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Thermochemistry

Thermochemistry. Some like it hot. Forms of Energy. Potential Energy (gravitational, magnetic, chemical) Kinetic Energy Work Light Heat. Heat. Heat is NOT temperature, but the two are related. Temperature Heat Average energy Total energy

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Thermochemistry

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  1. Thermochemistry Some like it hot...

  2. Forms of Energy Potential Energy (gravitational, magnetic, chemical) Kinetic Energy Work Light Heat

  3. Heat Heat is NOT temperature, but the two are related. TemperatureHeat Average energy Total energy High temp, low heatLow temp, high heat

  4. Heat TemperatureHeat Measure with thermometer Can't measure directly Is a property of an object Can only say how much was gained or lost

  5. Units Metric: since heat is a type of energy, we can use the metric units of energy: Joules (J) Non-metric: the calorie (cal) is a unit specific to heat.

  6. Unit Definitions Joules: 1 J = 1 kg*m2/s2 This looks awful, but has a simple meaning: One joule is the amount of work it takes to move a one kilogram object a distance of one meter, at an acceleration of one meter per second squared.

  7. Unit Definitions calories: One calorie is the amount of heat needed to heat one gram of pure water up by one degree. Note that it doesn't matter whether it's from 20oC to 21oC or from 80oC to 81oC...it's still one calorie.

  8. Formula Time! The symbol for heat is q The units are J or cal. This is just like the symbol for distance is d, and it can be measured in different units like m, miles, cm... If the value of q is positive, heat is going into the object (endothermic) If the value of q is negative, heat is going out of the object (exothermic)

  9. Formula Time! Because energy can't be created or destroyed, then if the hot thing loses 50 J (q=-50J), the cold thing must gain 50J (q=+50J) Or, more generally qhot + qcold = 0 WARNING! This assumes no heat was lost anywhere. We'll have ways of accounting for that later.

  10. Formula Time! Second formula q = m*c*T m is the mass (more stuff = more heat) c is the heat capacity (or specific heat capacity). This is a number that's specific to each substance—some things heat up more easily than others. T is the change in temperature (Tfinal – Tinitial). Bigger temperature changes = more heat.

  11. One More Relevant Fact Any two (passive) objects in contact with each other will eventually end up at the same temperature. So two different objects placed together may have different values for Tinitial, but will have the same value for Tfinal

  12. Summary • Heat is a form of energy. • Related to temperature, but is not the same thing. • Measured in Joules or calories • Symbol is q. Negative q is exothermic. Positive q is endothermic. • qhot + qcold = 0 • q = m*c*T

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