Heat and Energy

1. Heat and Energy Chp 10

2. Energy • The ability to do work or produce heat • 2 types: • Kinetic • Due to motion • Affected by mass and speed of object • Potential • Due to position or composition • Can be converted to mechanical energy

3. Law of Conservation of Energy • Energy can be converted to new forms, but it cannot be created or destroyed • A change in the energy of an object is a result of work being done on it • System – part of universe we are focused on • Surroundings – rest of universe • If one object loses energy, than another must gain it (if system gains, then surroundings lose)

4. Thermodynamics • Study of energy • Heat is a type of energy • When objects experience friction, some of their mechanical energy is converted to heat energy (no longer useful to us) • 2 reaction types: • Exothermic – heat is released to surroundings • Endothermic – heat is taken in from the surroundings

5. Heat and Temperature • NOT the same thing, but related • Temperature – the average kinetic energy of molecules in a substance • Heat – the total kinetic energy of molecules in a substance • Can have a lot of heat with little temperature if a lot of molecules are involved

6. Laws of Thermodynamics • The energy of the universe is constant. • The universe’s entropy (disorder) is always increasing. • Heat always flows from hot to cold, never the other way.

7. Energy Equation • E = q + w • The signs for q and w are from the systems point of view (+ indicates it was gained by the system, - indicates the system lost it) • Ex. How much energy is there if a system does 10 kJ of work and absorbs 15 kJ of heat? • Since the system does the work it is -10, since the heat is absorbed its +15 so • E = 15 + -10 = 5 kJ

8. Heat Equation • Heat is affected by temperature, mass and an object’s specific heat capacity (a measure of how quickly it changes temperature) • High shc means it changes temp slowly (ex. Water) • Low shc means it changes temp quickly (ex. Sand) • Q = smT • Q is quantity of heat (in joules) • S is specific heat capacity (in J/goC) • M is mass (in grams) • T is change in temperature (in celsius)

9. An Example • How much heat is needed to raise 7.4 g of water from 29 to 46 oC? Q = 4.184 (7.4)(46-29) Q = 526 J • This only works if no phase change occurs (cannot melt, boil, etc)

10. Enthalpy • The flow of energy in a reaction at constant pressure (symbol is H) • The same for a process whether it occurs in one step, or in a series of steps (Hess’s Law) • Measured using a calorimeter: • Burn the object and measure the change in temperature in a water bath that the object is submerged in

11. Using Hess’s Law • Rearrange equations to cancel out compounds not present in answer • To cancel, the compounds must have the same coefficient and be on opposite sides of the arrow (one product and one reactant) • Whatever is done to the equation, must also be done to its enthalpy • If you multiply to get a coefficient, multiply the enthalpy • If you flip the equation, you flip the sign (+ becomes -)

12. How we use energy • When we use energy, we degrade its usefulness only • Quantity stays the same, but quality decreases • Fossil fuels • Coal • Oil • Natural gas • Solar energy • Nuclear fusion