Energy and Physical ChangesEnergy and Physical Changes

Energy is transferred during both Energy is transferred during both chemical and physical changes, chemical and physical changes, most commonly in the form of most commonly in the form of

heatheat

EnergyEnergyEnergy can be kinetic – associated with Energy can be kinetic – associated with motion, such as thermal, mechanical, motion, such as thermal, mechanical, electric, soundelectric, soundEnergy can be potential – associated Energy can be potential – associated with an objectwith an object’’s position, such as in s position, such as in chemical bonds, gravitational, chemical bonds, gravitational, electrostatic electrostatic (size of charge and distance between (size of charge and distance between charges)charges)

Energy is converted from one form to Energy is converted from one form to anotheranother

GOAL: To be able to define energy and to recognize different types of energy

First Law of ThermodynamicsFirst Law of Thermodynamics

The total energy of the universe is The total energy of the universe is constantconstant

Energy is conservedEnergy is conserved-q=+q -q=+q (heat lost by something equals heat gained by (heat lost by something equals heat gained by something else)something else)

GOAL: To understand that energy is not created or destroy, but transferred between different places and between different types of energy

Temperature and Heat are NOT Temperature and Heat are NOT the Same Thing!the Same Thing!

Temperature is a measureTemperature is a measure of the of the average kinetic energies of the average kinetic energies of the particles in a substanceparticles in a substanceHeat is energyHeat is energy that can be that can be transferred between substances that transferred between substances that are at are at different temperaturesdifferent temperaturesHeat will transfer between two Heat will transfer between two objects in contact until thermal objects in contact until thermal equilibrium occursequilibrium occurs

GOAL: To understand the different between heat and temperature, and to use the two terms correctly

Heat transferHeat transferThe quantity of heat lost by a hotter The quantity of heat lost by a hotter object (-q) and the quantity of heat object (-q) and the quantity of heat gained by a cooler object (+q) when gained by a cooler object (+q) when they are in contact are numerically they are in contact are numerically equal (but opposite direction)equal (but opposite direction)Exothermic (-q) heat is transferred Exothermic (-q) heat is transferred from the system to the surroundingsfrom the system to the surroundingsEndothermic (+q) heat is transferred Endothermic (+q) heat is transferred from the surroundings to the systemfrom the surroundings to the system

GOAL: To be able to recognize exothermic and endothermic in a variety of situations

Heat Transfer during a Heat Transfer during a Temperature ChangeTemperature Change

The quantity of heat transferred to or The quantity of heat transferred to or from an object from an object when its temperature when its temperature changeschanges depends on: depends on:– Quantity of the material (m)Quantity of the material (m)– Size of the temperature change (Size of the temperature change (T)T)– Identity of the material (c)Identity of the material (c)

Specific heat capacity – the quantity Specific heat capacity – the quantity of heat required to raise the of heat required to raise the temperature of 1.00g of a substance temperature of 1.00g of a substance by one kelvin (J/gby one kelvin (J/g..K)K)

GOAL: To know the factors that determine temperature change when heat is applied or removed from an object

Consequences of Consequences of Specific Heat CapacitySpecific Heat Capacity

Objects with a Objects with a large c large c value value take more take more energy to change temperature energy to change temperature (compare (compare seat belt buckle with bottle of water, or seat belt buckle with bottle of water, or sand with water at the beach)sand with water at the beach)

If objects with different c values are If objects with different c values are allowed to reach thermal equilibrium, the allowed to reach thermal equilibrium, the object with the large c value contains object with the large c value contains more heat energy (aluminum foil vs baked more heat energy (aluminum foil vs baked potato)potato)

GOAL: To understand how specific heat capacity affects temperature change as heat is applied or removed from an object

Energy and Changes of State

Energy and Changes of StateEnergy and Changes of StateDuring a State Change, Energy is used During a State Change, Energy is used to change the position of particles (PE), to change the position of particles (PE), BUT NOT the Speed (KE)!BUT NOT the Speed (KE)!Heat of fusion – energy to convert a Heat of fusion – energy to convert a substance from solid to liquid (J/g)substance from solid to liquid (J/g)

Heat of vaporization – energy to convert a Heat of vaporization – energy to convert a substance from liquid to gas (J/g)substance from liquid to gas (J/g)

NOTICE: NOTICE: The energy required for a change The energy required for a change of state is determined by the type of of state is determined by the type of substance and its quantity (mass), BUT NOT substance and its quantity (mass), BUT NOT Temperature Change!!!Temperature Change!!!

GOAL: To be able to calculate the energy lost or gained in a state change

For a State Change: q = m (Hfus) or q = m (Hvap)EXAMPLE: The heat required to turn 10 degree water into 120 degree steam = (heat to raise temp of water to 100 degrees) + (heat to change state) + (heat to raise temp of steam to 120 degrees)

Observe: Cooling of Lauric Acid Observe: Cooling of Lauric Acid and Solid-Liquid Equilibrium and Solid-Liquid Equilibrium

(Freezing)(Freezing)DiscussionDiscussion– Use these terms: Use these terms:

heat/energy, heat/energy, temperature, temperature, kinetic energy, kinetic energy, potential energy, potential energy, states, solid, states, solid, liquid, state liquid, state change, change, equilibriumequilibrium

White boards:White boards:

Draw particle Draw particle view of a solidview of a solid

Draw particle Draw particle view of a view of a liquidliquid

Draw particle Draw particle view of the view of the melting/freezing melting/freezing

phase changephase change

GOAL: To be able to label a heating/cooling curve with: states (solid, liquid, gas), state/phase changes (boiling/evaporating, condensing, solidifying, freezing), types of energy changes (kinetic or potential), how heat is calculated at each segment (heat of fusion or vaporization, q=mcT)

Energy UnitsEnergy UnitsJoule is the SI unit for thermal energyJoule is the SI unit for thermal energy

1 J = 1 kg1 J = 1 kg..mm22/s/s22

Kilojoules are also commonly usedKilojoules are also commonly used

The calorie is an older unit for heat; 1 The calorie is an older unit for heat; 1 cal = 4.184 Jcal = 4.184 J

Dietary Calories are actually 1000 Dietary Calories are actually 1000 caloriescalories

GOAL: To use the unit Joules to solve energy problems

q = m c q = m c TTUse to find Heat when Temperature Use to find Heat when Temperature is Changing!is Changing!

q is heat in joulesq is heat in joules

m is mass in gramsm is mass in grams

T = TT = Tfinalfinal – T – Tinitialinitial

Water has a particularly high specific Water has a particularly high specific heat; metals have low specific heatsheat; metals have low specific heats

GOAL: To be able to calculate heat lost or gained when a temperature change occurs

AssumptionsAssumptionsHeat transfers until both substances Heat transfers until both substances are at the same temperatureare at the same temperatureWe assume no heat is transferred to We assume no heat is transferred to warm the surroundings (though this warm the surroundings (though this is not accurate)is not accurate)The heat that is lost by one The heat that is lost by one substance is equal and opposite in substance is equal and opposite in sign to the heat that is gained by the sign to the heat that is gained by the other substanceother substance

GOAL: To understand these assumptions and apply them to solve Calorimetry problems (begins on slide 17)

CalorimetryCalorimetryConstant pressure calorimetry measures Constant pressure calorimetry measures HH

Constant pressure calorimetry can be Constant pressure calorimetry can be done with a coffee-cup calorimeterdone with a coffee-cup calorimeter

A reaction changes the temperature of the A reaction changes the temperature of the solution in the calorimeter; measuring the solution in the calorimeter; measuring the change in the solution allows calculation of change in the solution allows calculation of the change in the reactionthe change in the reaction

qqrxnrxn + q + qsolutionsolution = 0 = 0GOAL: To understand that heat lost by one substance equals heat gained by another substance within a closed system; and to be able to use this concept to solve constant pressure calorimetry problems

Thermodynamics – the study of Thermodynamics – the study of heat and workheat and work

E = q + wE = q + w

E is the change in kinetic and potential E is the change in kinetic and potential energies of the systemenergies of the system

Positive q is heat going into the systemPositive q is heat going into the system

Negative q is heat leaving the systemNegative q is heat leaving the system

Positive w is work done on the systemPositive w is work done on the system

Negative w is work done by the systemNegative w is work done by the system

Work (of a gas): w = - P(Work (of a gas): w = - P(V)V)GOAL: To be able to assign signs to heat and work and solve for E

State FunctionsState FunctionsA quantity that is the same no matter A quantity that is the same no matter what path is chosen in going from what path is chosen in going from initial to finalinitial to final

Changes in internal energy and Changes in internal energy and enthalpy for chemical or physical enthalpy for chemical or physical changes are state functionschanges are state functions

Neither heat nor work individually Neither heat nor work individually are state functions, but their sum isare state functions, but their sum is

GOAL: To be able to define state function and recognize that E and H are state functions while q and w are not

Enthalpy Changes for Chemical Enthalpy Changes for Chemical ReactionsReactions

Measures the change in heat contentMeasures the change in heat contentEnthalpy changes are specific to the Enthalpy changes are specific to the identity and states of reactants and identity and states of reactants and products and their amountsproducts and their amountsH is negative for exothermic reactions H is negative for exothermic reactions and positive for endothermic reactionsand positive for endothermic reactionsValues of Values of H are numerically equal but H are numerically equal but opposite in sign for chemical reactions opposite in sign for chemical reactions that are the reverse of each otherthat are the reverse of each otherEnthalpy change depends on molar Enthalpy change depends on molar amounts of reactants and productsamounts of reactants and products

2 Methods to find 2 Methods to find HHrxnrxn

HessHess’’s Law (indirect method)s Law (indirect method)

If a reaction is the sum of two or If a reaction is the sum of two or more other reactions, more other reactions, H for the H for the overall process is the sum of the overall process is the sum of the H H values of those reactionsvalues of those reactions

GOAL: To be able to solve for Hrxn using both the indirect and direct methods

C(s) + O2(g) CO2(g) H=-393.5 kJ/mol

CO(g) + 1/2O2(g) CO2(g) H=-283.0 kJ/mol

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_________________________________C(s) + 1/2O2(g) CO(g) H=?

C(s) + O2(g) CO2(g) H=-393.5 kJ/mol

H2(g) + 1/2O2(g) H2O(l) H=-285.8 kJ/mol

2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l) H=-2598.8 kJ/mol

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__________________________________2C(s) + H2(g) C2H2(g) H=?

Standard Enthalpies of FormationStandard Enthalpies of FormationThe The standard molar enthalpy of standard molar enthalpy of formation formation ((HHff

oo) is the enthalpy ) is the enthalpy change for the formations of 1 mol of change for the formations of 1 mol of a compound directly from its a compound directly from its component elements in their component elements in their standard statesstandard statesThe The standard statestandard state of an element or of an element or a compound is the most stable form a compound is the most stable form of the substance in the physical state of the substance in the physical state that exists at standard atmosphere that exists at standard atmosphere at a specified temperatureat a specified temperature

Standard Enthalpy of FormationStandard Enthalpy of Formation

The standard enthalpy of formation The standard enthalpy of formation for an for an element in its standard state is element in its standard state is zerozero

Most enthalpies of formation values Most enthalpies of formation values are negative, indicating an are negative, indicating an exothermic processexothermic process

The most stable compounds have the The most stable compounds have the largest exothermic valueslargest exothermic values

Enthalpies of FormationEnthalpies of FormationEnthalpy change for a reaction can be Enthalpy change for a reaction can be calculated from the enthalpies of calculated from the enthalpies of formation of the products and formation of the products and reactants (direct method):reactants (direct method):

ffoo(products)] – (products)] – ff

oo(reactants)] = (reactants)] = rxnrxnoo

Reactions with negative values of Reactions with negative values of rxnrxnoo

are are generallygenerally product-favored, while product-favored, while positive positive rxnrxn

oo usually indicates a usually indicates a reactant-favored reactionreactant-favored reaction

2H2H22S(g) + 3OS(g) + 3O22(g) (g) 2H 2H22O(l) + 2SOO(l) + 2SO22(g) (g) H=?H=?

SubstanceSubstanceHHf f (kJ/mol)(kJ/mol)

HH22S (g)S (g) -20.15-20.15

HH22O (l)O (l) -285.8-285.8

SOSO2 2 (g)(g) -296.1-296.1

Application of EnthalpyApplication of Enthalpy

2 Al(s) + Fe2 Al(s) + Fe22OO33(s) (s) Al Al22OO33(s) + 2 Fe(l)(s) + 2 Fe(l)

HHrxnrxn= -822.8 kJ= -822.8 kJ•How much heat is released if 1 mole of Al is How much heat is released if 1 mole of Al is used?used?

How much heat is released if 4.2 moles of Al is How much heat is released if 4.2 moles of Al is used?used?

How much heat is released if 150. g of Al is How much heat is released if 150. g of Al is used?used?