CALCULATING HEATS OF RXNS o Any phase change requires energy. either energy is absorbed (melting or vaporizing) either energy is absorbed (melting or.

CALCULATING HEATS OF RXNSCALCULATING HEATS OF RXNSo Any phase change requires Any phase change requires

energy.energy.• either energy is absorbed either energy is absorbed

(melting or vaporizing)(melting or vaporizing)– endoendothermicthermic

• Or energy released (condensing Or energy released (condensing or solidifying)or solidifying)– exoexothermicthermic

o This energy is in the form of This energy is in the form of thermal or heat energy.thermal or heat energy.• symbolized by H and is generally symbolized by H and is generally

in units of calories or Joulesin units of calories or Joules

CALCULATING HEATS OF RXNSCALCULATING HEATS OF RXNSo There are two ways to calculate There are two ways to calculate

the energies involved in phase the energies involved in phase changes.changes.• heating the substance up to the heating the substance up to the

melting point or boiling pointmelting point or boiling point– Diagonal portion of the heat Diagonal portion of the heat

curvecurve– Change in kinetic energyChange in kinetic energy

• Performing the actual melting or Performing the actual melting or vaporizingvaporizing– Flat portion of the heat curveFlat portion of the heat curve– Change in potential energyChange in potential energy

HEATING UP A SUBSTANCEHEATING UP A SUBSTANCEo Quantitatively the energy is Quantitatively the energy is

dependent on 3 factorsdependent on 3 factors• Temperature changeTemperature change• Amount of the materialAmount of the material• Type of the materialType of the material

o When we heat up a substance When we heat up a substance the particles vibrate faster.the particles vibrate faster.• According to the KMT, temp is According to the KMT, temp is

the average kinetic energy of the the average kinetic energy of the particles that make up the particles that make up the substancesubstance

HEATING UP A SUBSTANCEHEATING UP A SUBSTANCEo We calculate the change in the We calculate the change in the

temperature (temperature (ΔΔT) by taking the T) by taking the final temp (Tfinal temp (Tff) – initial temp (T) – initial temp (Tii))– ΔΔT = TT = Tff – T – Tii

o How much of the substance we How much of the substance we are heating is also important.are heating is also important.• The more we are heating, the The more we are heating, the

more energy it will takemore energy it will take• The unit we will keep track of The unit we will keep track of

here is masshere is mass

HEATING UP A SUBSTANCEHEATING UP A SUBSTANCEo The last component important to The last component important to

measure in order to calculate measure in order to calculate heat energy is dependent on the heat energy is dependent on the material (type)material (type)• Metals absorb heat differently Metals absorb heat differently

than plastic or water or etc.than plastic or water or etc.– Different substances heat up and Different substances heat up and

cool off at different ratescool off at different rateso This difference is measured in a This difference is measured in a

constant (substance dependent) constant (substance dependent) called called specific heat capacityspecific heat capacity..

HEATING UP A SUBSTANCEHEATING UP A SUBSTANCE• Specific heat capacity (C) is Specific heat capacity (C) is

defined as the amount of energy defined as the amount of energy it takes 1 g of a substance to heat it takes 1 g of a substance to heat up or cool down by 1 °C (J/g°C).up or cool down by 1 °C (J/g°C).

oMetals tend to have relatively low Metals tend to have relatively low specific heat capacities.specific heat capacities.• It takes them less energy to feel It takes them less energy to feel

hotter and cool off quicklyhotter and cool off quicklyoWater has a relatively high Water has a relatively high

specific heat capacity.specific heat capacity.• Water heats up and cools off Water heats up and cools off

slowlyslowly

HEATING UP A SUBSTANCEHEATING UP A SUBSTANCEoPutting these three Putting these three

measurements together we get measurements together we get the heat equationthe heat equation• ΔΔH = mCH = mCΔΔTT• ΔΔT = TT = Tf f - T- TiiHow much energy is absorbed How much energy is absorbed

by 123.3 grams of water if it by 123.3 grams of water if it heats up from 2.50°C to 75.5°C?heats up from 2.50°C to 75.5°C?

HEATING & COOLING CWHEATING & COOLING CW1.1. How much heat is lost by an How much heat is lost by an

aluminum pan that weighs aluminum pan that weighs 3500 grams if it comes out of 3500 grams if it comes out of the oven at a temperature of the oven at a temperature of 177 °C and is put into a sink-177 °C and is put into a sink-full of water at 23.8 °C?full of water at 23.8 °C?

2.2. What is the specific heat of a What is the specific heat of a 50.0 g piece of metal that 50.0 g piece of metal that gains 2118 Joules of energy gains 2118 Joules of energy as it heats up 110°C? What as it heats up 110°C? What metal is it?metal is it?

CHANGING PHASECHANGING PHASEoThe other equation we use for The other equation we use for

phase change calculations is phase change calculations is the energy required to perform the energy required to perform a phase change a phase change

oThis calc is different than the This calc is different than the mCmCΔΔT eqn since phase changes T eqn since phase changes don’t involve a change in temp.don’t involve a change in temp.• This is the flat portion of the This is the flat portion of the

heating curveheating curve• Energy is still being absorbed or Energy is still being absorbed or

lost, but not kinetic energylost, but not kinetic energy

CHANGING PHASECHANGING PHASEo During a phase change, the During a phase change, the

energy added begins to break the energy added begins to break the IMFs that are holding the IMFs that are holding the molecules togethermolecules together

o To calculate the energy To calculate the energy necessary to change the necessary to change the phase of a substance we need phase of a substance we need two constants.two constants.• Heat of fusion (Heat of fusion (ΔΔHHfusfus), which is the ), which is the

energy it takes to melt 1 mol of a energy it takes to melt 1 mol of a substancesubstance

• Heat of vaporization (Heat of vaporization (ΔΔHHvapvap), ), which is the energy it takes to which is the energy it takes to vaporize1 mol of a substancevaporize1 mol of a substance

CHANGING PHASECHANGING PHASEo If you have more than a mole of If you have more than a mole of

material or less than a mole, it material or less than a mole, it will therefore, take proportionally will therefore, take proportionally more or less energy to cause the more or less energy to cause the phase changephase change• Heat energy it takes to melt a Heat energy it takes to melt a

given amount of material: given amount of material: mol(mol(ΔΔHHfusfus))

• Heat energy it takes to vaporize a Heat energy it takes to vaporize a given amount of material: given amount of material: mol(mol(ΔΔHHvapvap))

CHANGING PHASE EXAMPLECHANGING PHASE EXAMPLEHow much energy does it take How much energy does it take to completely melt a 75.0 gram to completely melt a 75.0 gram

cube of ice?cube of ice?

If a cloud of steam loses 175,000 JIf a cloud of steam loses 175,000 J of energy as it condenses, of energy as it condenses, how much water would be how much water would be

collected?collected?

CHANGING PHASE CWCHANGING PHASE CW1.1. How much energy must be How much energy must be

removed from 540 g of liquid removed from 540 g of liquid water at 0°C in order to water at 0°C in order to convert it to ice?convert it to ice?

2.2. If 13360 J of heat was added If 13360 J of heat was added to 1000 g of ice at 0°C, how to 1000 g of ice at 0°C, how much water at 0°C is much water at 0°C is produced, and how much ice produced, and how much ice remains?remains?

PUTTING IT ALL TOGETHERPUTTING IT ALL TOGETHERo We know how to calculate the We know how to calculate the

energy it takes to heat up a energy it takes to heat up a substancesubstance• ΔΔH = mCH = mCΔΔTT

o And how much energy it takes to And how much energy it takes to cause phase to changecause phase to change• mol(mol(ΔΔHHfusfus) or mol() or mol(ΔΔHHvapvap))

o Now we can calculate how much Now we can calculate how much energy it takes to heat up a energy it takes to heat up a substance all the way through substance all the way through phase changes phase changes

HHtotaltotal = =

mCmCsolidsolidT+n(T+n(HHfusfus))

++mCmCliquidliquidT+T+HHvapvap++HHgasgas

HHtotaltotal = =HHsolidsolid HHmeltingmelting HHliquidliquid HHvaporizingvaporizing HHgasgas

HHtotaltotal = =HHsolidsolid HHmeltingmelting HHliquidliquid HHvaporizingvaporizing HHgasgas

mCmCsolidsolid TT +n(+n(HHfusfus)) +mC+mCliquidliquidTT+n(+n(HHvapvap))+mC+mCgasgasTT

PUTTING IT ALL TOGETHERPUTTING IT ALL TOGETHERHow much energy is necessary to How much energy is necessary to heat up 125.5 grams of ice up to heat up 125.5 grams of ice up to

75 °C?75 °C?

PUTTING IT ALL TOGETHERPUTTING IT ALL TOGETHERHow much energy is lost as 313 How much energy is lost as 313 grams of steam is cooled down grams of steam is cooled down

from 150 °C and frozen into ice at -from 150 °C and frozen into ice at -15 °C?15 °C?