Chemistry but it's getting hot in here
K(s) + ½ Cl2(g) ⇆ KCl(s), ∆H° = −437 kJ/mol rxn. How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)? 87.4 kJ is released 43.7 kJ is released 43.7 kJ is absorbed 87.4 kJ is absorbed
43.7 kJ is absorbed
Endothermic curve
Delta E is positive - value of h (change of h) is shown - greater activation energy to get to products means more energy has to be absorbed into the system for it to work
4 ways we will determine enthalpy (ΔH)
1. Calorimetry (MCAT) 2. Bond energy data 3. Consider Heat of Formation tables Hf 4. Hess's law system of equations, essentially
What are the state functions? What does being a state function mean?
All energy changes are state functions which means they are pathway independent.' Internal Energy = E Pressure = P Volume = V Enthalpy = H Entropy = S Free Energy = G
Breaking bonds
endothermic, requires energy
∆Hf
enthalpy of formation -The amount energy released or absorbed in making that substances from its constituent element o = at standard conditions (1 M and 1 atm); measured at 25oC
Molar entropy
entropy content of one mole of substance under standard conditions
Surroundings
everything else
Making bonds
exothermic, releases energy
Exothermic phase change
freezing, condensation, deposition gas to solid
Which has least entropy at room temp conditions? 1 mole of gold 1 mole of mercury equal amounts of entropy since there is 1 mole of each substance not enough information to determine
gold would be solid at room temperature, mercury would be a liquid
energy (E)
the ability to do work or produce heat, potential or kinetic
ΔHvap
(∆Hvap)—heat absorbed to vaporize 1 mole of liquid
Why does a salt dissolve?
-There is enthalpy change when salt or ionic compounds are dissolved into solution -Lattice energy: energy required to separate ions of a salt -Energy of solvation is energy release when ions of salt are surrounded by water molecules
Entropy changes as temperature increases
-kinda linear, but as we add more temperature, entropy increases (linear). -entropy increases by a lot when we go through phase change from solid to liquid to gas.
At a certain temperature C(s) + O2(g) → CO2(g) has a ∆G of −339.4 kJ mol−1. Which statements from the following list are true at this temperature I. the reaction is exothermic II. gaseous carbon dioxide spontaneously forms III. gaseous carbon dioxide is unstable a. I onlyb. II onlyc. I and II onlyd. II and III only
G is negative, meaning that it is spontaneous -we know S is negative, and we know that H is negative because if it was positive, then it wouldn't be spontaneous AT ALL -so we know the reaction is exothermic c.
Delta E is negative
Net loss of energy in the system (exothermic)
Temperature
Temperature (T)—is proportional to the average kinetic energy of the molecules, KEave . "Heat 'em up and speed 'em up" as you saw in the states of matter animation.
Enthalpy (H)-
internal energy of a system and usually measured as a change
We have one block that is 99 degrees, 20g, another block that is 100 degrees 10g. Which block is hotter?
the smaller block
System
the thing we are studying (usually a chemical rxn or physical change)
Breaking bonds ---. What kind of reaction is this? Why is that?
-require energy endothermic -energy is needed to overcome coulombic attraction between nuclei and shared elextrons
What can be said about a chemical system that has reached a minimum in free energy?
-system as reached equilibrium
Gas absorbs 45 kJ of heat and does 29 kJ of work. Calculate E.
absorbs=+ve q does work=-w 45-29 16kJ (DeltaE= w + q)
Enthalpy of reaction (∆Hrxn) -
amount of heat released (negative values) or absorbed (positive values) by a chemical reaction at constant pressure in kJ/molrxn
Heat of reaction: bond energies
bonds broken-bonds formed (essentially reactants - products)
Which of the following statements is true about bond energies in the reaction shown above? a. The energy absorbed as the bonds in the reactants are broken is greater than the energy released as the bonds in the product are formed. b. The energy released as the bonds in the reactants are broken is greater than the energy absorbed as the bonds in the product are formed. c. The energy absorbed as the bonds in the reactants are broken is less than the energy released as the bonds in the product are formed. d. The energy released as the bonds in the reactants are broken is less than the energy absorbed as the bonds in the product are formed. .
c. remember, heat of reaction using bond energies is reactants - products, bonds broken - bonds formed. Because H is negative, that means products is greater. Bonds broken = endothermic, b/c energy is used to break bonds Bonds forms = exothermic, b/c energy is released when bonds are broken
What can potential energy graphs tell us
change of energy (deltaE) and H
w is negative
work is negative; work is done BY the system
Select the process(es) below in which entropy increases, ∆S > 0 2O3(g) → 3O2(g) 2CH4(g) → C2H6(g) + H2(g) NaCl(s) → Na+(aq) + Cl−(aq)
2O3(g) → 3O2(g) 2CH4(g) → C2H6(g) + H2(g) It would be highly unlikely that you would be asked to distinguish something like this as it is a VERY tough decision as demonstrated by the following slides showing that there is very small entropy decrease. NaCl(s) → Na+(aq) + Cl−(aq) Unless there is some other info in the problem indicating otherwise, you can assume ∆S is positive for dissolution.
Which describes the direction of changes in enthalpy and entropy for an endothermic dissolving process of an ionic solute in water at constant temperature? Enthalpy increases and entropy decreases. Enthalpy decreases and entropy increases. Both enthalpy and entropy decrease. Both enthalpy and entropy increase.
Both enthalpy and entropy increase, b/c solid goes to aqueous, (entropy inc.), and endothermic means enthalpy inc.
Exothermic curve
Delta E is negative -value of h is shown -activation energy is smaller than endothermic, just enough to push it over the edge, and then all that energy will go out
q is negative
system loses heat (exothermic)
w is positive
work is positive; work is done ON the system
1st law of thermodynamics
The energy of the universe is constant
Enthalpy of formation (∆Hf)
heat absorbed or released when ONE mole of compound is formed from elements in their standard states in kJ/molrxn
Enthalpy of fusion (∆Hfus)
heat absorbed to melt (overcome IMFs) 1 mole of solid to liquid @ MP expressed in kJ/molrxn
q is positive
system gains heat (endothermic)
Bonding formation ---. What kind of reaction is this? Why is that?
-releases energy exothermic -resultant coulombic attraction b/w bonded atoms lowers potential energy, causing a release.
Which has more entropy? 10 mL of 1 M NaCl 10 mL of 2 M NaCl 1 and 2 have the same entropy since they both represent the same ionic compound in solution.
10 mL of 2 M NaCl All other parameters being equal, higher concentration will have more entropy since there are more molecules present creating more positional possibilities.
enthalpy of reaction practice problem K(s) + ½ Cl2(g) ⇆ KCl(s), ∆H° = −437 kJ/mol rxn. How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)? 87.4 kJ is released 43.7 kJ is released 43.7 kJ is absorbed 87.4 kJ is absorbed
43.7 kJ is absorbed
Heat of fusion
Amount of energy required to change a substance from the solid phase to the liquid phase or vice versa
What does Enthalpy describe/measure/
Enthalpy (H) describes the change in the internal energy of the system based on a certain amount, H is intensive (Joules/mole)
How is Heat (q) measured
Heat (q) is measured by how much energy is lost/gained by the system (when it flows to or away from the system), q is extensive (Joules)
Kinetic energy
Kinetic energy - energy of motion (translational, rotational & vibrational motion of particles in our case), proportional to Kelvin temperature; kinetic energy depends on the mass and the velocity of the object: KE = 1⁄2 mv
Which gas has more entropy? NO NO2 they have the same entropy not enough information to determine I have absolutely no idea how I would even venture a guess.
NO2 (240 J/molK) All else being the same, larger molecules have higher entropy. There are more ways to distribute energy in a larger molecule.
Potential Energy
Potential energy - energy by virtue of position. In chemistry this is usually the energy stored in bonds (i.e., when gasoline burns there are differences in the attractive forces between the nuclei and the electrons in the reactants and the products) When bonded atoms are separated, the PE is raised because energy must be added to overcome the coulombic attraction between each nucleus and the shared electrons. When atoms bond, the above mentioned coulombic attraction results in energy being released and a subsequently lower PE.
Equal masses of two different substances absorb the same amount of heat. The temperature of substance B increases twice as much as the temperature of substance A. Which substance has the higher specific heat capacity? Explain.
Substance A has the higher specific heat capacity because it takes more energy to raise the temperature by one degree. The more energy it takes to raise the temperature of one gram of substance by one degree, the higher the specific heat capacity
Temperature vs. heat
Temperature: a measure of kinetic energy. Heat: the exchange of thermal energy caused by a temperature difference.
Heat of Vaporization
The amount of energy required for the liquid at its boiling point to become a gas or gas to liquid
2nd law of thermodynamics
The energy of the universe is constantly dispersing
We have one block that is 99 degrees, 20g, another block that is 100 degrees 10g. Which contains more thermal energy?
The larger one
What does it mean if a salt is exothermic
The salt is likely very easy to break apart into ions (less lattice energy), and there is more energy released from being surrounded by water molecules
The particle speed of four noble gas is plotted below at a constant temperature. Which gas is represented by the purple line? Helium Neon Argon Xenon
Xenon, because on the graph it shows that it has the slowest speed, but it has the highest probability density. While we don't know what probability density is, we know that out of all of the options Xenon is the biggest boi and therefore needs more energy to move quickly, and will likely have a smaller range of variance in movement
Enthalpy of combustion (∆Hcomb)
heat absorbed or released by burning (usually with O2) in kJ/molrxn; note that combustion reactions yield oxides of that which is combusted
Work on endothermic and exothermic reactions (work on this)
in an endothermic reaction, heat is absorbed, (q is positive), and w is therefore negative. -in an exothermic reaction, heat is released (q is negative)
A cube of ice is added to some hot water in a rigid, insulated container, which is then sealed. There is no heat exchange with the surroundings. What has happened to the total energy and the total entropy when the system reaches equilibrium? Energy remains constant and entropy remains constant Energy remains constant and entropy decrease Energy remains constant and entropy increases Energy decreases and entropy increases
insulated: energy remains constant, ice will become water, so entropy increases
Entropy (s)
measure of the dispersal of matter and energy; increase dispersal +ΔS; decrease dispersal −ΔS
Endothermic phase change
melting, vaporization, sublimation solid to gas
Endothermic
net absorption of energy (heat exchange) by the system; energy is a reactant; (i.e., baking soda and vinegar when mixed get very cold to the touch) ; +ΔH
Delta E is postiive
net gain of energy by the system (endothermic)
Exothermic
net release of energy (heat exchange) by the system; energy is a product; (i.e., burning methane gas in the lab burner produces heat; light sticks give off light which is also energy); −ΔH
We have one block that is 99 degrees, 20g, another block that is 100 degrees 10g. Which way would heat flow to reach thermal equilibrium?
small block to big block
So... when we are talking about system and surroundings regarding dissolving salts, what is the system, and what is the surroundings?
system- the solid and SOME water molecules surroundings- the rest of the water molecules and literally everything else
Free Energy (G)
the amount of work a system can perform on the surroundings
Specific heat capacity
the energy required to raise the temperature of one gram of a substance by one degree Celsius
What does it mean if a salt is endothermic
this means that there is greater energy given into breaking ion bonds (greater lattice energy) than energy released from ions being surrounded by water molecules.
Which has more entropy? two moles of NO2(g) one mole of N2O4(g) the same not enough information to determine
two moles of NO2 2(240.45 J/molK) since these molecules are not tied together with a bond, they have more "degrees of freedom" or positional possibilities. more molecules means more entropy.
A sample of ideal gas at 15.0 atm and 10.0 L is allowed to expand against a constant external pressure of 2.00 atm to a volume of 75.0 L. Calculate the work in units of kJ for the gas expansion (ch7 #35). 1 Latm = 101.3J
w=-p(change of volume) change of volume = 65.0 -p=-2.0 atm work = -13 kJ answer makes sense because the system does work on the surroundings.
Enthalpy of rxn equation/formation equation
ΔHrxn = Hproducts - Hreactants
3 equations to remember
1. ∆E = q + W 2. q = ∆H, at constant pressure (no volume change) 3. W = -P∆V
Mixing 50. g of alcohol at 20.°C with 50. g of hot water at 80.°C in an insulated container, describe the final temperature (thermal equilibrium) of the mixture (qualitatively). Above 80°C Below 20°C Exactly midway between the two temperatures (50°C) Between 50-80°C Between 20-50°C Not enough information
Not enough info: we need to know specific heat capacity for alcohol
Heat (q)
the amount of energy that is transferred from one system to its surroundings because of a temperature difference. -can only be measured when transferred -measured in Joules or Calories
