INTRO TO THERMOCHEMISTRY

Chemical reactions involve changes in energy

Breaking bonds requires energy Forming bonds releases energy

These energy changes can be in the form of heat

Heat is the flow of chemical energy

The study of the changes in energy in chemical reactions is called thermochemistry.

The energy involved in chemistry is real and generally a measurable value.

WHAT IS HEAT?Hot & cold, are automatically associated with the words heat and temperature

Heat & temp are NOT synonymsThe temperature of a substance is directly related to the energy of its particles, specifically its Kinetic Energy

Kinetic EnergyThe Kinetic Energy defines the temperature

– Particles vibrating fast = hot– Particles vibrating slow = cold

Vibrational energy is transferred from one particle to the next: One particle collides with the next particle and so on; and so on – down the line

Thermal energy is a form of kinetic energy that the particles have that make up a substance

Kinetic energy from vibrational energy (motion) in solids, and liquids and gases it is vibrational, rotational, and translational energy that contribute to the KE

POTENTIAL ENERGYPotential energy from molecular

attraction (within or between the particles)PE is the energy stored in the bonds

between the atoms and in the nuclear forces that hold the nucleus together. The PE of a molecule results from the

interactions between electrons and nuclei both between and within atoms. This interaction is a chemical bondThe energy changes that occur during a

chemical reaction are mainly due to the PE changes that occur during the breaking of chemical bonds in the reactants and the formation of new bonds in the products..

Thermal energy is also related to the type of material

Example: 2H2(g) + O2(g) → 2H2O(g) + heat

The bonds between the hydrogen atoms in the H2 and the oxygen atoms in the O2 must be broken in order to make the H-O bonds in H2O. This breaking of bonds requires energy and is therefore endothermic. However, in this example, more energy is released in the making of the H-O bonds than is required to break the H-H and the O=O bonds.

Therefore the overall reaction is exothermic. This means that the reverse reaction would be endothermic

2H2O(g) + heat → 2H2(g) + O2(g

Different type of materials– May have the same temp, same mass,

but different conductivity– Affected by the potential energy or the

intermolecular forces So it is possible to be at same temp (same

KE) but have very different thermal energies

The different abilities to hold onto or release energy is referred to as the substance’s heat capacity

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

from molecule to molecule

Different type of materials– May have the same temp, same mass,

but different conductivity– Affected by the potential energy or the

intermolecular forces So it is possible to be at same temp (same

KE) but have very different thermal energies

The different abilities to hold onto or release energy is referred to as the substance’s heat capacity

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

from molecule to molecule

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 CAPACITYThe measure of how well a material absorbs or releases heat energy is its heat capacity

It can be thought of as a reservoir to hold heat, how much it holds before it overflows is its capacity

Heat capacity is a physical property unique to a particular material

Water takes 1 calorie of energy to raise the temp 1 °CSteel takes only 0.1 calorie of energy to raise temp 1 °C

SPECIFIC HEAT CAPACITY(Cp) The amount of energy it takes to raise the

temp of a standard amount (1 g) of an object 1°C

Specific heats can be listed on data tablesSmaller the specific heat the less energy it takes the substance to feel hot and the less time it takes the substance to cool offLarger the specific heat the more energy it takes to heat a substance up (bigger the heat reservoir) the longer time it takes the substance to cool offhttp://www.engineeringtoolbox.com/specific-heat-capacity-food-d_295.html

SUBSTANCESUBSTANCE SPECIFIC HEAT SPECIFIC HEAT CAPACITY, CCAPACITY, CPP

WATER, HWATER, H22OO 4.184.18J/g°C OR J/g°C OR 11cal/g°Ccal/g°C

ALUMINUM, ALUMINUM, AlAl

.992.992J/g°C J/g°C OR OR .237.237cal/g°Ccal/g°C

TABLE SALT, TABLE SALT, NaClNaCl

.865 .865 J/g°C J/g°C OR OR .207.207cal/g°Ccal/g°C

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

MERCURY, MERCURY, HgHg

.139 .139 J/g°C J/g°C OR OR .033.033cal/g°Ccal/g°C

CHEMICAL RXNSCHEMICAL RXNS There are 2 types of chemical rxns

– Exothermic rxns rxns in which heat energy is a product

Exothermic rxns typically feel warm as the rxn proceeds– You might hear the word exergonic- these

are reactions that release energy, but not necessarily HEAT!!

Ex: the hydration of any strong acid or base

There are 2 types of chemical rxns– Exothermic rxns rxns in which heat

energy is a product Exothermic rxns typically feel warm as

the rxn proceeds– You might hear the word exergonic- these

are reactions that release energy, but not necessarily HEAT!!

Ex: the hydration of any strong acid or base

CH4CH4 ++ 2O2

2O2 2043kJ change 2043kJ change CO2CO2 2H2O2H2O++ ++

Exothermic rxn Exothermic 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

The other type of reaction is– Endothermic rxns rxns in which

heat energy is a reactant (absorbs heat energy)

Endothermic rxns typically feel cooler the longer the rxn proceeds

You might hear the word endergonic these are reactions that absorb energy, but not necessarily HEAT!!

Ex: Citric acid and baking soda

NH4NO3+H2O+ 752kJ NH4OH+HNO3NH4NO3+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

H2O (s) + 752kJ H2O (l)

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

chem rxn is called the enthalpy of the rxn– Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy

Enthalpy is a value that also contains a component of direction (energy in or energy out)

The energy used or produced in a chem rxn is called the enthalpy of the rxn– Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy

Enthalpy is a value that also contains a component of direction (energy in or energy out)

HEATHEATHEATHEAT HEATHEATHEATHEAT HEATHEATHEATHEAT HEATHEATHEATHEAT

Most common version of enthalpy is when we have a change in enthalpy (H)

The enthalpy absorbed or gained (changed) in a rxn is dependent on the amount of material reacting

– Amount is usually in the form of moles

– We can use the coefficient ratios of the balanced chemical reactions to energy ratios to calculate how much energy a reaction used or produced

Most common version of enthalpy is when we have a change in enthalpy (H)

The enthalpy absorbed or gained (changed) in a rxn is dependent on the amount of material reacting

– Amount is usually in the form of moles

– We can use the coefficient ratios of the balanced chemical reactions to energy ratios to calculate how much energy a reaction used or produced

CHANGE IN ENTHALPYCHANGE IN ENTHALPY

Endothermic Versus Exothermic ReactionsTo further understand the difference between the two types of reactions (exothermic and endothermic), we need to explore a couple of other concepts. In addition to kinetic energy (vibrational, rotational and translational motion), molecules also have potential energy. Potential energy is energy due to position and composition. It is stored in molecular bonds that exist within molecules (intramolecular); between different molecules (intermolecular), between different atoms of an element and finally within atoms.

In endothermic reactions the reactants have less potential energy than the products do. Energy must be added to the system from the surroundings in order to raise the particles up to the higher energy level.Energy + A + B --> AB

In exothermic reactions the reactants have more potential energy than the products have. The extra energy is released to the surroundings. A + B --> AB + Energy

EXAMPLE 1:How much heat will be released if 1.0g of H2O2 decomposes in a bombardier beetle to produce a defensive spray of steam

EXAMPLE 1:How much heat will be released if 1.0g of H2O2 decomposes in a bombardier beetle to produce a defensive spray of steam

2H2O2 2H2O + O2 Hº =-190kJ2H2O2 2H2O + O2 Hº =-190kJ

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

powerful tools. – Given a rxn equation with an energy

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

Chemical reaction equations are very powerful tools. – Given a rxn equation with an energy

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

THINK Moles and ratios! From the balanced chemical equation, for every 2 mols of H2O2 that decomposes, 190kJ of heat is produced. Now, calculate how much energy is produced when1.0 g of H2O2 decomposes.

THINK Moles and ratios! From the balanced chemical equation, for every 2 mols of H2O2 that decomposes, 190kJ of heat is produced. Now, calculate how much energy is produced when1.0 g of H2O2 decomposes.

Convert 1.0 g of H2O2 to moles of H2O2Convert 1.0 g of H2O2 to moles of H2O2

2H2O2 2H2O + O2 Hº = -190kJ

2H2O2 2H2O + O2 Hº = -190kJ

Again, with 2 moles of H2O2, 190 kJ of energy is produced but since there is only 0.02941 mols of H2O2 calculate how much energy the bug produces?

Again, with 2 moles of H2O2, 190 kJ of energy is produced but since there is only 0.02941 mols of H2O2 calculate how much energy the bug produces?

2H2O2 2H2O + O2 Hº = -190kJ

2H2O2 2H2O + O2 Hº = -190kJ

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 Hº=-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 Hº=-1366.7kJ

Example #2Example #2

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:

Example #3:If you drink 4 cups of ice water at 0°C, how much heat energy is transferred as this water is brought to body temp? (each glass contains 250 mL of water & body temp is 37°C). Density of water is 1g/mL.

Example #3:If you drink 4 cups of ice water at 0°C, how much heat energy is transferred as this water is brought to body temp? (each glass contains 250 mL of water & body temp is 37°C). Density of water is 1g/mL.

Enthalpy is dependent on the conditions of the rxn– It’s important to have a standard set of

conditions– This allow us to compare the affect of

temps, pressures, etc. On different substances

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

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

Enthalpy is dependent on the conditions of the rxn– It’s important to have a standard set of

conditions– This allow us to compare the affect of

temps, pressures, etc. On different substances

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

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

Standard enthalpies can be found on tables of data measured as standard enthalpies of formations (pg 799-800)

Standard enthalpies of formations are measured values for the energy to form chemical compounds (Hf°)

– H2 gas & O2 gas can be ignited to produce H2O and a bunch of energy

– The amount of energy produced by the rxn is 285kJ for every mol of water produced

Standard enthalpies can be found on tables of data measured as standard enthalpies of formations (pg 799-800)

Standard enthalpies of formations are measured values for the energy to form chemical compounds (Hf°)

– H2 gas & O2 gas can be ignited to produce H2O and a bunch of energy

– The amount of energy produced by the rxn is 285kJ for every mol of water producedH2(g) + ½02(g)

H2O(g)

H2(g) + ½02(g) H2O(g)

Hf°=-285.8kJ/molHf°=-285.8kJ/mol

STANDARD ENTHALPIES OF FORMATION

SYMBOLSYMBOL FORMULASFORMULAS HHff°°kJ/molkJ/mol

AlClAlCl33(s)(s) Al + 3/2ClAl + 3/2Cl22 AlCl AlCl33 -705.6-705.6

AlAl22OO33(s)(s) 2Al + 3/2O2Al + 3/2O22 Al Al22OO33 -1676.0-1676.0

COCO22(g)(g) C + OC + O22 CO CO22 -393.5-393.5

HH22O(g)O(g) HH22 + + 1/21/2OO22 H H22OO -241.8-241.8

CC33HH88(g)(g) 3C + 4H3C + 4H22 C C33HH88 -104.7-104.7

CALORIMETRYCALORIMETRY Calorimetry is the process of measuring heat

energy – Measured using a device called a calorimeter

– Uses the heat absorbed by H2O to measure the heat given off by a rxn or an object

The amount of heat soaked up by the water is equal to the amount of heat released by the rxn

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

– Uses the heat absorbed by H2O to measure the heat given off by a rxn or an object

The amount of heat soaked up by the water is equal to the amount of heat released by the rxn

HSYS=-HSURHSYS=-HSURHsys is the reaction that is

taking place in the main chamber (rxn etc.) And Hsur

is the surroundings which is generally water.

Hsys is the reaction that is taking place in the main

chamber (rxn etc.) And Hsur is the surroundings which is

generally water.

HSYS=±│q│HSYS=±│q│

You calculate the amount of heat absorbed by the water (using q= mCT)

Which leads to the amount of heat given off by the rxn HSYS=±│q│– you know the mass of the water (by weighing it)

– you know the specific heat for water (found on a table)

– and you can measure the change in the temp of water (using a thermometer)

You calculate the amount of heat absorbed by the water (using q= mCT)

Which leads to the amount of heat given off by the rxn HSYS=±│q│– you know the mass of the water (by weighing it)

– you know the specific heat for water (found on a table)

– and you can measure the change in the temp of water (using a thermometer)

CALORIMETRYCALORIMETRY

A chunk of Al that weighs 72.0g is heated to 100.0°C is dropped in a calorimeter containing 120ml of water at 16.6°C. the H2O ’s temp rises to 27.0°C.

A chunk of Al that weighs 72.0g is heated to 100.0°C is dropped in a calorimeter containing 120ml of water at 16.6°C. the H2O ’s temp rises to 27.0°C.

- mass of Al = 72.0g

- Tinitial of Al = 100.0°C

- Tfinal of Al = 27.0°C

- CAl = .992J/g°C (from table)

- mass of Al = 72.0g

- Tinitial of Al = 100.0°C

- Tfinal of Al = 27.0°C

- CAl = .992J/g°C (from table)

HSYSHSYS

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

HH == -5214J-5214J

We can do the same calculation with the water info

– Mass of H2O= 120g– Tinitial of H2O= 16.6°C– Tfinal of H2O = 27°C– CH2O= 4.18J/g°C (from table)

We can do the same calculation with the water info

– Mass of H2O= 120g– Tinitial of H2O= 16.6°C– Tfinal of H2O = 27°C– CH2O= 4.18J/g°C (from table)

HSURHSUR

HH == 5216J5216J

Equal but opposite, means that since the Al decreased in temp, it released heat causing the H2O to increase in temp.

Equal but opposite, means that since the Al decreased in temp, it released heat causing the H2O to increase in temp.

HH == 120g120g 4.18J/g°C 4.18J/g°C 27°C-16.6°C27°C-16.6°C