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THERMOCHEMISTRY

THERMOCHEMISTRY. CALCULATING PHASE CHANGES. CALCULATING HEATS OF RXNS. Any phase change requires energy. either energy is absorbed (melting or vaporizing) endo thermic Or energy released (condensing or solidifying) exo thermic This energy is in the form of thermal or heat energy.

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THERMOCHEMISTRY

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  1. THERMOCHEMISTRY CALCULATING PHASE CHANGES

  2. CALCULATING HEATS OF RXNS • Any phase change requires energy. • either energy is absorbed (melting or vaporizing) • endothermic • Or energy released (condensing or solidifying) • exothermic • This energy is in the form of thermal or heat energy. • symbolized by H and is generally in units of calories or Joules

  3. CALCULATING HEATS OF RXNS • There are two ways to calculate the energies involved in phase changes. • heating the substance up to the melting point or boiling point • Diagonal portion of the heat curve • Change in kinetic energy • Performing the actual melting or vaporizing • Flat portion of the heat curve • Change in potential energy

  4. boils condenses melts freezes

  5. HEATING UP A SUBSTANCE • Quantitatively the energy is dependent on 3 factors • Temperature change • Amount of the material • Type of the material • When we heat up a substance the particles vibrate faster. • According to the KMT, temp is the average kinetic energy of the particles that make up the substance

  6. HEATING UP A SUBSTANCE • We calculate the change in the temperature (ΔT) by taking the final temp (Tf) – initial temp (Ti) • ΔT = Tf – Ti • How much of the substance we are heating is also important. • The more we are heating, the more energy it will take • The unit we will keep track of here is mass

  7. HEATING UP A SUBSTANCE • The last component important to measure in order to calculate heat energy is dependent on the material (type) • Metals absorb heat differently than plastic or water or etc. • Different substances heat up and cool off at different rates • This difference is measured in a constant (substance dependent) called specific heat capacity.

  8. HEATING UP A SUBSTANCE • Specific heat capacity (C) is defined as the amount of energy it takes 1 g of a substance to heat up or cool down by 1 °C (J/g°C). • Metals tend to have relatively low specific heat capacities. • It takes them less energy to feel hotter and cool off quickly • Water has a relatively high specific heat capacity. • Water heats up and cools off slowly

  9. HEATING UP A SUBSTANCE • Putting these three measurements together we get the heat equation • ΔH = mCΔT • ΔT = Tf- Ti How much energy is absorbed by 123.3 grams of water if it heats up from 2.50°C to 75.5°C?

  10. HEATING & COOLING CW How much heat is lost by an aluminum pan that weighs 3500 grams if it comes out of the oven at a temperature of 177 °C and is put into a sink-full of water at 23.8 °C? What is the specific heat of a 50.0 g piece of metal that gains 2118 Joules of energy as it heats up 110°C? What metal is it?

  11. CHANGING PHASE • The other equation we use for phase change calculations is the energy required to perform a phase change • This calc is different than the mCΔT eqn since phase changes don’t involve a change in temp. • This is the flat portion of the heating curve • Energy is still being absorbed or lost, but not kinetic energy

  12. CHANGING PHASE • During a phase change, the energy added begins to break the IMFs that are holding the molecules together • To calculate the energy necessary to change the phase of a substance we need two constants. • Heat of fusion (ΔHfus), which is the energy it takes to melt 1 mol of a substance • Heat of vaporization (ΔHvap), which is the energy it takes to vaporize1 mol of a substance

  13. CHANGING PHASE • If you have more than a mole of material or less than a mole, it will therefore, take proportionally more or less energy to cause the phase change • Heat energy it takes to melt a given amount of material: mol(ΔHfus) • Heat energy it takes to vaporize a given amount of material: mol(ΔHvap)

  14. CHANGING PHASE EXAMPLE How much energy does it take to completely melt a 75.0 gram cube of ice? If a cloud of steam loses 175,000 J of energy as it condenses, how much water would be collected?

  15. CHANGING PHASE CW How much energy must be removed from 540 g of liquid water at 0°C in order to convert it to ice? If 13360 J of heat was added to 1000 g of ice at 0°C, how much water at 0°C is produced, and how much ice remains?

  16. PUTTING IT ALL TOGETHER • We know how to calculate the energy it takes to heat up a substance • ΔH = mCΔT • And how much energy it takes to cause phase to change • mol(ΔHfus) or mol(ΔHvap) • Now we can calculate how much energy it takes to heat up a substance all the way through phase changes

  17. DHtotal = +DHmelting +DHliquid +DHvaporizing +DHgas DHsolid DHtotal = mCsolidDT+n(DHfus)+mCliquidDT+DHvap+DHgas

  18. +n(DHfus) DHtotal = mCsolidDT +mCliquidDT +DHmelting +DHliquid +n(DHvap) +DHvaporizing +DHgas DHsolid +mCgasDT

  19. PUTTING IT ALL TOGETHER How much energy is necessary to heat up 125.5 grams of ice up to 75 °C?

  20. PUTTING IT ALL TOGETHER How much energy is lost as 313 grams of steam is cooled down from 150 °C and frozen into ice at -15 °C?

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