Chemical rxns involve changes in energy – Breaking bonds requires energy – Forming bonds...

Chemical rxns involve changes in Chemical rxns involve changes in energy energy

– Breaking bondsBreaking bonds requires energy requires energy – Forming bondsForming bonds releases energyreleases energy

The study of the changes in energy in The study of the changes in energy in chem rxns is called chem rxns is called thermochemistry.thermochemistry.

The energy involved in chemistry is The energy involved in chemistry is real and generally measurable, and real and generally measurable, and can be thought of as can be thought of as heatheat

– Energy units are numerous, but our Energy units are numerous, but our focus will be focus will be JouleJoule (SI base unit) and (SI base unit) and the the caloriecalorie

– 1 calorie = 1 calorie = 4.184 Joules4.184 Joules

Chemical rxns involve changes in Chemical rxns involve changes in energy energy

– Breaking bondsBreaking bonds requires energy requires energy – Forming bondsForming bonds releases energyreleases energy

The study of the changes in energy in The study of the changes in energy in chem rxns is called chem rxns is called thermochemistry.thermochemistry.

The energy involved in chemistry is The energy involved in chemistry is real and generally measurable, and real and generally measurable, and can be thought of as can be thought of as heatheat

– Energy units are numerous, but our Energy units are numerous, but our focus will be focus will be JouleJoule (SI base unit) and (SI base unit) and the the caloriecalorie

– 1 calorie = 1 calorie = 4.184 Joules4.184 Joules

INTRO TO THERMOCHEMISTRYINTRO TO THERMOCHEMISTRY

WHAT IS HEAT?WHAT IS HEAT?Hot & cold, are automatically associated Hot & cold, are automatically associated

with the words heat and temperaturewith the words heat and temperature– Heat & temperature are Heat & temperature are NOTNOT synonyms synonyms– The temperature of a substance is The temperature of a substance is

directly related directly related to the vibrational energy to the vibrational energy of its particlesof its particles, specifically its:, specifically its:

Hot & cold, are automatically associated Hot & cold, are automatically associated with the words heat and temperaturewith the words heat and temperature

– Heat & temperature are Heat & temperature are NOTNOT synonyms synonyms– The temperature of a substance is The temperature of a substance is

directly related directly related to the vibrational energy to the vibrational energy of its particlesof its particles, specifically its:, specifically its:

The Kinetic Energy defines the The Kinetic Energy defines the temperaturetemperature– Particles vibrating fast = Particles vibrating fast = hothot– Particles vibrating slow = Particles vibrating slow = coldcold

KineticKinetic energy is transferred from one energy is transferred from one particle to the next (a.k.a. conduction)particle to the next (a.k.a. conduction)

– Sometimes this energy can be Sometimes this energy can be transferred from one object to another transferred from one object to another and influence physical propertiesand influence physical properties

– The more energy an object has The more energy an object has the more energy is transferredthe more energy is transferred

KineticKinetic energy is transferred from one energy is transferred from one particle to the next (a.k.a. conduction)particle to the next (a.k.a. conduction)

– Sometimes this energy can be Sometimes this energy can be transferred from one object to another transferred from one object to another and influence physical propertiesand influence physical properties

– The more energy an object has The more energy an object has the more energy is transferredthe more energy is transferred

An Ice Cold Spoon A Hot Spoon

2 Hot Spoons2 Hot Spoons

Thermal energy (Thermal energy (qq) is the total ) is the total energy of all the particles that make energy of all the particles that make up a substanceup a substance

– Kinetic energy fromKinetic energy from vibration of vibration of particlesparticles

– Potential energy fromPotential energy from molecular molecular attractionattraction (within or between the (within or between the particles)particles)

Thermal energy is dependent upon Thermal energy is dependent upon the amount or mass of the amount or mass of material present material present ( (KE =KE =½mv½mv22))

Thermal energy is Thermal energy is also related to the also related to the type of materialtype of material

Different types of materials may Different types of materials may have the same temp, same mass, have the same temp, same mass, but differentbut different connectivityconnectivity– Affected by the potential energy Affected by the potential energy

stored in chemical bonds or the stored in chemical bonds or the IMFs IMFs holding molecules togetherholding molecules together

So it is possible to be at same temp So it is possible to be at same temp (same KE) (same KE) but have very different but have very different thermal energiesthermal energies

The ability to hold onto or release The ability to hold onto or release thermal (heat) energy is referred to thermal (heat) energy is referred to as the substance’s as the substance’s heat capacityheat capacity

Thermal energy can be transferred Thermal energy can be transferred from object to object through direct from object to object through direct contactcontact– Molecules collide,Molecules collide, transferring energy transferring energy

from molecule to moleculefrom molecule to molecule– The flow of thermal energy is called The flow of thermal energy is called

heatheat

DEFINITION

THE FLOW OF THERMAL ENERGY FROM SOMETHING WITH A

HIGHER TEMP TO SOMETHING WITH A LOWER TEMP

UNITS MEASURED IN JOULES OR CALORIES

TYPES

THROUGH WATER OR AIR = CONVECTION

THROUGH SOLIDS = CONDUCTION

TRANSFERRED ENERGY BY COLLISION WITH PHOTON =

RADIANT ENERGY

HEAT CAPACITYHEAT CAPACITY The measure of how well a material The measure of how well a material

absorbs or releases heat energy is absorbs or releases heat energy is its heat capacityits heat capacity– It can be thought of as a reservoir to It can be thought of as a reservoir to hold heat, how much it holds before hold heat, how much it holds before it overflows is its it overflows is its capacitycapacity

Heat capacity is a physical property Heat capacity is a physical property unique to a particular materialunique to a particular material– Water takesWater takes 1 calorie1 calorie of energy of energy to raise temp 1 to raise temp 1 °C°C

– Steel takesSteel takes onlyonly 0.1 calorie0.1 calorie of energy to raise temp of energy to raise temp 1 1 °C°C

The measure of how well a material The measure of how well a material absorbs or releases heat energy is absorbs or releases heat energy is its heat capacityits heat capacity– It can be thought of as a reservoir to It can be thought of as a reservoir to hold heat, how much it holds before hold heat, how much it holds before it overflows is its it overflows is its capacitycapacity

Heat capacity is a physical property Heat capacity is a physical property unique to a particular materialunique to a particular material– Water takesWater takes 1 calorie1 calorie of energy of energy to raise temp 1 to raise temp 1 °C°C

– Steel takesSteel takes onlyonly 0.1 calorie0.1 calorie of energy to raise temp of energy to raise temp 1 1 °C°C

SPECIFIC HEAT CAPACITYSPECIFIC HEAT CAPACITY The amount of energy it takes to raise

the temp of 1 gram of an object 1°C is that object’s specific heat capacity (C or s)

Specific heats can be listed on data tables

– Smaller the specific heat the less energy it takes the substance to feel hot• They heat up quickly and cool down

quickly– Larger the specific heat the more

energy it takes to heat a substance up (bigger the heat reservoir)• They heat up slowly and cool down slowly

The amount of energy it takes to raise the temp of 1 gram of an object 1°C is that object’s specific heat capacity (C or s)

Specific heats can be listed on data tables

– Smaller the specific heat the less energy it takes the substance to feel hot• They heat up quickly and cool down

quickly– Larger the specific heat the more

energy it takes to heat a substance up (bigger the heat reservoir)• They heat up slowly and cool down slowly

Specific Heat Equation Q = MCSpecific Heat Equation Q = MCTTQ = Quantity of heat (joules)Q = Quantity of heat (joules)M = Mass of substance (grams)M = Mass of substance (grams)C = Specific heat capacityC = Specific heat capacityT = Change in temperature T = Change in temperature

TTfinalfinal – T – Tinitialinitial = Change in temp. = Change in temp.http://www.youtube.com/watch?http://www.youtube.com/watch?

v=XLWP03pwTYY v=XLWP03pwTYY

Specific Heat Equation Q = MCSpecific Heat Equation Q = MCTTQ = Quantity of heat (joules)Q = Quantity of heat (joules)M = Mass of substance (grams)M = Mass of substance (grams)C = Specific heat capacityC = Specific heat capacityT = Change in temperature T = Change in temperature

TTfinalfinal – T – Tinitialinitial = Change in temp. = Change in temp.http://www.youtube.com/watch?http://www.youtube.com/watch?

v=XLWP03pwTYY v=XLWP03pwTYY

SPECIFIC HEAT CAPACITYSPECIFIC HEAT CAPACITY

SUBSTANCESUBSTANCE SPECIFIC HEAT CAPACITY, SPECIFIC HEAT CAPACITY, CCPP

WATERWATER 4.184.18J/g°C OR J/g°C OR 11cal/g°Ccal/g°C

ICE ICE 2.10 2.10 J/g°C OR J/g°C OR .502.502cal/g°Ccal/g°C

STEAMSTEAM 1.871.87J/g°C OR J/g°C OR .447.447cal/g°Ccal/g°C

MERCURY, HgMERCURY, Hg .139 .139 J/g°C OR J/g°C OR .033.033cal/g°Ccal/g°C

ALCOHOL ALCOHOL (Ethyl)(Ethyl) 2.40 2.40 J/g°C OR J/g°C OR .580.580cal/g°Ccal/g°C

CALCIUM, CaCALCIUM, Ca .647 .647 J/g°C OR J/g°C OR .155.155cal/g°Ccal/g°C

ALUMINUM, AlALUMINUM, Al .992.992J/g°C OR J/g°C OR .237.237cal/g°Ccal/g°C

TABLE SALT, TABLE SALT, NaClNaCl .865 .865 J/g°C OR J/g°C OR .207.207cal/g°Ccal/g°C

AMMONIA, NHAMMONIA, NH33 2.09 2.09 J/g°C OR J/g°C OR .500.500cal/g°Ccal/g°C

SILVER, AgSILVER, Ag .235 .235 J/g°C OR J/g°C OR .056.056cal/g°Ccal/g°C

LEAD, PbLEAD, Pb .129.129J/g°C OR J/g°C OR .031.031cal/g°Ccal/g°C

There are three methods used to There are three methods used to transfer heat/thermal energy transfer heat/thermal energy

– ConductionConduction – transfer of heat – transfer of heat through direct contactthrough direct contact

– ConvectionConvection – transfer of heat – transfer of heat through a medium like air or through a medium like air or waterwater

– RadiantRadiant – transfer of heat by – transfer of heat by electromagnetic radiationelectromagnetic radiation

There are three methods used to There are three methods used to transfer heat/thermal energy transfer heat/thermal energy

– ConductionConduction – transfer of heat – transfer of heat through direct contactthrough direct contact

– ConvectionConvection – transfer of heat – transfer of heat through a medium like air or through a medium like air or waterwater

– RadiantRadiant – transfer of heat by – transfer of heat by electromagnetic radiationelectromagnetic radiation

CHANGE IN HEAT ENERGY (ENTHALPY)CHANGE IN HEAT ENERGY (ENTHALPY) The energy used or produced in a The energy used or produced in a

chem rxn is called the enthalpy of chem rxn is called the enthalpy of the rxn (the rxn (HHrxnrxn))– Burning a 15 gram piece of paper Burning a 15 gram piece of paper

produces a particular amount of produces a particular amount of thermal energy or heat energy thermal energy or heat energy ((enthalpyenthalpy))

Enthalpy is a value that also contains Enthalpy is a value that also contains a component of direction (energy in a component of direction (energy in or energy out)or energy out)– Heat gained by the surroundings Heat gained by the surroundings

is the is the out-of/exoout-of/exo direction direction– Heat lost by the surroundings Heat lost by the surroundings

is the is the in-to/endoin-to/endo direction direction

HEATHEATHEATHEAT HEATHEATHEATHEAT HEATHEATHEATHEAT HEATHEATHEATHEAT

Chemical rxns can be classified as either:Chemical rxns can be classified as either:– ExothermicExothermic a reaction in which heat a reaction in which heat

energy is generated (aenergy is generated (a productproduct))– EndothermicEndothermic reaction in which heat reaction in which heat

energy is absorbed (aenergy is absorbed (a reactantreactant))Exothermic rxns typically feel Exothermic rxns typically feel warmwarm as as

the rxn proceeds (from the perspective the rxn proceeds (from the perspective of the surroundings)of the surroundings)– Give off heat energy (Give off heat energy (light, fire, heatlight, fire, heat))

Endothermic rxns typically feel Endothermic rxns typically feel coolercooler the longer the rxn proceeds (from the the longer the rxn proceeds (from the perspective of the surroundings)perspective of the surroundings)– Absorb heat energy, sometimes Absorb heat energy, sometimes

enough to get very coldenough to get very coldhttp://www.youtube.com/watch?http://www.youtube.com/watch?

v=XgiCn1IpvzM&list=PL65159266CFCv=XgiCn1IpvzM&list=PL65159266CFC74682 74682

C3H

8

C3H

8

++5O25O2 2043kJ 2043kJ3CO23CO2 4H2O4H2O++ ++

Exothermic rxnExothermic rxn

– To a cold camper, the important product here is the heat energy

– To a cold camper, the important product here is the heat energy

C3H8 + O2

In an exothermic process the amount of In an exothermic process the amount of energy given off is more than the initial energy given off is more than the initial

energy invested. So the products are always energy invested. So the products are always lower in energy than the reactants.lower in energy than the reactants.

NH4NO3+H2O+ 752kJ NH4OH + HNO3

NH4NO3+H2O+ 752kJ NH4OH + HNO3

Endothermic rxn Endothermic rxn

– Similar system as what is found in cold packs

– Similar system as what is found in cold packs

NH4NO3 + H2O

NH4OH + HNO3

In an endothermic process more energy is In an endothermic process more energy is required to cause the rxn to proceed than required to cause the rxn to proceed than obtained in return. So the products are obtained in return. So the products are

always higher in energy than the always higher in energy than the reactants.reactants.

Most common measurement of the Most common measurement of the energy or enthalpy in a reaction is energy or enthalpy in a reaction is actually a actually a change in enthalpychange in enthalpy ( (HH) )

– HHrxnrxn = = ∑H∑Hproductsproducts - ∑H - ∑Hreactantsreactants The enthalpy absorbed or gained The enthalpy absorbed or gained

(changed) in a rxn is dependent on (changed) in a rxn is dependent on the the number of moles number of moles of material of material reactingreacting

– We can stoichiometrically calculate We can stoichiometrically calculate how much energy a rxn uses or how much energy a rxn uses or produces produces

– H values can be provided with a H values can be provided with a rxn equn and have magnitude & rxn equn and have magnitude & direction of transfer (+ or -)direction of transfer (+ or -)

CHANGE IN ENTHALPYCHANGE IN ENTHALPY

(For Example)(For Example)How much heat will be How much heat will be

absorbed for 1.0g of Habsorbed for 1.0g of H22OO22 to to decompose in a bombardier decompose in a bombardier

beetle to produce a beetle to produce a defensive spray of steamdefensive spray of steam

2H2H22OO2 2 +190kJ+190kJ 2H 2H22O + OO + O22

USING H IN CALCULATIONSUSING H IN CALCULATIONS Chemical reaction equations are Chemical reaction equations are

very powerful tools. very powerful tools. – Given a rxn equation with an energy Given a rxn equation with an energy

value, We can calculate the amount value, We can calculate the amount of energy produced or used for any of energy produced or used for any given amount of reactants.given amount of reactants.

Analyze: we know that if we had 2 mols of H2O2 decomposing we would use 190kJ of heat, but how much would it be if only 1.0 g of H2O2

Analyze: we know that if we had 2 mols of H2O2 decomposing we would use 190kJ of heat, but how much would it be if only 1.0 g of H2O2

Therefore: we have to convert our given 1.0 g of H2O2 to moles of H2O2

Therefore: we have to convert our given 1.0 g of H2O2 to moles of H2O2

1.0g H1.0g H22OO22

1mol H1mol H22OO22

34g H34g H22OO22

2H2O2 +190kJ 2H2O + O22H2O2 +190kJ 2H2O + O2

= .02941 mol= .02941 mol

Molar massMolar mass

Therefore: with 2 moles of H2O2 it requires the use of 190 kJ of energy, but we don’t have 2 moles we only have .02941 mols of H2O2, so how much energy would the bug require?

Therefore: with 2 moles of H2O2 it requires the use of 190 kJ of energy, but we don’t have 2 moles we only have .02941 mols of H2O2, so how much energy would the bug require?

190kJ190kJ

2molH2molH22OO22

= 2.8kJ= 2.8kJ.02941 mol.02941 mol

Rxn equationRxn equation

2H2O2 +190kJ 2H2O + O22H2O2 +190kJ 2H2O + O2

How much heat will be released when 4.77 g of ethanol (C2H5OH) react with excess O2 according to the following

equation:

C2H5OH+3O2 2CO2+3H2O H = -1366.7kJ

How much heat will be released when 4.77 g of ethanol (C2H5OH) react with excess O2 according to the following

equation:

C2H5OH+3O2 2CO2+3H2O H = -1366.7kJ

Example #2Example #2

4.77g C4.77g C22HH55OHOH1mol C1mol C22HH55OHOH

46g C46g C22HH55OHOH

-1366.7kJ-1366.7kJ

1mol C1mol C22HH55OHOH

= -142 kJ= -142 kJ

1. Ethanol, C2H5OH, is quite flammable and when 1 mole of it burns it has a reported H of -1366.8 kJ. How much energy is given off in the combustion of enough ethanol to produce 12.0 L of Carbon dioxide @ 755 mmHg and 25.0°C?

1. Ethanol, C2H5OH, is quite flammable and when 1 mole of it burns it has a reported H of -1366.8 kJ. How much energy is given off in the combustion of enough ethanol to produce 12.0 L of Carbon dioxide @ 755 mmHg and 25.0°C?

Classroom Practice 1Classroom Practice 1

1 C2H5OH+ 3 O2 2 CO2+ 3 H2O H= -1366.8 kJ

1 C2H5OH+ 3 O2 2 CO2+ 3 H2O H= -1366.8 kJ

H =H = FINAL TEMP – INITIAL TEMPFINAL TEMP – INITIAL TEMP

SPECIFICHEAT

SPECIFICHEATMASSMASS

We can also track energy changes due to temp changes, using H=mCT:

We can also track energy changes due to temp changes, using H=mCT:

If the temp difference is positive– The rxn is exothermic because the

final temp is greater than the initial temp

– So the enthalpy ends up positive

If the temp difference is positive– The rxn is exothermic because the

final temp is greater than the initial temp

– So the enthalpy ends up positive if the temp change is negative– the enthalpy ends up negative– the rxn absorbed heat into the

system, so it’s endothermic

if the temp change is negative– the enthalpy ends up negative– the rxn absorbed heat into the

system, so it’s endothermic

Example: If you drink 4 glasses of ice water at 0°C, how much heat energy is transferred as this water is brought to body temp? Each glass contains 250 g of water & body temp is 37°C.

Example: If you drink 4 glasses of ice water at 0°C, how much heat energy is transferred as this water is brought to body temp? Each glass contains 250 g of water & body temp is 37°C. mass of 4 glasses of water:

– m = 4 x 250g = 1000g H2O change in water temp:

– Tf – Ti = 37°C - 0°C specific heat of water:

–CH2O = 4.18 J/g•C°(from previous slide)

mass of 4 glasses of water:– m = 4 x 250g = 1000g H2O

change in water temp:– Tf – Ti = 37°C - 0°C

specific heat of water:–CH2O = 4.18 J/g•C°(from previous slide)H=mCH2OTH=mCH2OTH=(1000g)(4.18J/g•°C)(37°C)H=(1000g)(4.18J/g•°C)(37°C)H= 160,000JH= 160,000J

Example 2: 500 g of a liquid is heated from 25°C to 100°C. The liquid absorbs 156,900 J of energy. What is the specific heat of the liquid and identify it.

Example 2: 500 g of a liquid is heated from 25°C to 100°C. The liquid absorbs 156,900 J of energy. What is the specific heat of the liquid and identify it. H = mCTH = mCT

C= H/mTC= H/mT

C = 156,900J/(500g)(75°C)C = 156,900J/(500g)(75°C)

C = 4.184 J/g°CC = 4.184 J/g°C H2OH2O

2. An orange contains 445 kJ of energy. What volume of water could this same amount of energy raise from a temp of 25.0°C to the boiling point?

3. Water at 0.00°C was poured into 30.0g of water in a cup at 45.0°C. The final temp of the water mixture was 19.5°C. What was the mass of the 0.00°C water?


3. Water at 0.00°C was poured into 30.0g of water in a cup at 45.0°C. The final temp of the water mixture was 19.5°C. What was the mass of the 0.00°C water?



H= 445 KJ x 1000 = 445000J

T = 100.0 – 25.0 = 75.0

C = 4.18 J/g°C

M = H / C T

445000J / (4.18 x 75.0) = 1420 g H2O = 1420 ml H2O

Enthalpy is dependent on theEnthalpy is dependent on the conditionsconditions of the rxnof the rxn– It’s important to have aIt’s important to have a standard setstandard set

of conditions, which allows us to of conditions, which allows us to compare Cthe affect of temps, compare Cthe affect of temps, pressures, etc. On different pressures, etc. On different substancessubstances

Chemist’s have defined a standard Chemist’s have defined a standard set of conditionsset of conditions– Stand. Temp =Stand. Temp = 298K or 25°C298K or 25°C– Stand. Press =Stand. Press = 1atm or 760mmHg1atm or 760mmHg

Enthalpy produced in a rxn under Enthalpy produced in a rxn under standard conditions is the standard conditions is the standard standard enthalpy (enthalpy (H°)H°)

Standard enthalpies can be found on Standard enthalpies can be found on tables measured as standardtables measured as standard enthalpies of formations, enthalpies of formations, enthalpies enthalpies of combustion,of combustion, enthalpies of solution, enthalpies of solution, enthalpies of fusion, and enthalpies enthalpies of fusion, and enthalpies of vaporizationof vaporization

– Enthalpy of formation (Enthalpy of formation (HHff) is the ) is the amount of energy involved in the amount of energy involved in the formation of a compound from its formation of a compound from its component elements.component elements.

– Enthalpy of combustion (Enthalpy of combustion (HHcombcomb) is ) is the amount of energy produced in a the amount of energy produced in a combustion rxn.combustion rxn.

– Enthalpy of solution (Enthalpy of solution (HHdissdiss) is the ) is the amount of energy involved in the amount of energy involved in the dissolving of a compound dissolving of a compound

– Enthalpy of fusion (Enthalpy of fusion (HHfusfus) is the ) is the amount of energy necessary to melt amount of energy necessary to melt a substance.a substance.

– Enthalpy of vaporization (Enthalpy of vaporization (HHvapvap) is ) is the amount of energy necessary to the amount of energy necessary to convert a substance from a liquid convert a substance from a liquid to a gas.to a gas.

All of these energies are measured All of these energies are measured very carefully in a laboratory setting very carefully in a laboratory setting under specific conditionsunder specific conditions

– At 25 At 25 °°C and 1atm of pressureC and 1atm of pressure These measured energies are These measured energies are

reported in tables to be used in reported in tables to be used in calculations all over the world.calculations all over the world.

Calorimetry is the process of Calorimetry is the process of measuring heat energy measuring heat energy – Measured using a device called a Measured using a device called a

calorimetercalorimeter– Uses the heat absorbed by HUses the heat absorbed by H22O to O to

meas-ure the heat meas-ure the heat given offgiven off by a rxn by a rxn or an objector an object

The amount of The amount of heat soaked upheat soaked up by by the water is the water is equal toequal to the amount of the amount of heat heat releasedreleased by the rxn by the rxn

HHSYSSYS=-=-HHSURSUR

HHsyssys is the system or what is the system or what is taking place in the is taking place in the

main chamber (rxn etc.) main chamber (rxn etc.) And And HHsursur is the is the

surroundings which is surroundings which is generally water. generally water.

A COFFEE CUPA COFFEE CUPCALORIMETERCALORIMETER

A BOMB A BOMB CALORIMETERCALORIMETER

used when tryingused when tryingto find the amountto find the amountof heat produced of heat produced

bybyburning burning

something.something.

Used for a Used for a reactionreaction

In water, or just a In water, or just a transfer of heat.transfer of heat.

With calorimetry we use the sign of With calorimetry we use the sign of what happens to the waterwhat happens to the water– When the water loses heat into the When the water loses heat into the

system it obtains a system it obtains a negative change negative change (- (-HHsurrsurr))– Endothermic (+Endothermic (+HHsyssys))

– When the water gains heat from the When the water gains heat from the system it obtains a system it obtains a positive change positive change (+(+HHsurrsurr))– Exothermic (-Exothermic (-HHsyssys))

CALORIMETRYCALORIMETRY

HEATHEATHEATHEATHEATHEATHEATHEAT

HEATHEATHEATHEATHEATHEATHEATHEAT

--HH sys sys = = HHsurrsurr

- SIGN MEANS- SIGN MEANSHEAT WASHEAT WAS

RELEASED BY RELEASED BY THE RXNTHE RXN

+ SIGN MEANS+ SIGN MEANSHEAT WASHEAT WAS

ABSORBED BYABSORBED BYWATERWATER

HHsys sys ==--HHsurr surr

+ SIGN MEANS+ SIGN MEANSHEAT WASHEAT WAS

ABSORBED BY ABSORBED BY THE RXNTHE RXN

- SIGN MEANS- SIGN MEANSHEAT WASHEAT WAS

RELEASED BYRELEASED BYWATERWATER

You calculate the amount of heat You calculate the amount of heat absor-bed by the water (using absor-bed by the water (using H= H= mCmCT)T)

Which leads to the amount of heat Which leads to the amount of heat given off by the rxngiven off by the rxn– you know the mass of the water (you know the mass of the water (by by

weighing itweighing it))– you know the specific heat for water you know the specific heat for water

((found on a tablefound on a table))– and you can measure the change in and you can measure the change in

the temp of water (the temp of water (using a using a thermometerthermometer))

CALORIMETRYCALORIMETRY

A chunk of Al that weighs 72.0g is A chunk of Al that weighs 72.0g is heated to 100°C is dropped in a heated to 100°C is dropped in a calorimeter containing 120ml of calorimeter containing 120ml of

water at 16.6°C. water at 16.6°C. the Hthe H22O’s temp rises to 27°C.O’s temp rises to 27°C.- mass of Al mass of Al == 72g 72g

- TTinitialinitial of Al = of Al = 100°C 100°C

- TTfinalfinal of Al = of Al = 27°C 27°C

- CCAl Al == .992J/g°C .992J/g°C (from table)(from table)

HHAlAl

HHAlAl = = 72g72g .992J/g°C.992J/g°C 27°C-100°C27°C-100°C

HHAlAl== -5214J-5214J

We can do the same calc with the We can do the same calc with the water infowater info

HH2OHH2O

HHHH22OO== 5216J5216J

Equal but opposite, means that the Al Equal but opposite, means that the Al decreased in temp, it released its stored decreased in temp, it released its stored

heat into the Hheat into the H22O, causing the temp of the O, causing the temp of the HH22O to increase.O to increase.

HHHH22OO ==120g120g 4.18J/g°C4.18J/g°C 27°C-16.6°C27°C-16.6°C

– Mass of HMass of H22O= O= 120g120g– TTinitialinitial of H of H22O= O= 16.6°C16.6°C– TTfinalfinal of H of H22O = O = 27°C27°C– CCH2OH2O= = 4.18J/g°C 4.18J/g°C (from table)(from table)

When a 4.25 g sample of solid When a 4.25 g sample of solid NHNH44NONO33 dissolves in 60.0 g of water dissolves in 60.0 g of water in a calori-meter, the temperature in a calori-meter, the temperature

drops from 21.0drops from 21.0°°C to 16.9C to 16.9°°C. C. Calculate the energy involved in the Calculate the energy involved in the

dissolving of the NHdissolving of the NH44NONO33. . Hwater = (mwater)(Cwater)(Twater)Hwater = (mwater)(Cwater)(Twater)

Hwater = (60g)(4.18J/g°C)(16.9°C-21.0°C)

Hwater = (60g)(4.18J/g°C)(16.9°C-21.0°C)Hwater = -1.03 x 103 JHwater = -1.03 x 103 J

Hwater = HNH4NO3Hwater = HNH4NO3

HNH4NO3 = 1.03 x 103 JHNH4NO3 = 1.03 x 103 J

4.4. A coffee-cup calorimeter with a mass of A coffee-cup calorimeter with a mass of 4.8 g is filled with water to mass of 250 4.8 g is filled with water to mass of 250 g. The water temperature was 24.2g. The water temperature was 24.2C C before 3.2 g of NaOH pellets was before 3.2 g of NaOH pellets was added to the water. After the NaOH added to the water. After the NaOH pellets had dissolved the temp of the pellets had dissolved the temp of the water registered 85.8water registered 85.8C. How much C. How much heat did the Hheat did the H22O absorb, and how much O absorb, and how much heat did the NaOH produce?heat did the NaOH produce?

5.5. 41.0g of glass at 95°C is placed in 175 g 41.0g of glass at 95°C is placed in 175 g of Water at 19.5°C in a calorimeter. The of Water at 19.5°C in a calorimeter. The temps are allowed to equalize. What is temps are allowed to equalize. What is the final temp of the glass/water the final temp of the glass/water mixture? (Water = 4.18J/g°C; Glass = mixture? (Water = 4.18J/g°C; Glass = 8.78J/g°C)8.78J/g°C)


Chemical rxns involve changes in energy – Breaking bonds requires energy – Forming bonds...

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Transcript of Chemical rxns involve changes in energy – Breaking bonds requires energy – Forming bonds...