1. First Law of Thermodynamics
College Chemistry

2. Energy Many types of energy  must obey law of conservation of energy
Energy cannot be created or destroyed, it can only change forms
energy – capacity to do work
Work = Force x distance
Kinetic energy, potential energy, radiant (solar) energy, thermal energy (heat), chemical energy (stored in bonds)

3. Systems & Surroundings
In thermodynamics, the world is divided into a system and its surroundings
A system is the part of the world we want to study (e.g. a reaction mixture in a flask)
The surroundings consist of everything else outside the system
SYSTEM
CLOSED
OPEN
ISOLATED

4. OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction flask, rocket engine)
CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains fixed) e.g. a sealed reaction flask
ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask)

5. EXOTHERMIC & ENDOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction) that releases heat.
Exothermic process: heat < 0 (at constant pressure)
Burning fossil fuels is an exothermic reaction

6. Endothermic process: heat > 0 (at constant pressure)
Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.
Endothermic process: heat > 0 (at constant pressure)
Forming Na+ and Cl- ions from NaCl is an endothermic process
Photosynthesis is an endothermic reaction (requires energy input from sun)

7. Measuring Heat
reaction
Exothermic reaction, heat given off & temperature of water rises
reaction
Endothermic reaction, heat taken in & temperature of water drops

9. Thermodynamics
Thermodynamics – scientific study of the interconversions of heat and energy
Thermochemistry is a state function
Thermochem depends only on where you started and where you ended, does NOT matter how you get there

10. First Law of Thermodynamics
Conservation of energy, energy cannot be created or destroyed but only change forms
DE = Ef – Ei
If we consider a reaction, we can consider the products and the reactants
S(s) + O2(g)  SO2(g)
DE = Eproducts - Ereactants
Also, since we know that energy must be conserved, the energy lost (or gained) by the system (the reaction), must be equal to the energy gained (or lost) by the surroundings
0 = DEsystem + DEsurrounding OR DEsys = - DEsurr

11. The First Law of Thermodynamics
Relating DE to Heat(q) and Work(w)
Energy cannot be created or destroyed.
Energy of (system + surroundings) is constant.
Any energy transferred from a system must be transferred to the surroundings (and vice versa).
We normally don’t care about the surroundings, we just care about the work done (or by) the surroundings on the system
From the first law of thermodynamics:

12. The First Law of Thermodynamics

13. First Law of Thermodynamics
Calculate the energy change for a system undergoing an exothermic process in which 15.4 kJ of heat flows and where 6.3 kJ of work is done on the system.
Work/energy/heat (q) is almost ALWAYS given in Joules (or kJ), sometimes calories
DE = q + w

15. Work and Heat The concept of work can be applied to gases
Pistons move gases up and down
If gases are expanded – work is negative
If gases contrast – work is positive
Work done on gases is in atm (usually), we need a conversion factor to convert to Joules
1 L*atm = J

16. Example 6.1
A certain gas expands in volume from 2.0 L to 6.0 L at constant temperature. Calculate the work done by the gas if it expands (a) against a vacuum and (b) against a constant pressure of 1.2 atm
(a) w = -PDV
w = -(0 atm)(6 – 2 L) = 0 J
No work done
(b) w = -PDV
w = -(1.2 atm) (6.0 – 2.0 L) = -4.8 L*atm
-4.8 L*atm x (101.3 J/1 L*atm) = -4.9 x 102 J
Work is negative, gas expanded
This example shows work (or heat) is NOT a state function, beginning and ending volume are the same, but the external pressure affects it

17. Example 6.2
The work done when a gas is compressed in a cylinder is 462 J. During this process, there is a heat transfer of 128 J from the gas to the surroundings. Calculate the energy for this process.
DE = q + w
q = J (exothermic)
w = J (compressed)
YOU MUST FIRST ALWAYS FIND THE SIGNS!
Now plug it in
DE = -128 J J = 334 J