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 synonyms The temperature of a substance is directly related to the energy of its particles, specifically its Kinetic Energy

Kinetic Energy  The 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 ENERGY Potential 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 bond The 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 dependent upon the amount or mass of material present  (KE =½mv 2 )  Thermal energy is also related to the type of material  Example: 2H 2(g) + O 2(g) → 2H 2 O (g) + heat  The bonds between the hydrogen atoms in the H 2 and the oxygen atoms in the O 2 must be broken in order to make the H-O bonds in H 2 O. 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  2H 2 O (g) + heat → 2H 2(g) + O 2(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 CAPACITY The 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 °C Steel takes only 0.1 calorie of energy to raise temp 1 °C

SPECIFIC HEAT CAPACITY(C p ) 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 tables Smaller the specific heat  the less energy it takes the substance to feel hot and the less time it takes the substance to cool off Larger 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 off

SUBSTANCE SPECIFIC HEAT CAPACITY, C P WATER, H 2 O 4.18 J/g°C OR 1 cal/g°C ALUMINUM, Al.992 J/g°C OR.237 cal/g°C TABLE SALT, NaCl.865 J/g°C OR.207 cal/g°C SILVER, Ag.235 J/g°C OR.056 cal/g°C MERCURY, Hg.139 J/g°C OR.033 cal/g°C

CHEMICAL 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

CH O kJ change   CO 2 2H 2 O  Exothermic rxn –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

NH 4 NO 3 +H 2 O+ 752kJ  NH 4 OH+HNO 3  Endothermic rxn –Similar system as what is found in cold packs H 2 O (s) + 752kJ  H 2 O (l)

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)

HEATHEATHEATHEATHEATHEATHEATHEAT

 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 ENTHALPY

Endothermic Versus Exothermic Reactions To 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 H 2 O 2 decomposes in a bombardier beetle to produce a defensive spray of steam EXAMPLE 1: How much heat will be released if 1.0g of H 2 O 2 decomposes in a bombardier beetle to produce a defensive spray of steam 2H 2 O 2  2H 2 O + O 2  Hº =-190kJ USING  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 H 2 O 2 that decomposes, 190kJ of heat is produced. Now, calculate how much energy is produced when1.0 g of H 2 O 2 decomposes. Convert 1.0 g of H 2 O 2 to moles of H 2 O 2 2H 2 O 2  2H 2 O + O 2  Hº = -190kJ

Again, with 2 moles of H 2 O 2, 190 kJ of energy is produced but since there is only mols of H 2 O 2 calculate how much energy the bug produces? 2H 2 O 2  2H 2 O + O 2  Hº = -190kJ

How much heat will be released when 4.77 g of ethanol (C 2 H 5 OH) react with excess O 2 according to the following equation: C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O  Hº= kJ How much heat will be released when 4.77 g of ethanol (C 2 H 5 OH) react with excess O 2 according to the following equation: C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O  Hº= kJ Example #2

 H = FINAL TEMP – INITIAL TEMP FINAL TEMP – INITIAL TEMP SPECIFIC HEAT SPECIFIC HEAT MASS  We can also track energy changes due to temp changes, using  H=mC  T:

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 )  Standard enthalpies of formations are measured values for the energy to form chemical compounds (  H f °) – H 2 gas & O 2 gas can be ignited to produce H 2 O 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 )  Standard enthalpies of formations are measured values for the energy to form chemical compounds (  H f °) – H 2 gas & O 2 gas can be ignited to produce H 2 O and a bunch of energy – The amount of energy produced by the rxn is 285kJ for every mol of water produced H 2(g) + ½0 2(g)  H 2 O (g)  H f °=-285.8kJ/mol

STANDARD ENTHALPIES OF FORMATION SYMBOLFORMULAS  H f °kJ/mol AlCl 3 (s) Al + 3/2Cl 2  AlCl Al 2 O 3 (s) 2Al + 3/2O 2  Al 2 O CO 2 (g) C + O 2  CO H 2 O(g) H 2 + 1/2O 2  H 2 O C 3 H 8 (g) 3C + 4H 2  C 3 H

CALORIMETRY  Calorimetry is the process of measuring heat energy – Measured using a device called a calorimeter – Uses the heat absorbed by H 2 O 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 H 2 O 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  H SYS =-  H SUR  H sys is the reaction that is taking place in the main chamber (rxn etc.) And  H sur is the surroundings which is generally water.  H SYS =± │ q │

 You calculate the amount of heat absorbed by the water (using q= mC  T)  Which leads to the amount of heat given off by the rxn  H SYS =± │ 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= mC  T)  Which leads to the amount of heat given off by the rxn  H SYS =± │ 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) CALORIMETRY

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 H 2 O’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 H 2 O’s temp rises to 27.0°C. -mass of Al = 72.0g -T initial of Al = 100.0°C -T final of Al = 27.0°C -C Al =.992J/g°C (from table) -mass of Al = 72.0g -T initial of Al = 100.0°C -T final of Al = 27.0°C -C Al =.992J/g°C (from table)  H SYS q= 72g.992J/g°C 27°C-100°C HH HH = = -5214J

 We can do the same calculation with the water info – Mass of H 2 O= 120g – T initial of H 2 O= 16.6°C – T final of H 2 O = 27°C – C H2O = 4.18J/g°C (from table)  We can do the same calculation with the water info – Mass of H 2 O= 120g – T initial of H 2 O= 16.6°C – T final of H 2 O = 27°C – C H2O = 4.18J/g°C (from table)  H SUR HH HH = = 5216J Equal but opposite, means that since the Al decreased in temp, it released heat causing the H 2 O to increase in temp. HH HH = = 120g 4.18J/g°C 27°C-16.6°C