THERMOCHEMISTRY HEAT CAPACITY, SPECIFIC HEAT, ENDOTHERMIC/EXOTHERMIC, ENTHALPY, STANDARD ENTHALPIES, CALORIMETERY
INTROTOTHERMOCHEMISTRY • Chemical rxns involve changes in energy • Breaking bondsrequires energy • Forming bondsreleases energy • The study of the changes in energy in chemrxns is called thermochemistry. • The energy involved in chemistry is real and generally a measurable value • Energy units are numerous, but we will concentrate on the Joule (SI base unit) and the calorie (little c, big C is the food Calorie or a kilocalorie) • 1 calorie = 4.184 Joules
There are three methods used to transfer heat energy • Conduction – transfer of heat through direct contact • Convection – transfer of heat through a medium like air or water • Radiant – transfer of heat by electromagnetic radiation
WHAT IS HEAT? • Hot & cold, are automatically associated with the words heat and temperature • Heat & temperature are NOT synonyms • The temperature of a substance is directly related to the energy of its particles, specifically its: • The Kinetic Energy defines the temperature • Particles vibrating fast = hot • Particles vibrating slow = cold
A Hot Spoon An Ice Cold Spoon energy transfer • Kinetic energy is transferred from one particle to the next (a.k.a. conduction) • Sometimes this energy can be transferred from one object to another and influence physical properties • The more energy an object has the more energy is transferred
2 Hot Spoons • Thermal energy is the total energy of all the particles that make up a substance • Kinetic energy fromvibration of particles • Potential energy frommolecular attraction(within or between the particles) • Thermal energy is dependent upon the amount or mass of material present (KE =½mv2) • Thermal energy is also related to the type of material
Different type of materials • May have the same temp, same mass, but different connectivity • Affected by the potential energy stored in chemical bonds or the IMFs holding molecules together • 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
Thermal energy can be transferred from object to object through direct contact • Molecules collide,transferring energy from molecule to molecule AKA: HEAT
HEAT RESERVOIR 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 takes1 calorieof energy to raise temp 1 °C • Steel takesonly0.1 calorieof energy to raise temp 1 °C
SPECIFIC HEAT CAPACITY • The amount of energy it takes to raise the temp of a standard amount of an object 1°C is that object’s specific heat capacity (C) • The standard amount =1 gram • Specific heats can be listed on data tables • Smaller the specific heat the less energy it takes the substance to feel hot • Larger the specific heat the more energy it takes to heat a substance up (bigger the heat reservoir)
Specific heats and heat capacities work for gains in heat and in losses in heat • Smaller the specific heat theless time it takes the substance to cool off • Larger the specific heat thelonger time it takes the substance to cool off • Specific heat capacity values are used to calculate changes in energy for chemical rxns • It’s important for chemists to know how much energy is needed or produced in chemical rxns
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) • Heat gained is the out-of direction; ie exo-
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) • Heat gained is the out-of direction; ie exo- • Heat lost is the into direction; ie endo-
SURROUNDINGS HEAT HEAT HEAT HEAT SYSTEM SYSTEM EXOTHERMIC ENDOTHERMIC
Chemical rxns can be classified as either: • Exothermic a reaction in which heat energy is generated (aproduct) • Endothermic reaction in which heat energy is absorbed (areactant) • Exothermic rxns typically feel warm as the rxn proceeds • Give off heat energy, sometimes quite alot • Endothermic rxns typically feel cooler the longer the rxn proceeds • Absorb heat energy, sometimes enough to get very cold
2043kJ + + + C3H8 5O2 3CO2 4H2O • Exothermic rxn • To a cold camper, the important product here is the heat energy
In an exothermic process the amount of energy given off is more than the initial energy invested. So the products are less in energy than the reactants.
Endothermic rxn NH4NO3+H2O+ 752kJ NH4OH+HNO3 • Similar system as what is found in cold packs
Activation Energy Energy Lost ENDOTHERMIC RXN In an endothermic process more energy is required to cause the rxn to proceed than obtained in return. So the products are less in energy than the reactants.
CHANGE IN ENTHALPY • Most common measurement of the energy or enthalpy in a reaction is actually a change in enthalpy (H) • DHrxn = ∑Hproducts - ∑Hreactants • The enthalpy absorbed or gained (changed) in a rxn is dependent on the number of moles of material reacting • We can stoichiometrically calculate how much energy a rxn uses or produces • DH values can be provided with a rxnequn and have magnitude & direction of transfer (+ or -)
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. (For Example) How much heat will be absorbed for 1.0g of H2O2 to decompose in a bombardier beetle to produce a defensive spray of steam 2H2O2 +190kJ 2H2O + O2
2H2O2 +190kJ 2H2O + O2 Molar mass 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 1mol H2O2 1.0g H2O2 = .02941 mol 34g H2O2
2H2O2 +190kJ 2H2O + O2 Rxn equation 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? 190kJ .02941 mol = 2.8kJ 2molH2O2
Example #2 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 analyze: we know that if we had 1 mol of ethanol(assuming coefficient of 1 in rxn equation) burning we would produce 1366.7kJ of heat, but how much would it be if only we only had 4.77 g of ethanol?
C2H5OH+3O22CO2+3H2O H = -1366.7kJ 1mol C2H5OH -1366.7kJ 4.77g C2H5OH 1mol C2H5OH 46g C2H5OH = -142 kJ
Classroom Practice 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 C2H5OH+ 3 O2 2 CO2+ 3 H2O H= -1366.8 kJ
FINAL TEMP – INITIAL TEMP SPECIFIC HEAT MASS • We can also track energy changes due to temp changes, using H=mCpT: H = • 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 upnegative • the rxn absorbed heat into the system, soit’sendothermic
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) H=(1000g)(4.18J/g•°C)(37°C) H=mCH2OT H= 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. DH = mCDT C= DH/mDT C = 156,900J/(500g)(75°C) H2O C = 4.184 J/g°C
Classroom Practice 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? 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?
Enthalpy is dependent on theconditionsof the rxn • It’s important to have astandard setof conditions, which allows 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 measured as standardenthalpies of formations, enthalpies of combustion, enthalpies of solution, enthalpies of fusion, and enthalpies of vaporization • Enthalpy of formation (Hf) is the amount of energy involved in the formation of a compound from its component elements. • Enthalpy of combustion (Hcomb) is the amount of energy produced in a combustion rxn. • Enthalpy of solution (Hdiss) is the amount of energy involved in the dissolving of a compound
Enthalpy of fusion (Hfus) is the amount of energy necessary to melt a substance. • Enthalpy of vaporization (Hvap) is the amount of energy necessary to convert a substance from a liquid to a gas. • All of these energies are measured very carefully in a laboratory setting under specific conditions • At 25 °C and 1atm of pressure • These measured energies are reported in tables to be used in calculations all over the world.
Calorimetry is the process of measuring heat energy • Measured using a device called a calorimeter • Uses the heat absorbed by H2O to meas-ure 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 is the system or what is taking place in the main chamber (rxn etc.) And Hsur is the surroundings which is generally water. HSYS=-HSUR
A COFFEE CUP CALORIMETER USED FOR A REACTION IN WATER, OR JUST A TRANSFER OF HEAT. A BOMB CALORIMETER USED WHEN TRYING TO FIND THE AMOUNT OF HEAT PRODUCED BY BURNING SOMETHING.
CALORIMETRY • With calorimetry we use the sign of what happens to the water • When the water loses heat into the system it obtains a negative change (-Hsurr) • Endothermic (+Hsys) • When the water gains heat from the system it obtains a positive change (+Hsurr) • Exothermic (-Hsys)
- D H -D H rxn + D H water -H sys Hsys -Hsurr Hsurr water HEAT HEAT + D H HEAT HEAT rxn = = WATER = SURROUNDINGS WATER = SURROUNDINGS + SIGN MEANS HEAT WAS ABSORBED BY THE RXN - SIGN MEANS HEAT WAS RELEASED BY WATER SYSTEM SYSTEM - SIGN MEANS HEAT WAS RELEASED BY THE RXN + SIGN MEANS HEAT WAS ABSORBED BY WATER EXOTHERMIC ENDOTHERMIC
CALORIMETRY • You calculate the amount of heat absor-bed by the water (using H= mCT) • Which leads to the amount of heat given off by the rxn • 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)
A chunk of Al that weighs 72.0g is heated to 100°C is dropped in a calorimeter containing 120ml of water at 16.6°C. the H2O’s temp rises to 27°C. HAl HAl= = 72g .992J/g°C 27°C-100°C -5214J • mass of Al = 72g • Tinitial of Al = 100°C • Tfinal of Al = 27°C • CAl = .992J/g°C (from table) HAl
We can do the same calc with the water info = = HH2O HH2O 5216J 120g 4.18J/g°C 27°C-16.6°C • Mass of H2O= 120g • Tinitial of H2O= 16.6°C • Tfinal of H2O = 27°C • CH2O= 4.18J/g°C(from table) HH2O Equal but opposite, means that the Al decreased in temp, it released its stored heat into the H2O, causing the temp of the H2O to increase.
When a 4.25 g sample of solid NH4NO3 dissolves in 60.0 g of water in a calori-meter, the temperature drops from 21.0°C to 16.9°C. Calculate the energy involved in the dissolving of the NH4NO3. DHwater = (mwater)(Cwater)(DTwater) DHwater = (60g)(4.18J/g°C)(16.9°C-21.0°C) DHwater = -1.03 x 103 J - DHwater = DHNH4NO3 DHNH4NO3 = 1.03 x 103 J
Classroom Practice 3 A coffee-cup calorimeter with a mass of 4.8 g is filled with water to mass of 250 g. The water temperature was 24.2C before 3.2 g of NaOH pellets was added to the water. After the NaOH pellets had dissolv-ed the temp of the water registered 85.8C. How much heat did the H2O absorb, and how much heat did the NaOH produce? 41.0g of glass at 95°C is placed in 175 g of Water at 19.5°C in a calorimeter. The temps are allowed to equalize. What is the final temp of the glass/water mixture? (Water = 4.18J/g°C; Glass = 8.78J/g°C)