Energy and Chemical Reactions

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

heat

EnergyEnergy can be kinetic – associated with motion, such as thermal, mechanical, electric, soundEnergy can be potential – associated with an object’s position, such as chemical, gravitational, electrostaticEnergy is converted from one form to another

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

First Law of Thermodynamics

The total energy of the universe is constantEnergy is conserved

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

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

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

Heat transferThe quantity of heat lost by a hotter object and the quantity of heat gained by a cooler object when they are in contact are numerically equal (but opposite direction)Exothermic – heat is transferred from the system to the surroundingsEndothermic – heat is transferred from the surroundings to the system

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

Energy UnitsJoule is the SI unit for thermal energy

1 J = 1 kg.m2/s2

Kilojoules are also commonly usedThe calorie is an older unit for heat; 1 cal = 4.184 JDietary Calories are actually 1000 calories

GOAL: To use the unit Joules to solve energy problems

Specific Heat Capacity and Heat Transfer

The quantity of heat transferred to or from an object when its temperature changes depends on:– Quantity of the material– Size of the temperature change– Identity of the material

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

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

Consequences of Specific Heat Capacity

Objects with a large c value take more energy to change temperature (compare seat belt buckle with bottle of water, or sand with water at the beach)If objects with different c values are allowed to reach thermal equilibrium, the object with the large c value contains more heat energy (aluminum foil vs baked potato)

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

q = m c DTUse to find Heat when Temperature is Changing!q is heat in joulesm is mass in gramsDT = Tfinal – Tinitial

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

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

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

GOAL: To understand these assumptions and apply them to solve Calorimetry problems (begins on slide 14)

Energy and Changes of StateHeat of fusion – energy to convert a substance from solid to liquid (J/g)Heat of vaporization – energy to convert a substance from liquid to gas (J/g)The energy required for a change of state is determined by the type of substance and its quantity (mass)

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: Solid-Liquid Equilibrium

Discussion– Use these terms:

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

White boards:Draw particle

view of a solidDraw particle

view of a liquidDraw particle

view of the melting/freezing phase change

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

CalorimetryConstant pressure calorimetry measures DHConstant pressure calorimetry can be done with a coffee-cup calorimeterA reaction changes the temperature of the solution in the calorimeter; measuring the change in the solution allows calculation of the change in the reaction

qrxn + qsolution = 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

Calorimetry

Constant volume calorimetry measures DEA bomb calorimeter is used for constant volume calorimetryqrxn +qbomb +qwater = 0

Thermodynamics – the study of heat and work

DE = q + w

DE is the change in kinetic and potential energies of the system

Positive q is heat going into the system

Negative q is heat leaving the system

Positive w is work done on the system

Negative w is work done by the system

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

State FunctionsA quantity that is the same no matter what path is chosen in going from initial to finalChanges in internal energy and enthalpy for chemical or physical changes are state functionsNeither heat nor work individually are state functions, but their sum is

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

Enthalpy Changes for Chemical Reactions

Measures the change in heat contentEnthalpy changes are specific to the identity and states of reactants and products and their amountsDH is negative for exothermic reactions and positive for endothermic reactionsValues of DH are numerically equal but opposite in sign for chemical reactions that are the reverse of each otherEnthalpy change depends on molar amounts of reactants and products

2 Methods to find DHrxn

Hess’s Law (indirect method)

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

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

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

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

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

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

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

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

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

Standard Enthalpies of FormationThe standard molar enthalpy of formation (DHf

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

Standard Enthalpy of Formation

The standard enthalpy of formation for an element in its standard state is zeroMost enthalpies of formation values are negative, indicating an exothermic processThe most stable compounds have the largest exothermic values

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

[S DHfo(products)] – [S DHf

o(reactants)] = Dhrxno

Reactions with negative values of DHrxno

are generally product-favored, while positive DHrxn

o usually indicates a reactant-favored reaction

2H2S(g) + 3O2(g) 2H2O(l) + 2SO2(g) DH=?

Substance DHf (kJ/mol)

H2S (g) -20.15

H2O (l) -285.8

SO2 (g) -296.1

Application of Enthalpy

2 Al(s) + Fe2O3(s) Al2O3(s) + 2 Fe(l)

DHrxn= -822.8 kJ• How much heat is released if 1 mole of Al is

used?

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

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