chemical compounds

  7. Heat is energy (Q) • flows from an area of high heat to an area of low heat • Can be measured in calories, Calories, or Joules • 1000 cal = 1 Cal = 4.184 J

  8. Heat energy (Q) • System: part of the universe being studied • Surroundings: everything outside of the system • Universe: system + surroundings

  9. Heat energy (Q) Endothermic: • feels cold to touch • temperature decreases • heat absorbed by system • Q = +

  10. Heat energy (Q) Exothermic: • feels warm to touch • temperature increases • heat released by system • Q = -

  11. Specific heat capacity • C or Cp • The amount of energy required to raise the temperature of 1 gram of a substance 1ºC • Water= 4.18 J/g 0C • Glass=.50 J/g 0C

  12. Specific heat capacity • Measure of a substance’s resistance to temperature change • High=resistant (water) • Low=easy (metals, glass, etc.)

  13. Equation Q = m C ∆T - Qis the heat absorbed or released in Joules -m is mass in grams -C is the specific heat in J/g/ºC -∆T is the temperature change in ºC

  14. (Q) = m C (DT) Temperature change (final temp minus initial temp) Specific heat capacity in J/g-oC mass

  15. When 1982 g of water underwent a temperature change from 23.677oC to 27.482oC, how much energy in kJ did the water absorb? The specific heat of water is 4.184 J/(g-oC). Show all work!

  16. How much energy in J is required to raise the temperature of 500.0 g of copper from 22.8oC to 100.0oC? The specific heat of copper is 0.387 J/g/oC. Show all work!

  17. Measuring heat exchange Calorimeter- insulated device used for measuring the amount of heat absorbed or released during a reaction

  18. Calorimeter

  19. Calorimeter In a closed system: Heat loss = Heat gain Q lost = Q gained m C∆ T = m C ∆ T

  20. Enthalpy • (∆H) heat content of a system at constant pressure • Can measure changes in enthalpy during a reaction

  21. Enthalpy heat of reaction∆ Hrxn ∆ Hrxn = Hfinal - Hinitial Or ∆ Hrxn = Hproducts - Hreactants

  22. Thermochemical Equations • Used to calculate and express heat changes. • The physical states of the reactants and products in this equation is important! • Example: CH4(g) + 2O2(g)→ CO2(g) + 2H2O(l) ∆H = -890.2 kJ

  23. Exothermic rxn: ∆ Hrxn is < 0 ∆ Hrxn is negative

  24. Endothermic rxn: ∆ Hrxn is > 0 ∆ Hrxn is positive

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