Thermal Energy, Endothermic and Exothermic

Celsius, Fahrenheit, and Kelvin Formulas

• Celsius to Kelvin: 0°C + 273.15 = 273.15K
• Celsius to Fahrenheit: (0°C × 9/5) + 32 = 32°F
• Water freezes at 0°C and boils at 100°C
  • You can convert to K and °F easily from there

Difference between temperature and thermal energy

• Temperature is a measure of the average kinetic energy of moving particles of matter
• Thermal energy is a measure of the total kinetic energy of moving particles of matter
  • Kinetic energy is a measurement of movement; anything that moves has kinetic energy
  • The slower the particles are moving, the less kinetic energy they have, therefore the less thermal energy they have
• A 20 gallon tub of water at 30°C has a lower temperature than a drop of water at 50°C
• However, the same tub has higher thermal energy than the same drop of water because its mass is larger

Thermal energy depends on:

• mass

• temperature

• number of particles

  

Specific Heat - the amount of energy needed to raise the temp. of 1 g of a substance by 1°C

• Different for each substance
• Specific heat of liquid water - 4.186 J/g°C
  • one thermochemical (small/lowercase) calorie is also equal to 4.186 Joules

Formula°C

• heat (J) = specific heat (J/g°C) × mass (g) × temp. change (°C)
• ]]Don’t forget to convert the units in practice problems]]
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Sample Problem

Vocabulary

• system - a specified portion of matter in a given region of space that has been selected for study during an experiment or observation

  • usually where a chemical reaction is taking place; the reactants and products of a chemical reaction

• surrounding - the area around the system; everything but the system itself

• endothermic - absorbing heat

• exothermic - releasing heat (exo = exit)

How many Joules are in one…

Example: convert 10 calories (small) to Joules

1 calorie = 4.184 J

10 calories = 41.84 J

Example 2: convert 4184 Joules to calories (small)

4184/4.184 = 1000 calories

Molar mass

• Example: the molar mass of Substance 1 is 10 g/mol. How many grams are in 2 moles of Substance 1?
  • The answer is 20 g. Multiply the # of moles given by the molar mass (2 moles * 10 g/mol = 20 g).