Thermochemistry

The study of energy changes that accompany physical or The study of energy changes that accompany physical or chemical changes in matterchemical changes in matter

All thermodynamics depends on the LAW OF All thermodynamics depends on the LAW OF CONSERVATION OF ENERGY: the total energy of a CONSERVATION OF ENERGY: the total energy of a systemsystem and its and its surroundingssurroundings remains the same remains the same

Changes that occur in matter may be classified as Changes that occur in matter may be classified as physical, chemical or nuclearphysical, chemical or nuclear

Heat and Energy ChangesPhysical, chemical and nuclear changes are all accompanied by a Physical, chemical and nuclear changes are all accompanied by a

change in energychange in energy–PhysicalPhysical: a change in the form of a substance. No chemical : a change in the form of a substance. No chemical bonds are broken.bonds are broken.Ex. hydrogen boils at -252Ex. hydrogen boils at -252°C °C HH2 (l)2 (l) + heat + heat H H2 (g)2 (g)

–ChemicalChemical: a change in the chemical bonds between atoms : a change in the chemical bonds between atoms resulting in new substances resulting in new substances Ex. hydrogen is burned as fuel in the space shuttle’s engines.

2H2 (g) + O2 (g) 2H2O (l) + heat

–NuclearNuclear: a change in the protons or neutrons in an atom : a change in the protons or neutrons in an atom resulting in a new atom.resulting in a new atom.Ex. hydrogen undergoes nuclear fusion in the Sun, producing Ex. hydrogen undergoes nuclear fusion in the Sun, producing helium.helium.

He e2 H 4 42

01-

11

Heat and Energy Changes cont’d Chemical system – the substances undergoing a

change

Surroundings – the system’s environment

Thermal Energy – energy available from a substance as a result of the motion of its molecules

Temperature (T) – the average kinetic energy of the molecules in a sample, measured in °C or K

Heat (q) – amount of thermal energy transferred between substances (hot to cool), measured in Joules (J)

Energy flows between substances because of their difference in temperature.

Heat and Energy Changes cont’dA. A. ExothermicExothermic: releasing thermal energy, heat (q) flows from the : releasing thermal energy, heat (q) flows from the

system to the surroundings, usually causing an increase in the system to the surroundings, usually causing an increase in the temperature of the surroundings. temperature of the surroundings.

q has a negative valueq has a negative value

B. B. EndothermicEndothermic: absorbing thermal energy, heat (q) flows into the : absorbing thermal energy, heat (q) flows into the system from the surroundings, usually causing a decrease in the system from the surroundings, usually causing a decrease in the temperature of the surroundings.temperature of the surroundings.

q has a positive valueq has a positive value

AA BB

Heat Transfer and Enthalpy Change

Thermal Energy – total quantity of potential energy and kinetic Thermal Energy – total quantity of potential energy and kinetic energy of a substance. energy of a substance.

Kinetic EnergyKinetic Energy– Energy of motionEg.– moving electrons in atoms– vibration, rotation and translation of atoms and molecules

Potential EnergyPotential Energy– Energy due to the position or composition of an objectEg:– nuclear potential energy of protons and neutrons– bond energy– intra and intermolecular forces

Endothermic & Exothermic Reactions Revisited… If more energy is If more energy is releasedreleased from the formation of new from the formation of new

bonds in the products than is required to break bonds in bonds in the products than is required to break bonds in the reactants – heat/energy is released –the reactants – heat/energy is released – Exothermic Exothermic

Products of an exothermic reaction have lower Products of an exothermic reaction have lower potential energy (and usually stronger bonds) than the potential energy (and usually stronger bonds) than the reactants.reactants.

The temperature of the surroundings must then…The temperature of the surroundings must then…

IncreaseIncrease

Endothermic Endothermic reactions – opposite, products have a reactions – opposite, products have a higher potential energy than reactants and usually higher potential energy than reactants and usually weaker bonds.weaker bonds.

A fire is started in a fireplace by striking a match and A fire is started in a fireplace by striking a match and lighting crumpled paper under some logs. Explain all the lighting crumpled paper under some logs. Explain all the energy transfers in this scenario using the terms energy transfers in this scenario using the terms ‘exothermic’, ‘endothermic’, ‘system’, ‘surroundings’, ‘exothermic’, ‘endothermic’, ‘system’, ‘surroundings’, ‘potential energy’, and ‘kinetic energy’.‘potential energy’, and ‘kinetic energy’.

Explain why the water in a swimming pool at 24°C has Explain why the water in a swimming pool at 24°C has more thermal energy than a cup of boiling water at 100°C.more thermal energy than a cup of boiling water at 100°C.

More Terms

Measuring Energy Changes

CalorimetryCalorimetry: : experimental technique used to measure energy changes/transfers (the quantity of heat (q) in chemical systems

different substances vary in their ability to absorb amounts of heatdifferent substances vary in their ability to absorb amounts of heat

Substances used in a system have a specific heat capacity:

amount of energy required to raise the temperature of amount of energy required to raise the temperature of 1 g 1 g of a of a substance by substance by 1 1 °C or 1K°C or 1K

(see Table 1, pg.301)(see Table 1, pg.301)

Equation used to determine the value of q:

The amount of heat transferred (The amount of heat transferred (qq) depends on) depends on

» measured in grams

» measured in J/g•°C or J/g•K

–measured in °C or K

Measuring Energy Changes

Calorimeters

Laboratory calorimeter: Laboratory calorimeter: –Can provide precise data for changes of state or chemical Can provide precise data for changes of state or chemical reactions that do not involve gases.reactions that do not involve gases.

Bomb Calorimeter:Bomb Calorimeter:–Container inside which a fuel is burnedContainer inside which a fuel is burned–Much more accurate measurements of heat transfer.Much more accurate measurements of heat transfer.–Pressure changes depending on the reactionPressure changes depending on the reaction–Often described as ‘isolated’Often described as ‘isolated’

Calorimeters

Calorimetry cont… Therefore:Therefore:

HEAT LOST by the PROCESS = HEAT GAINED by the WATERHEAT LOST by the PROCESS = HEAT GAINED by the WATER

OROR

HEAT GAINED by the PROCESS = HEAT LOST by the WATER HEAT GAINED by the PROCESS = HEAT LOST by the WATER

+q

-q

How much energy is required to raise the How much energy is required to raise the temperature of 250mL of water in a 500g kettle that temperature of 250mL of water in a 500g kettle that is 100% iron from 20°C to 99°C?is 100% iron from 20°C to 99°C?

Determine the final temperature of an isolated Determine the final temperature of an isolated system if a 300g lead rock at 150°C is dropped system if a 300g lead rock at 150°C is dropped into 150g of water at 4°C.into 150g of water at 4°C.

If 350.0 mL of 0.50 mol/L HCl at 20.0°C is combined with If 350.0 mL of 0.50 mol/L HCl at 20.0°C is combined with 250.0mL of 2.50 mol/L NaOH at 35.0°C, the final temperature 250.0mL of 2.50 mol/L NaOH at 35.0°C, the final temperature of the mixture is 36.5°C. Find the heat released in this of the mixture is 36.5°C. Find the heat released in this dilution process.dilution process.

qqtotaltotal = = qqacidacid ++ qqbasebase

== mcΔTmcΔTacidacid ++ mcΔTmcΔTbasebase

== (350.0g (350.0g .. 4.18J/(g*°C) 4.18J/(g*°C) .. (36.5°C – 20.0°C) + (36.5°C – 20.0°C) + 250g 250g .. 4.18J/G°C 4.18J/G°C .. (36.5-35.0°C) (36.5-35.0°C)

== 24, 140J24, 140J ++ 1568J1568J

== 25, 708J 25, 708J or 25.7kJor 25.7kJ

Calorimetry of an Acid/Base Dilution

Heat Transfer and Enthalpy Change

ΔH > 0, endothermic reaction (q= negative)ΔH > 0, endothermic reaction (q= negative)

ΔH < 0, exothermic reaction (q=positive)ΔH < 0, exothermic reaction (q=positive)

This system is exothermic b/c product have lower enthalpy than the This system is exothermic b/c product have lower enthalpy than the reactants so ΔH<0.reactants so ΔH<0.

This example of This example of exothermic exothermic

change change shows that the shows that the change in change in

potential potential energy of the energy of the system (system (ΔΔH) H)

equals equals the change in the change in kinetic energy of kinetic energy of

the the surroundings (q)surroundings (q)

Energy

Reaction Progress

Changes in Kinetic and Potential Energyhigh potential energy

low potential energylow kinetic energy

high kinetic energy

ΔH q

ΔHsystem = ±lq surroundingsl

Investigation 1: Thermal Energy Transfer in a Coffee-Cup CalorimeterThermal Energy Transfer in a Coffee-Cup Calorimeter