Unit 2 Flashcards
system
chemical reaction
surroundings
everything that is not the chemical reaction
open system
freely exchanges energy and matter with surroundings
closed system
does not allow certain transfers in/out of system
isolated system
cannot exchange either energy or matter outside the system
work
work performed by a system is the energy transferred by the system to its surroundings
energy
the capacity to do work or produce heat
kinetic energy
energy due to the motion of particles like thermal energy
potential energy
stored in the chemical bonds
chemical energy
energy released when bonds form in a reaction
first law of thermodynamics
energy cannot be created or destroyed, it can only be transferred in different forms
internal energy
sum of potential and kinetic energy of a system
state function
depends only on initial and final stated, not the way to get there
path function
depends on the path you take to get somewhere
when measuring internal energy
you are measuring changes in it (delta E)
work is done on system
compression +ve sign
work is done by system
expansion -ve sign
q heat is gained from surroundings
+ve
q heat is lost to surroundings
exothermic -ve
when a gas is compressed
work is done on system so it is +ve, p is always positive
exothermic
head is going out, surroundings get warmer
endothermic
head is going in, the surroundings get cooler
how to tell if ends or exo
- sign of Q and delta H (+ve is ends, -ve is exo)
- surroundings get cooler/warmer
- where e (energy) is placed in a reaction (reactant is ends, products is exo)
- potential energy diagrams
potential energy diagrams
y axis- potential energy
x-axis- reaction progress
Include states!
- see diagram for more
enthalpy units
delta h is in kj per mole
q units
heat is in j or kj
sources of error in a calorimetry lab
1) * heat loss to calorimeter or environment
2) assuming that c is the same as water or assuming that density is the same as water
3) ** purity in the reactants, slight loss of mass due to gasses leaving, not all limiting reagent reacted
- super important ones!
ways to find delta h
- calorimetry
- hess’s law
- enthalpy of formation
- bond energy
hess’s law
a state function that says that equations of enthalpy changes can be manipulated
enthalpy of formation
the standard enthalpy of formation is, delta H f nought, is the change in enthalpy the accompanies the formation of one mole of compound from its elements in their standard states P-R
reminder of states
metals are solid except for mercury
nonmetals are mostly gases except for (liquid: bromine) (solid: C, I2, P, S, Se)
- don’t forget HOFBrINCl diatomic!
the enthalpy of formation of an element in its standard state is
zero!
standard state
state at 25 degrees celsius at 1 atm
delta h of formation units
kj per mol
bond energy
- energy needed to break the bond between two atoms in the gas phase (convert to gas if need be)
- can be used to approximate standard enthalpy of reaction
breaking bonds is a __ process
endothermic, delta H is +ve
forming bonds is a ___ process
exothermic, delta H is -ve
for bond energy it is reactants ___ products
minus products since it takes energy to break the reactant bonds (+ve) and there is an input of energy to create the product bonds (-ve)
sublimation
going from solid to gas directly
bond energy is useful when
there is no other data to base equations on
bond energy data
only given positive values to break the bond
true only for the gas phase
are an average as C-C bond strength varies depending on the adjacent bonds
- see notes for this
larger reactants in bond energy yields a
endothermic reaction
larger products in bond energy yields a
exothermic reaction
when temperature goes down
that means that reaction is absorbing energy and is endothermic
Endothermic
+w, +q, delta E>0, delta H>0. Gas is compressed, work is done on the system and there is cooler surroundings, energy is a reactant
Exothermic
-w, -q, delta E<0, delta H<0, gas expands, the surroundings get warmer and energy is a product
m
mass of solution
delta h units
kj/mol unless it is in hess’s law, its just kj
Joules
kpa x L
density
g/ml
potential energy diagrams
** see notes
L times ml/l times g/ml
gets you g
in therefore statements
-ve work turns into “work done by system is +ve value J”
+ve work turns into “ work inputted to system is +ve value J”
delta H measures…
change of energy, not measuring the constant amount
sources of error in a calorimetry lab
- heat loss to calorimeter and environment
- assuming same specific heat capacity and density as water
- purity in reactants/not all limiting reagent reacted
- make sure that you can explain effect on delta h1, h2, and delta H rxn.
- make sure that the error is actually possible!
enthalpy
the amount of heat released or absorber per mole of reactions
coal
- formed from the remains of plants that were buried and subjected to high pressure and heat over long periods of time
coal disadv and adv
- pros: important and plentiful in the US
cons: expensive and dangerous to mine underground and strip mining causes a lot of environmental damage - cons: the burning of coal, especially high-sulfur coal, yields air pollutants such as sulfur dioxide which can lead to acid rain. even if the coal were pure carbon, the carbon dioxide produced when burned would still have significant effects on earth’s climate
is bond energy reliable?
bond energy is the least accurate measure of delta h
are all C-C bonds equal?
no! sometimes, the atoms around the c will pull/draw away the electrons from the bond which means more/less electrons present in that bond and if there are less electrons, it has less energy. The value listed is just an average bond energy
bond energy larger reactants
endothermic
bond energy larger products
exothermic
why is the nought important?
shows that the process occurs in standard state. this is important for thermodynamics since you are measuring only the heat flow, not the absolute values. this is a common reference state to compare the thermodynamic properties of the two substances
heat of formation is reactants_ products
products - reactants. this is because it takes energy to
petrol and natural gas
- formed from the remains of marine organisms a long time ago. plentiful but trapped in shale rock.
petroleum: dark, thick liquid composed mostly of compounds called hydrocarbons that contain carbon and hydrogen. - natural gas: usually associated with petroleum depositis, consists mostly of methane, but it also contains signigicant amounts of ethane, propane and butane
petrol and natural gas pros and cons
- pros: access so much energy!
- cons: potential contamination of ground water, risks to air quality and possible mishandling of wastes associated with the process
hydrogen fuel
combustion of hydrogen gas releases so much energy
hydrogen fuel pros and cons
pros: so much energy! only real waste product is water\
cons: barely any hydrogen exists as free gas, takes a lot of water to produce, transporting the energy is dangerous as the atoms can make the pipelines themselves brittle and accidental leakage could be so bad for environment
state functions
energy, enthalpy
path functions
heat, work
standard enthalpies of formation are relative values and are based on____
the zero points fo enthalpy of formation are the elements in standard state and all substances are measured relative to this point
if just a balanced chemical eqn was given and you had to determine the sign of W….
you would check the number of moles on either side of eqn. more moles in products means expansion which is -ve w. less moles in products means compression which is +ve w
bond energy is most accurate when
both products and reactants are gases
going from solid to gas
requires energy to break bonds. change of state is heating or cooling. is therefore endothermic!!!!
why is the heat of formation products- reactants
has to do with the definition. the heat of formation is… AHH just look at the picture
when talking about bond energy and heat of formation
make sure you say, “enthalpy calculated using bond energy”
q and moles?
q is dependent on moles but delta h is not
delta t and q
if q is less, the delta t is also less if everything is unchanged
when doing q and w signs for a reaction:
write out balanced chemical equation and q is based on exo or endothermic while w is based on moles of gas before and after
when doing specific heat capacity questions:
use q=mc delta t to explain it carefully. check the polarity, then the number of molecules per gram.
if given the number: say its___ as hard to increase the kinetic energy of particles of this compared to this
nonpolar vs polar: A non-polar molecule would only have van der Waals’ forces present between molecules. The weaker the intermolecular forces, the easier it is to raise the kinetic energy of the molecules. Hence, compound X will have a lower specific heat capacity than Y
subtration and addition
keep the SMALLEST DECIMAL PLACE as the significant digits!!!
surroundings vs system
make sure you know what is the surroundings and which are the reactions
using the delta h to get q (delta h times n eqn)
- delta h: is the moles of the coefficient of the reaction
- n: moles of actual LR not of coefficient. actual amount ex. 0.33!
