5.1 The first law of therodynamics and Enthalpy Flashcards
First law of thermodynamics = energy cannot be created or destoryed
* if i have 1000J of E in the room then i come back and there 2000J of E –> I wouldnt think it was created, but transfered into the room
Way to express the first law of thermodynamics
Delta E = q + w
* Delta E = Change in Internal energy (in a sample of something)
* q = Heat
* w = Work
* Heat and Work are the two chielf ways we transfer energy
* q/w+ = system gained E
* q/w- = system lost E
When something does work it loses energy
* Think going out in the woods and swinging an axe, you’re doing work and losing E
* When the surroundings are doing work on the system, the surroundings are losing E and the system is gaining E (w+)
* If the system is doing work on the surroundings, then the system is now doing the work, thereby losing the E (w-)
q/w signs (+/-) always relaive to whats happening to the system, not the surroundings
EX: 10J of work is done by the surroundings on the system
* This would be +W, because the surroundings are doing the work (losing W) which means the system is having work done on it, gaining W –> and the sign is from the perspective of the system always, and its +W because the system is gaining W.
Energy = The ability to do work
* SI unit = Joul (n/m)
* Energy also measured in Calories (4.184 J)
Calories: 4.184 J
* amount of heat needed to raise 1 gram of h2o 1 degree celcius
* not an SI unit
* 1cal = 4.184 J
* 1Cal (cap C) = 1 kcal = 1000cal (this is the nutritional calorie)
Temperature of the gas is all dependent on its internal energy
* if its delta E doesnt change, than its E doesnt change
* If its internal E goes up, than its temperature goes up
* this is all true for a gas
* its not if heat comes in or out of a system, its the direction that delta E deviates
* If delta E is positive than the temperature of that gas goes up
* If delta E goes down than the temperature of that gas goes down
* Delta E = 0 = Isothermal = no change
Special type of work we talk about for gases is PV work
Pv work
* w = -P(ext) * Delta V
* P external
* Delta V = the change in volume
* If the volume of the gas goes up, than delta V is positive
* Pressure is always positive (but it does have a negative sign infront of it in the formula [however, pressure itself cannot be negative, think about it IRL])
* So Running through this equation, when gas expands (positive Delta V) that means work comes out negative (because pressure is always positive and theres a negative sign infront of the entire thing)
* When a gas expands W is going to be a negative term –> the idea is when gas expands, gas is doing the work on the surroundings, it is pushing the surroungs back to expand. Meaning the system is doing work on the surroundings (which means its negative work for the system because its losing work, or giving it away)
* However, if the surroundings are doing work on the gas, pushing it in, causing it to get compressed, delta V is negative making W positive in this equation. This is because the surroundings are doing work on the gas, which means the system is gaining work (+W). Surroundings doing work on system = +W because system is gaining work or reciving work, or having work done on it.
Expanding gas = -W
Compressed gas = +W
If heats transfered from system –> surroundings and the gas expands what would you know about delta E?
* Well its a -q, and its a -w meaning delta E would be negative as well
* This means the temperature of the gas is going down
* delta E = q + w
Enthalpy = The same as heat WHEN the reaction is carried out under a constant pressure
Delta H = q p
* Delta H = the change in enthalpy
* q(sub p) = heat when the rxn is carried out at a constant pressure
* This is pretty common, the pressure in this room is roughly 1 atm, and it will stay that way most likely - with a reaction at constant pressure the change in enthalpy will = the change in heat (because pressure stayed constant)
1atm = pressure at sea level
Any reaction in an enclosed container thats producing or consuming moles of gas will actaully be changing the pressure - Delta H would not = heat (q)
Enthalpy is a state function
* state function = it only depends on the inital state and the final state of the system
* EX: Altitude change. You drive up to the top of a mountain w. your car. Person 2 hops on an airplain and flies there and aparshoots down. We both started in the same spot, and we both ended up in the same spot - so we both had the same alititude change. This alititude change is indepdent of path (much like displacement).
* All this being said really all that matters is the inital state of the system and the final state of the system. If thats all you need to know to calculate something than its a state function - path indepdent
2 major things that are not state functions
* q (heat)
* w (work)
* shown below
* most things are state functions
Endothermic
* Endo = within or to bring heat inside
* This is when Delta H is positive - makes since, the system is bringing in heat - it is getting that heat from the surroundings
* Delta H > 0
* Delta H = q(sub p). If delta H is positive, than q is positive, meaning theres increased heat in the system
* The hallmark of an endothermic rnx is that everything around it gets colder, because its sucking up all the heat from everything around it.
Exothermic
* Delta H < 0
* Delta H = qp
* If delta H is < 0 that means q is negative, meaning theres decreased heat in the system (this system is losing heat to the surroundings)
* Exothermic - Exo = outside, thermic = heat
* So its heat outside
* The hallmark of an exothermic reaction is that everything on the surroundings gets hotter because the system is giving off heat
The picture below is showing an exothermic reaction. The water is getting hotter (which is the surroundings), meaning the system is cooling down. Delta H < 0, meaning we get -q (decreased heat in the system)
This is an example of an endothermic rxn. Its showing that the Delta H > 0, meaning we have a positive q. Positive q means the system is gaining heat, and the surroundings are losing it
What is the enthalpy change when 3 moles of O2 are consumed in the Rxn above
What is the enthalpy change when 9g H2O (l) are produced in the rxn below
This is showing how to set up conversion factors between the delta H number and a certain number of moles in the equation
if 2 H2O are produced, than the rxn must use -572 kj. So you can relate 2 mol H2O to -572 kj. Thats the only new thing
Phase changes
* there are 6 different phase changes
* going to need to know the names for all these
* going to need to know if its endo or exothermic
Melting: (fusion)
* Solid –> Liquid
* You’re providing the system heat to help it melt.
* System = +q = endothermic (within the system temperature)
Vaporization: (boiling)
* Liquid –> Gas
* Endothermic
Sublimiation
* solid –> gas
* Doesnt really happen much under normal conditions –> think dry ice, which is solid carbon dioxide (never goes through liquid phase = never feels wet, = why its called dry ice)
* Endothermic
The ones below are the exact opposite of the ones above, and are going to release heat into the environment, making them exothermic.
* demonstrated in the picture below
Condinsation
* Gas –> Liquid
* Exothermic (the environment gains heat, the system gives off heat, meaning we get a -q = -Delta H)
* This is drops of water on a glass of water on a hot day. Whats happening is moisture in the air is condensing on that cold glass (thats at a lower temperature), is causing the water in the atmosphere to go from the gas phase to the liquid phase. So note, its not water from inside the glass going out, but water from the atmopshere going from gas phase to liquid phase on the cold glass
Crystalization: Freezing (more common)
* Liquid –> Solid
* Exothermic
Deposition
* gas –> solid
* Exothermic
probs chatgpt these last 3 to understand the exothermic phase
Whats another term for melting used in chemistry?
* is this endo or exothermic
Fusion
So when were talking about the delta H of fusion, that means the delta H of melting
endothermic
What is fusion (changing between what states)
Solid –> Liquid
Vaporization is the change in what states?
* Endo or exothermic
Liquid –> Gas
* could also be called boiling
Endothermic. Heat is being added to the system (+q)
* think about it, if you want to boil water you’re proably going to put it in a pot and turn the heat on (adding heat from the surroundings to the system)
Sublimiation
* what phases is it going between?
* Is it normal? give an example
* Endo or exothermic
Solid –> Gas
Doesnt really happen under normal conditions. However, it does happen w/ dry ice
Endothermic
What is condensation?
* Endo or exo thermic
Gas –> Liquid
Exothermic (system is losing heat)
Crystalization
* Other name
* Phase change
* Endo vs exo
Freezing
Liquid –> Solid
Exothermic
Deposition
* phase change
* Endo vs exo
Gas –> Solid
Exothermic
