Heat and Energy Chp 10

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

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)

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

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

Laws of Thermodynamics 1.The energy of the universe is constant. 2.The universe’s entropy (disorder) is always increasing. 3.Heat always flows from hot to cold, never the other way.

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 = = 5 kJ

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/g o C) –M is mass (in grams) –  T is change in temperature (in celsius)

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

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

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 -)

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