Heat and EnergyHeat and Energy

Chp 10Chp 10

EnergyEnergy

The ability to do work or produce heatThe ability to do work or produce heat 2 types:2 types:

– KineticKinetic Due to motionDue to motion Affected by mass and speed of objectAffected by mass and speed of object

– PotentialPotential Due to position or compositionDue to position or composition Can be converted to mechanical energyCan be converted to mechanical energy

Law of Conservation of EnergyLaw of Conservation of Energy

Energy can be converted to new forms, but Energy can be converted to new forms, but it cannot be created or destroyedit cannot be created or destroyed– A change in the energy of an object is a result A change in the energy of an object is a result

of work being done on itof work being done on it– System – part of universe we are focused on System – part of universe we are focused on – Surroundings – rest of universeSurroundings – rest of universe– If one object loses energy, than another must If one object loses energy, than another must

gain it (if system gains, then surroundings lose)gain it (if system gains, then surroundings lose)

ThermodynamicsThermodynamics

Study of energyStudy of energy Heat is a type of energyHeat is a type of energy When objects experience friction, some of When objects experience friction, some of

their mechanical energy is converted to heat their mechanical energy is converted to heat energy (no longer useful to us)energy (no longer useful to us)

2 reaction types:2 reaction types:– Exothermic – heat is released to surroundingsExothermic – heat is released to surroundings– Endothermic – heat is taken in from the Endothermic – heat is taken in from the

surroundingssurroundings

Heat and TemperatureHeat and Temperature

NOT the same thing, but relatedNOT the same thing, but related Temperature – the average kinetic energy of Temperature – the average kinetic energy of

molecules in a substancemolecules in a substance Heat – the total kinetic energy of molecules Heat – the total kinetic energy of molecules

in a substancein a substance Can have a lot of heat with little temperature Can have a lot of heat with little temperature

if a lot of molecules are involvedif a lot of molecules are involved

Laws of ThermodynamicsLaws of Thermodynamics

1.1. The energy of the universe is constant.The energy of the universe is constant.

2.2. The universe’s entropy (disorder) is always The universe’s entropy (disorder) is always increasing.increasing.

3.3. Heat always flows from hot to cold, never Heat always flows from hot to cold, never the other way.the other way.

Energy EquationEnergy Equation

E = q + wE = q + w– The signs for q and w are from the systems The signs for q and w are from the systems

point of view (+ indicates it was gained by the point of view (+ indicates it was gained by the system, - indicates the system lost it)system, - indicates the system lost it)

Ex. How much energy is there if a system Ex. How much energy is there if a system does 10 kJ of work and absorbs 15 kJ of does 10 kJ of work and absorbs 15 kJ of heat?heat?– Since the system does the work it is -10, since Since the system does the work it is -10, since

the heat is absorbed its +15 sothe heat is absorbed its +15 so E = 15 + -10 = 5 kJE = 15 + -10 = 5 kJ

Heat EquationHeat Equation

Heat is affected by temperature, mass and an Heat is affected by temperature, mass and an object’s specific heat capacity (a measure of how object’s specific heat capacity (a measure of how quickly it changes temperature)quickly it changes temperature)– High shc means it changes temp slowly (ex. Water)High shc means it changes temp slowly (ex. Water)– Low shc means it changes temp quickly (ex. Sand)Low shc means it changes temp quickly (ex. Sand)

Q = smQ = smTT– Q is quantity of heat (in joules)Q is quantity of heat (in joules)– S is specific heat capacity (in J/gS is specific heat capacity (in J/gooC)C)– M is mass (in grams)M is mass (in grams) T is change in temperature (in celsius)T is change in temperature (in celsius)

An ExampleAn Example

How much heat is needed to raise 7.4 g of How much heat is needed to raise 7.4 g of water from 29 to 46 water from 29 to 46 ooC?C?

Q = 4.184 (7.4)(46-29)Q = 4.184 (7.4)(46-29)Q = 526 JQ = 526 J

This only works if no phase change occurs This only works if no phase change occurs (cannot melt, boil, etc)(cannot melt, boil, etc)

EnthalpyEnthalpy

The flow of energy in a reaction at constant The flow of energy in a reaction at constant pressure (symbol is H)pressure (symbol is H)

The same for a process whether it occurs in The same for a process whether it occurs in one step, or in a series of steps (Hess’s one step, or in a series of steps (Hess’s Law)Law)

Measured using a calorimeter:Measured using a calorimeter:– Burn the object and measure the change in Burn the object and measure the change in

temperature in a water bath that the object is temperature in a water bath that the object is submerged insubmerged in

Using Hess’s LawUsing Hess’s Law

Rearrange equations to cancel out Rearrange equations to cancel out compounds not present in answercompounds not present in answer

To cancel, the compounds must have the To cancel, the compounds must have the same coefficient and be on opposite sides of same coefficient and be on opposite sides of the arrow (one product and one reactant)the arrow (one product and one reactant)

Whatever is done to the equation, must also Whatever is done to the equation, must also be done to its enthalpybe done to its enthalpy– If you multiply to get a coefficient, multiply the If you multiply to get a coefficient, multiply the

enthalpyenthalpy– If you flip the equation, you flip the sign (+ If you flip the equation, you flip the sign (+

becomes -)becomes -)

How we use energyHow we use energy

When we use energy, we degrade its When we use energy, we degrade its usefulness onlyusefulness only– Quantity stays the same, but quality decreasesQuantity stays the same, but quality decreases

Fossil fuelsFossil fuels– CoalCoal– OilOil– Natural gasNatural gas

Solar energySolar energy– Nuclear fusionNuclear fusion