THERMOCHEMISTRY

1. THERMOCHEMISTRY COLLISION THEORY, ENDOTHERMIC/EXOTHERMIC, ENTHALPY, STANDARD ENTHALPIES, CALORIMETERY

2. INTRO TO THERMOCHEMISTRY • The study of the changes in energy in chemrxns is called thermochemistry. • The energy used or produced in a chemrxn is called the enthalpy of the rxn (ΔHrxn) • Burning a 15 g piece of paper produces a particular amount of thermal or heat energy (enthalpy) • Enthalpy is a value that also contains a component of direction (energy in or energy out)

3. INTRO TO THERMOCHEMISTRY • Where does that energy come from? • When bonds are broken that requires energy. • When new bonds are formed that produces energy. • Let’s look at a simple reaction of NO + O3 NO2 + O2 • Unpacking it…there is 1 O2-O bondbeing broken • And 1 new bond being formed, NO-O.

4. Energy change, ΔH = Hproducts- Hreactants Energy (kJ) Activated complex Exothermic Activation energy Hproducts > Hreactants reactants Energy transferred to break bond Energy released as heat to surroundings ΔH = negative products Time (s)

5. INTRO TO THERMOCHEMISTRY • Sometimes it takes more energy to break the old bonds than we get back when the new bonds are formed • These reactions absorb energy and if there isn’t enough energy they don’t happen naturally • If we get back more energy than needed to break the old bonds • These reactions produce energy and if there is enough it will be in the form of heat and light.

6. COLLISION THEORY • To visualize the bonds being broken and formed in a chemrxn chemists use a model called collision theory • According to collision theory, atoms, ions, and molecules form products when they collide • provided that the particles have enough kinetic energy • And that the molecules are oriented properly

7. COLLISION THEORY

8. COLLISION THEORY • The minimum amount of energy that the particles or reactants must have in order to react is called the rxn’sactivation energy. • In a sense the activation energy is a barrier that reactants must get over to be converted to products • The higher the barrier the larger the investment of energy in order to get the rxn to proceed

10. Activated complex

11. COLLISION THEORY • During a rxn, a particle that is neither reactant nor product forms momentarily, called an activated complex • if there is sufficient energy and if the atoms are oriented properly • An activated complex is a kind of transition molecule which has similarities to reactants & products • An activated complex is the arrangement of atoms at the peak of the activation-energy barrier

13. COLLISION THEORY • We can determine the change in enthalpy of a reaction (ΔH) • ΔHrxn = ∑Hproducts - ∑Hreactants • If Hproducts < Hreactants then -ΔHrxn • Indicates an exothermic reaction • Always get the initial activation energy back and then some • If Hproducts > Hreactants then +ΔHrxn • Indicates an endothermic reaction • Constantly need to put energy into the rxn

14. ENDOTHERMIC VS. EXOTHERMIC • Chemical rxns can be classified as either: • Exothermic  a rxn in which heat energy is generated (a product) • These rxns would feel warm to the touch • ΔHrxnis negative • Endothermic rxn in which heat energy is absorbed (a reactant) • These rxns would feel cool to the touch • ΔHrxnis positive

15. 2043kJ  + + + C3H8 5O2 3CO2 4H2O ENDOTHERMIC VS. EXOTHERMIC • Exothermic rxn • To a cold camper, the important product here is the heat energy C3H8 + O2

16. ENDOTHERMIC VS. EXOTHERMIC • Endothermic rxn NH4NO3+H2O+ 752kJ  NH4OH + HNO3 • Similar system as what is found in cold packs NH4OH + HNO3 NH4NO3 + H2O

17. ENTHALPIES • There are literally as many enthal-pies as there are types of rxns and processes. • The ones published are standard enthalpies symbolized by a degree • Standardized means measured at 25 °C and 1 atm and 1 Molar • Enthalpy of formation (ΔH°f) is the amnt of energy involved in the formation of a compnd from its component elements.

18. ENTHALPIES • Enthalpy of combustion (ΔH°comb) is the amount of energy produced in a combustion rxn. • Enthalpy of solution (ΔH°sol) is the amount of energy involved in the dissolving of a compound • Enthalpy of fusion (ΔH°fus) is the amount of energy necessary to melt a substance. • Enthalpy of vaporization (ΔH°vap) is the amount of energy necessary to convert a substance from a liquid to a gas.

19. ENTHALPIES • Enthalpy of neutralization (ΔH°neut) is the amount of energy produced when an acid reacts with a base. • Enthalpy of sublimation (ΔH°sub) is the amount of energy involved in the subliming of a substance.

20. CALCULATING HEATS OF RXNS • There are four ways to calculate the energy of a reaction. • Stoichiometry (uses ΔHrxnpropor-tionally with a given amount of material) • ΔH=mCΔT (conservation of energy and a ΔT to calculate a ΔHrxn) • Enthalpy of Formation (takes data from a table to calculate the ΔHrxn) • Hess’s Law (energies of several reactions can be combined to find ΔHrxn for a more complex rxn)

21. CALCULATING HEATS OF RXNS • Chemrxn 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. How much heat will be released when 4.77 g of ethanol (C2H5OH) react with O2according to the following eqn: C2H5OH+3O22CO2+3H2O H= -1366.7kJ

22. CALCULATING HEATS OF RXNS C2H5OH+3O22CO2+3H2O H= -1366.7kJ 1mol C2H5OH 4.77g C2H5OH 46g C2H5OH -1366.7kJ 1mol C2H5OH = -142 kJ

23. CALCULATING HEATS OF RXNS Ethyl alcohol burns according to the following balanced equation: C2H5OH+3O22CO2+3H2O H= -1366.7kJ How many molecules of water are pro-duced if 5000 KJ of energy are released? 6.02x1023 molecules 3mol H2O 5000 kJ -1366.7 kJ 1mol H2O = 6.61 x 1024 molecules

24. CLASSROOM PRACTICE 1 Ethanol, C2H5OH, is quite flammable and when 1 mole of it burns it has a reported ΔH of -1366.8 kJ. How much energy is given off in the combustion of enough ethanol to produce 12.0 L of CO2 @ 755 mmHg and 25.0°C?

25. MEASURING ENTHALPY • The only way to measure enthalpy is through a process called calorimetry (measuring heat). • 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 is the system/rxn Hsuris generally H2O HSYS=-HSUR

26. A COFFEE CUP CALORIMETER Used for a rxn in water, or just a transfer of heat. A BOMB CALORIMETER used when trying to find the amount of heat produced by burning something.

27. MEASURING ENTHALPY • 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 (-ΔHsurr) • Endothermic (+ΔHsys) • When the water gains heat from the system it obtains a positive change (+ΔHsurr) • Exothermic (-ΔHsys)

28. MEASURING ENTHALPY • You calculate the amount of heat absorbed by the H2O (using ΔH= mCΔT) • 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)

29. MEASURING ENTHALPY When a 4.25 g sample of solid NH4NO3 dissolves in 60.0 g of water in a calorimeter, the temp drops from 21.0°C to 16.9°C. Calculate the energy involved in the dissolving of the NH4NO3. ΔHH2O= (mwater)(Cwater)(ΔTwater) ΔHH2O=(60g)(4.18J/g°C)(16.9°C-21.0°C) ΔHH2O= -1.03 x 103 J - ΔHH2O= ΔHNH4NO3 ΔHNH4NO3= 1.03 x 103 J

30. MEASURING ENTHALPY A chunk of Al that weighs 72.0g is heated to 100°C is dropped in a cal-orimeter containing 120ml of water at 16.6°C. The water ends up at 27°C. What is the specific heat of Al? ΔHH2O= (mwater)(Cwater)(ΔTwater) ΔHH2O=(72g)(4.18J/g°C)(27°C -16.6°C) ΔHH2O= +5216 J ΔHAl= -5216 J = (72.0g)(x)(27°C -100°C) CAl= .992 J/g°C

32. CLASSROOM PRACTICE 2 A coffee-cup calorimeter is filled with 250g of H2O. The H2O temp was 24.2°C before 3.2 g of NaOH pellets was added to the H2O. After the NaOH pellets had dissolved the temp of the H2O registered 85.8°C. How much heat did the H2O absorb, & how much heat did the NaOH produce? 41.0g of glass at 95°C is placed in 175g of H2O at 19.5°C in a calorimeter. The temps are allowed to equalize. What is the final temp of the glass/water mixture? (H2O = 4.18J/g°C; Glass = 8.78 J/g°C)