Chapter 6The Nature of EnergyWhat is Energy?The capacity to do work or to produce heat

The Law of Conservation Of Energy- energy is neither created or destroyed but converted from one form to another

Types of Energy1. Potential Energy-energy due to position

2. Kinetic Energy- energy due to motionKE = mv2

More TermsHeat- the transfer of energy between two objects due to temperature differences

Temperature- a property which determines the direction heat will flow when two objects are brought into contact

Con.Work- a force acting over a distance

State Function- a property that depends only on its present stateEnergy is a state functionSystem- part of the universe which we are focusing on( usually the reactants and products)

More Terms!!!Surroundings- everything else in the universe(examples could be the reaction container, the room, and anything else other than the reactants and products)Exothermic-energy flows out of the system( heat is produced)Endothermic-energy flows into the system( heat is absorbed

Internal Energy(E)E = q + w E is the change in the systems internal energyq represents heatw represents workq= +x (endothermic, heat flows in)q= -x (exothermic, heat flows out)w is neg. if energy flows out(system does work on the surroundings)w is pos. if energy flows in( surroundings do work on the system

ExampleCalculate E for a system where 1.40 kJ of work is done on the system undergoing an endothermic process in which 15.6 kJ of heat flows into the system.

E = q + w= 15.6kJ + 1.4kJ= 17.0kJ

PV Workw = -PV

Example: Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external pressure of 15 atm.

Examplep.247 6.3A balloon is being inflated to its full extent by heating the air inside it. In the final stages of this process, the volume of the balloon changes from 4.00x106 L to 4.50x106 L by the addition of 1.3x108 J of energy as heat. Assuming that the balloon expands against a constant pressure of 1.0 atm, calculate E for the process. ( To convert between Latm and J, use 1 Latm=101.3 J.

Enthalpy and CalorimetryEnthalpy (H) = E + PV

Remember thatE = qp + w so E = qp - P V so qp = E + PVNow ifH = E + (PV) so H = E + PV

H = qp

ExampleWhen one mole of methane is burned at a constant pressure, 890 kJ of energy is released as heat. Calculate H for a process in which a 5.8 g sample of methane is burned at constant pressure.

Calorimetry- a device use to determine the heat associated with a chemical reaction

Heat Capacity(C)= heat absorbedincrease in temp.Specific heat capacity- the energy required to raise the temperature of one gram of a substance by one degree CelsiusMolar heat capacity- the energy required to raise the temperature of one mole of a substance by one degree Celsius

Constant Pressure CalorimetryIs used in determining the changes in enthalpy(heats of reactions) for reactions occurring in solution

Energy released= s x m x T whereS= specific heat capacityM= mass of solutionT= increase in temperature

Example p. 252, 6.5When 1.00 L of 1.00 M Ba(NO3)2 solution at 25.0C is mixed with 1.00 L of 1.00 M Na2SO4 solution at 25.0C in a calorimeter, the white solid BaSO4 forms and the temperature of the mixture increases to 28.1C. Assuming that the calorimeter absorbs only a negligible quantity of heat, that the specific heat capacity of the solution is 4.18J/Cg, and that the density of the final solution is 1.0 g/ml, calculate the enthalpy change per mole of BaSO4 formed.

Hesss LawIn going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

Characteristics of Enthalpy ChangesIf a reaction is reversed, the sign of H is also reversed.If the coefficients in a balanced reaction are multiplied by an integer, the value of H is multiplied by the same integer.

ExampleTwo forms of carbon are graphite and diamond. Using the enthalpies of combustion for graphite and diamond, calculate H for the conversion of graphite to carbon:Cgraphite(s) Cdiamond(s)The combustion reactions areCgraphite(s) +O2(g) CO2(g) H =-394kJCdiamond(s) + O2(g) CO2(g) H =-396kJ

ExampleCalculate H for the synthesis of diborane from its elements according to the following:2B(s) + 3H2(g) B2H6(g)Use the following reactions:2B(s) +3/2O2(g) B2O3(s) H =-1273kJB2H6(g) +3O2(g) B2O3(s)+3H2O(g) H =-2035 kJ H2(g) + O2(g) H2O(l) H= -286 kJH2O(l) H2O(g) H = 44kJ

Energy is conserved.There are 4 types of Potential EnergyGravitational- Electrical- you can remove electrons from one area and sent them to another. The area losing electrons become more and more positive and the area gaining electrons becomes more negative. The greater and greater the charge difference, the more energy is stored within the system.Chemical- certain chemicals have bonds with require little energy to break. This energy must be put into the bond to break it. So in other words it takes energy to break bonds and then rearranging the atoms into new products form new bonds which gives off energy4. Nuclear- E=mc2Heat is not a substance but a process. Energy is transferred.Example of state function: You can drive from NY to LA via many different routes. No matter which one you take, you are still going from NY to LA. The actual distance between the cities is the sameInternal energy E if a system can be defined as the sum of the kinetic and potential energies of all the particles in the system.

Thermodynamic quantities always consists of 2 parts: a number giving the magnitude of the change and a sign indicating the direction of the flow

The sign reflects the systems point of view.

If a quantity of energy flows into a system via heat(endothermic process) q is equal to +x where the positive sign indicates that the systems energy is increasing.

When energy flows out of the system via heat(exothermic process) , q = -x where the negative sign indicates that the system energy is decreasing

If the system does work on the surrounding(energy flows out of the system) w is negative.If the surroundings do work on the system(energy flows into the system) w is positive

q= + 15.6kJ because it is endothermin

w = +1.4kJ since work is done on the system

The Joule(J) is the fundamental SI unit for energy

J= kgm2/s2A common type of work associated with chemical processes is work done by a gas(through expansion) or work done to a gas(compression)This equation gives us the magnitude(size) of the work required to expand a gas V against a pressure P.

What about the sign of the work?

W and PV have opposite signs because when the gas expands (V is positive), work flows into the surroundings ( w is negative)

Note that for a gas expanding against an external pressure P, w is a negative quantity as required, since work flows out of the system.

When a gas is compressed, V is a negative quantity( the volume decreases) which makes w a positive quantity( work flows into the system) In dealing with PV WORK keep in mind that the P always refers to the external pressure the pressure that causes a compression or that resists an expansionSince P is constant the change in PV is due only to a change in volume

Since this equation is identical o the one for qp we get the followingCalorimetry is based on observing the temp change when a body absorbs or discharges energy as heat

Different substances respond differently to being heated. This is called heat capacity,CH is directly proportional to the quantities of the reactants and products in a reaction so they must also be multiplied by the integer.Hw. : P