1. Chapter 6: Thermochemistry

2. Energy
Law of conservation of energy - energy can not be created or destroyed, but it can be transformed
Similar to how matter can be transformed
Energy - the capacity to do work
Work is the result of a force acting over a distance
Types of Energy
Kinetic
Potential

3. Kinetic Energy
Kinetic energy - Energy associated with the motion of an object
Mechanical energy – the energy of moving macroscale objects
Moving cars, planes, baseballs etc.
Thermal energy (heat) – the energy of moving microscale objects
Moving ions, atoms, molecules etc.

4. Potential Energy
Potenial energy – energy associated with position or composition, stored energy
Has the potential to be transformed into kinetic energy
Ball held at the top of building, potential to fall and land on something with kinetic energy
Chemical energy – relative positions of electrons and nuclei in atoms/molecules represents potential energy

5. Energy: system vs surroundings
To track the energy and transferring of energy you can define a system interacting with its surrounding
System - A component or set of components of interest
You define the system
Surroundings – everything that a system can exchange energy with, everything touching/connected to it
That which “surrounds” the system

6. Units of energy Joule (J) - basic unit of energy in the metric system
1J =1kg •m2/s2
1kJ = 1000 J
calorie (cal) – 1cal = the energy required to raise the temp of 1g of H2O by 1ºC
4.184 J = 1 cal
1000 cal = 1kcal = 1Cal
Calorie (Cal) – unit associated with “food” calories
Calorie = kcal = food calorie

7. First law of thermodynamics
Thermodynamics – the study of energy and its interconversions
First law of thermodynamics – the total energy of the universe is constant
Law of conservation of energy is built into the definition
Perpetual motion machine (a device that puts out constant energy) cannot exist

8. Internal energy
Internal energy (E) – the sum of the kinetic and potential energies of all particles that compose a system
E is a state function which is only dependant on the state of the system, not how it got there

9. Energy of a system
Where does lost or gained energy go or come from? The surroundings!
Negative change in energy = loss (released) of energy from the system to the surroundings
Positive change in energy = gaining (absorbed) of energy by the system from the surroundings

10. Heat and work
Heat and work can be exchanged between the surroundings and the system
q = heat, the flow of energy caused by a temperature difference
w = work

11. Heat flow
Why does thermal energy (heat) flow due to a temperature difference?
Thermal equilibrium – heat is distributed to molecules in contact with them until the thermal energy of the system and surroundings is the same temperature

12. Quantifying heat We can measure the heat gained or lost by a system
Substances can transfer heat, but have different capacities to do so
Heat capacity (C) – the quantity of heat required to change a substances temperature by 1 °C
Based on the mass, the heat capacity and the change in temperature, we can quantify the heat transferred to or from the system

13. Problems
A thief breaks into a safe at a bank to steal a block of gold in the shape of a coin from Super Mario Bros. The safe keeps the gold at 14.0 °C. Once the thief grabs the gold coin in his hand, the coin eventually reaches body temperature at 37.0°C. How much heat was transferred to the gold coin which has a mass of 11.25g?

14. Thermal energy transfer
When two substances of different temperatures reach thermal equilibrium with each other, the heat from the hotter substance is transferred to the colder substance
The hotter substance cools down, as the colder substance warms up, with their final temperatures being the same

15. Thermal energy transfer problems
With the masses and initial temperatures of the metal and water, what will the final temperature be when they both reach thermal equilibrium?

16. Distribute Kmetal and Kwater
Collect Tfinal terms on the leftside, with other terms going to right side
Factor out Tfinal
Isolate Tfinal

17. Practice
A 32.5g block of aluminum initially at 45.8 °C is submerged into g of water initially at 15.4 °C. What is the final temperature once the two substances reach thermal equilibrium?

18. Pressure-volume work
Pressure-volume work – when the force caused is from a volume change against an external pressure
If the system does work to increase the volume against a external pressure, the ΔV is positive
If ΔV = positive, then w = negative, the system does work

19. Using the equation Units of pressure = atm Units of volume = L
Pressure x volume = L x atm
You can convert units of L x atm to units of J
1 L x atm = J

20. ΔE for chemical reactions
If a system is at constant volume, then there is no work done
If a chemical reaction does no work….
qv = the heat at constant volume

21. Calorimetry
Calorimetry- measurement of the external energy exchanged between a reaction(system) and the surrounding by monitoring the temperature change
Bomb calorimeter is a device used in calorimetry
The chemical reaction(system) transfers heat to the bomb (surrounding)

22. Enthalpy (H)
Enthalpy – the sum of the internal energy of a system and the product of its pressure and volume
For a process at a constant pressure…..
ΔH is the heat absorbed or evolved by the system
If a reaction produces a significant amount of gas, this can increase the volume, which will do work

23. Endothermic vs Exothermic
If the ΔH of a chemical reaction is positive, the system absorbs heat from the surroundings, this is an endothermic reaction
If the ΔH of a chemical reaction is negative, the system releases heat to the surroundings, this is an exothermic reaction

24. How does a chemical reaction give off heat???
Chemical “potential” energy is stored in the bonds
Chemical energy arises from the electrostatic forces between protons and electron that compose atoms and molecules
Breaking bonds absorbs energy, while making bonds releases energy
Breaking stronger bonds absorbs more energy than breaking weaker bonds

25. ΔH and thermochemical equations
ΔHrxn – the enthalpy of reaction or heat of reaction, the heat transferred in a reaction
Dependant on amounts of the reactants that react
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) ΔHrxn = kJ
When the reaction occurs with the corresponding moles from the equation, 2044kJ of heat is released from the system (exothermic reaction)
1 mol C3H8: -2044kJ or 5 mol O2: -2044kJ
The relationship acts as a conversion factor to get from moles of a compound to the enthalpy of the reaction

26. Practice
An LP gas tank in a homebarbeque contains 13.2kg of propane, C3H8. Calculate the heat (in kJ) associated with the complete combustion of all the propane in the tank.
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) ΔHrxn = kJ

27. Measuring ΔHrxn Coffee-cup calorimetry occurs at constant pressure
Can calculate ΔHrxn using a coffee-cup calorimeter
By recording the change in temperature after areaction occurs, depending on the mass and solution heat capacity, you can calculate ΔHrxn
Coffee-cup calorimetry occurs at constant pressure

28. Calorimetry recap
Bomb calorimetry occurs at constant volume and measures ΔE for a reaction
Coffee-cup calorimetry occurs at constant pressure and measures ΔH for a reaction.

29. Chemical equations and ΔHrxn
Rules for chemical equations and ΔHrxn
1. If a chemical equation is multiplied bysome factor, then ΔHrxn is also multiplied by the same factor
2. If a chemical equation is reversed, then ΔHrxn changes sign.

30. 3. If a chemical equation can be expressed by the sum of a series of steps, then ΔHrxn for the overall equation is the sum of the heats of reaction for each step
Anything on both the product and reactant side cancels out
This is known as Hess’s law

31. Practice
The rules can allow us to calculate the ΔHrxn of a new reaction using known reactions that can build up to the new reaction