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Regents Chemistry

Regents Chemistry. 11/05/2008 Mrs. Nephew. Measuring Heat. The amount of heat given off or absorbed in a rxn can be calculated by: q=mC∆T q=heat (Joules) m=mass of the substance C=specific heat capacity of substance T=(Temp initial-Temp final).

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Regents Chemistry

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  1. Regents Chemistry 11/05/2008 Mrs. Nephew

  2. Measuring Heat • The amount of heat given off or absorbed in a rxn can be calculated by: q=mC∆T q=heat (Joules) m=mass of the substance C=specific heat capacity of substance T=(Temp initial-Temp final)

  3. Ex. How many joules are absorbed when 50 g of water are heated from 30.2C to 58.6C? • Substitute the known values into the equation q=mC∆T .

  4. Ex. How many joules are absorbed when 50 g of water are heated from 30.2C to 58.6C? • Substitute the known values into the equation q=mC∆T . Q=5936J=5.94 x 103J

  5. Calorimeter • An insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process. • Remember in lab we used a “coffee cup calorimeter” earlier in the semester?

  6. In a calorimeter, the heat gained by the water is equal to the heat lost by the metal. That amount can be calculated using the calculation we just learned. • By measuring the temp increase of the water, the heat given off in a reaction can be calculated.

  7. Thermochemistry • The study of heat changes that accompany chemical reactions and phase changes.

  8. We can’t determine the amount of heat required to melt or boil a substance. Temperature remains constant during a phase change, so there is no ∆T, so we can’t use our equation.

  9. Heat of Fusion • The amount of heat needed to convert a unit of a substance from solid to liquid at its melting point. • Heat of fusion of water at 0C and 1 atm is 334 J/g. • No change in kinetic energy, no change in temperature.

  10. Melting • Endothermic (requires heat). • So freezing would be exothermic-in freezing water releases 334J/g of heat and its potential energy decreases.

  11. Ex. How many joules are required to melt 255 g of ice at 0C? • Multiply the heat of fusion by the total mass of ice to determine the heat required. Q= (255g)(334 J/g)=85,170 J =85.2 kJ

  12. Heat of Vaporization • The amount of heat needed to convert a unit mass of a substance from its liquid phase to its vapor phases at constant temperature.

  13. As heat is added, the particles absorb sufficient energy to overcome the attractive forces holding them in the liquid phase. • The potential energy of the system increases and the temp remains constant. • Endothermic process.

  14. Condensation is the reverse of boiling. It is an exothermic process.

  15. Heat of vaporization of water at 0 C and 1 atm is 2260 J/g.

  16. How many joules of energy are required to vaporize 423 g of water at 100 C and 1 atm? • Multiply the heat of vaporization by the total mass of water to determine the heat required. Q=(423g)(2260J/g)=955,980 J =956J

  17. Behavior of Gases • Kinetic molecular theory A model or theory that is used to explain the behavior of gases. It describes the relationships among pressure, volume, temperature, velocity, frequency, and force of collisions.

  18. Kinetic Theory • Relates pressure and the number of collisions per unit time for a gas.

  19. Discovery Lab Balloon Experiment Let’s demonstrate the relationship between temperature and volume of a gas in a closed system.

  20. Let’s try a worksheet on calculating heat…

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