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Archive – Q3 week 6 – 19-20

Week of 3/9 – 3/13 – Always REFRESH this page!

 
3/9  – Monday – Period 2
 
Review homework:
 
thermo 1 calorimetry key 0809.pdf
View Download

Calorie problems key complete p.pdf

 
Is it Entropy???? NOOOOOOO its Enthapy!!!!
1:  1st law of thermodynamics reviewed = infinite possible path functions 
      but only 1 value of  ΔE (STATE FUNCTION!)
 
2: Calorimetry reviewed with notes =  q chemical = – q calorimeter
     because q sys = q chemical + q calorimeter  = 0
 
       Cannot measure energy directly from chemical like cheeto!
 
       defined C cal so that we can get q cal —> q chemicals
 
3.  Constant Volume calorimetry vs. constant volume calorimetry
 
3. Derived ΔH
 
4. INtro to Hess Law
 
 
                                period 3/4 –
 
 
– Complete note-taking up to calorimetry
1:  1st law of thermodynamics reviewed = infinite possible path functions 
      but only 1 value of  ΔE (STATE FUNCTION!)
 
2: Calorimetry reviewed with notes =  q chemical = – q calorimeter
     because q sys = q chemical + q calorimeter  = 0
 
       Cannot measure energy directly from chemical like cheeto!
 
       defined C cal so that we can get q cal —> q chemicals
 
3.  Constant Volume calorimetry vs. constant volume calorimetry
 
3. Derived ΔH
 
4. INtro to Hess Law
 
3/9 – Monday – Homework:  PRE-LAB – Calorimetry / Hess Law Lab
 
Watch Lecture on Lab calculations, and complete worksheet given and answer in form below.  
 
I use this worksheet to model the problem – TAN color!.
Hess Law Lab Pre lab for lecture USE.pdf
View Download
 
THis is the white one – have this one: (which the form is based on)
Hess Law Lab Pre lab p.pdf
View Download
 
Hess Law Calculations Lab:
 
****In the video I make a small mistake. The density of all the solutions used in the calorimetry are 1.02 g/ml ! Thanks JP!
 
 
**** I also made a mistake where I did not carry over a negative sign from my q —-> ΔH
 
MONDAY’S Homework Form:

Hess Law Lab Calculations Form 1920

 

 
 
End of Monday..

3/10  – Tuesday – Period 2/3
 
1.  Quick Review of Mondays’ Form
 
Key from Mondays’ Form
Hess Law lab form 1 key complete p.pdf
View Download
 
2: Calorimetry and Hess Law Lab – Complete take the L worksheet.
3. Enter into the form.
4. Start data collection – start with Ccal -.
                                        Period 3/4- 
 
1.  Quick Review of Mondays’ Form
 
Key from Mondays’ Form
Hess Law lab form 1 key complete p.pdf
View Download
 
2: Calorimetry and Hess Law Lab – Complete take the L worksheet.
3. Enter into the form.
4: Hess Law – complete lab data collection for all of the 3 reactions and the Ccal.
 
 
Lab packet:
Calorimetry and Hess law p.pdf
 
 
1:  Anyone who did not get a 100% on the Pre-lab calculations will not start lab until you redo the assignment and get a 100%. Some of you made small errors.  I made all of you redo it with a new version (“take the L”).
 
Those of you who did not do the assignment, for whatever reason, will have a zero for the assignment.
 
2. You are complete the lab with your lab partners to complete the Lab. You are to use the lab station computers with temperature probes and Logger Pro.
 
a) Your  experiment should be set to take temperatures at 5 sec intervals for about 180 seconds. You will need to make a regression line of the best linear part of your line and determine the y-intercept, which will be = Tmix.
 
b) Once you complete the Calibration of the calorimeter, (determine Ccal), you will need to complete 3 trials using 3 the different reactions and calculate the H of each reaction (just like the HW).  
 
VERSION 2 – For those who did not did not get 100 or did not complete the homework – This will be completed by all students before we start the lab.  I will take the best out of 2 grades for those that did the assignment over the weekend.
 
I used this worksheet to model the problem this weekend.
Hess Law Lab Pre lab for lecture USE.pdf
View Download
 
I use this worksheet for the new form (“take the L” ) below:
Hess Law Lab Pre lab Version 2 – take the L p.pdf
View Download
 

Hess Law Lab Calculations Form Version 2 – take the L

 
3/10  – Tuesday Homework:  HESS LAW with Reactions!
 
*Remember that ΔH is a really important thermodynamic state function that will help us interconnect other Δquantities from other reaction if these reactions are connected. 
 
Period 2/3:  
1: Watch lecture on Hess Law (from 8:00 on!)  and complete with me the Hess Law problems that I model from the thermo 4 – Hess Law ditto.pdf worksheet. We will apply this to our lab tomorrow.
 
2. Complete your Hrxn for each of the three reactions from your Lab packet using today’s data.
thermo 4 – Hess Law ditto.pdf
View Download
 
thermo 4 – Hess Law ditto key.pdf
View Download
 
Period 3:
1: Watch lecture on Hess Law (from 8:00 on!)  and complete with me the Hess Law problems that I model from the thermo 4 – Hess Law ditto.pdf worksheet.
We will apply this Hess Law to our third reaction Tomorrow!
Homework Lecture from 8: 00 min mark on! 
I derive the H in the first 8 minutes.
We defined H (enthalpy) Monday (yesterday!!)
 
Is it Entropy???? NOOOOOOO its Enthalpy!!!!
End of Tuesday..

3/11  – Wednesday – Hess Law – The interconnectivity of ΔH
 
Todays notes:
 

 

Remember that we only obtain qrxn values from calorimetry and it is equal and opposite in value from the qcal.
 
qrxn = – qcal
 
Since most reactions occur in constant pressure conditions (NASA – Grade calorimeter) then qrxn values at constant pressure are equivalent to Enthalpy.
qrxn = ∆H 
 
∆H is a state function that depends only on final conditions – initial conditions AND NOT THE pathway!
Because ∆H is a state function then we can use its interconnectivity to solve for other ∆H values.
In the simple example to the left if we know the direct distance from North Riverhead to Westhampton and the direct distance from Westhampton to Northhampton we can use both values to determine the Overall change of distance from North Riverhead to Northhampton.
 
The interconnectivity of the state function ∆H is called Hess’s Law!
 
This law allows for chemists to calculate ∆H values without doing calorimetry that might be too impractical or expensive.
We learn today that this interconnectivity works because all chemicals are formed in reactions that start from the same baseline that I will call a free atom zone (FAZ).
The ∆H for formation of chemicals from their atoms (free atom zone) is called Heat of Formation, Hf .  The sum of all the Hf of the reactants equals the starting position in an energy diagram or in the case of the example above the starting position of the North Riverhead.
We will see how changes in the Heat of Formation of compounds (Hf0) can be connected to the overall change of the Enthalpy (H) of an entire reaction.  The power in this connectivity called Hess’s Law allows us to calculate the  Hrxn  from just a table of Hf(heats of formations) without having to do any calorimetry.  We need to keep in our heads that the reason for this interconnectivity is due to the using state functions (H) and that all Hfbegin from the same baseline of (elemental states).  This baseline leads us to the following formula that is posted in your reference table:

 

Since Hrxn is a state function we are not tied to just one way or pathway to calculates the change in Enthapy (Hrxn) of a chemical reaction.

 


 
Reactions in our lab:
 
1.                           HCl (aq)  +  NaOH (aq)       —>    NaCl (aq)   +   H2O (l)
 
Net Ion:            H+ (aq)    +    OH  (aq)     —->     H2O (l)
 
 
2.        NH4Cl (aq)  + NaOH (aq)    —–>   NH3 (g)   +     NaCl (aq)  +  H2O (l)      
 
Net ion:           NH4+ (aq)  +  OH–  (aq)   —->  NH3 (aq)  +  H2O (l)
 
 
3.                                       NH3 (aq)  + HCl (aq) —-> NH4Cl (aq)
 
Net Ion:                 NH3 (aq)   +  H+ (aq)  —->   NH4 (aq)
 
 
                                                                                                                                                          Theoretical Values
1.                              HCl (aq)  +  NaOH (aq)     —–>    NaCl (aq)   +   H2O (l)                            ∆H = -55.9 kJ/mol
 
2.                         NH4Cl (aq)  + NaOH (aq)     —–>   NH3 (g)   +     NaCl (aq)  +  H2O (l)     ∆H = -3.7 kJ/mol 
 
3.                                       NH3 (aq)  + HCl (aq)  —-> NH4Cl (aq)                                                  ∆H = -52.2 kJ/mol
 
Can you use the first 2 reactions ∆H values to obtain the third by manipulating the first 2 reactions? Is Hess Law supported? 
 
When you rearrange and cancel:
 
1.                              HCl (aq)  +  NaOH (aq)     —–>    NaCl (aq)   +   H2O (l)                            ∆H = -55.9 kJ/mol
 
2.    flipped               NH3 (g)   +     NaCl (aq)  +  H2O (l)  —-> NH4Cl (aq)  + NaOH (aq)            ∆H = +3.7 kJ/mol 
_____________________________________________________________________________
3.                                       NH3 (aq)  + HCl (aq)  —-> NH4Cl (aq)                                                  ∆H = -52.2 kJ/mol
 
                                       first reaction    +     second reaction        =   third overall reaction
                         ∆H = -55.9 kJ/mol  +  ∆H = +3.7 kJ/mol  =   ∆H = -52.2.9 kJ/m
 
Hess Law is supported!
 
So ∆H of reactions that cancel out to give the overall reaction clearly support the interconnectivity of Hess’s Law but the question remains why?  
 
∆H is a state function yes but why does each reaction have a unique starting position  What explains its starting value?  We know the difference between the reactions creates an overall ∆H for the overall reaction as in the 3rd reaction above but it does not explain the positions in the energy diagram that creates the interconnectivity.  
 
For instance from today’s Lab we get the following energy diagram for the 1st reaction using heats of formation values from the Thermo tables:
 
Heats of formation, ∆H0 or the enthalpy change of heat that occurs when a compound is made from its elements.  Every compound is made from elements combining to form bonds between atoms thus every compound in EVERY chemical reaction is contains a certain potential energy associated with how much energy is released if forming that(those) bond(s). 
 
For example the ∆H for HCl that is one of the reactants in reaction 1 from our lab is represented by the following synthesis (composition or formation) reaction:
 
                                                                        H2   +      Cl2   —>     2HCl
 
∆H  are always posted in tables in kJ/mol ( kilojoule per mol ).  Since the energy posted to form HCl from its elements is always PER 1 MOLE we reduce the above reaction to produce 1 mole of HCl.  Thus the reaction now becomes:
 
                                                                1/2 H2   +     1/2 Cl2   —>     HCl
 
When we look up the ∆H for HCl in thermodynamic tables we obtain ∆H = -167.2 kJ/mol.  The thermodynamic tables do not provide the reaction but only the producthus you need to be aware what the table is implying the energy change from the formation of the listed chemical from its elements. 
 
∆H = -167.2 kJ/mol  for HCl means that -167.2 kJ/mol of energy is released when elements combine to form 1 mole of HCl.  Its negative because the energy is released as bonding of the elements creates increased stability. 
Notice the arrow moving down in the diagram below for Every chemical in the reaction for reaction 1. Some move farther down than others because some compounds release more energy when they are formed WHICH MEANS THEY ARE MORE STABLE and have LOWER potential energy than the chemicals that do not release as much energy when they are formed.
 

 

So the starting point for the reactants in a chemical reaction is the SUM of the individual reactants heats of formations
 
     Notice the Sum of all of the ∆Hf of products – the Sum of all of the ∆Hf of products = ∆H rxn
 
              That is the new skill tonight: determining the ∆H rxn using ∆H f values.
 
END OF NOTES…

 
                        Period 2-
 
1: Reviewed the North Riverhead Digram and used metaphors with Chemical Reactions/stability and introduced Heats of Formation.
 
c) Verify Hess LAW:  (We complete this complete this once we learn about Hess Law)
 
Using the Hrxn of  reaction 1 and reaction 2 determine the Hrxn for reaction 3 using Hess Law.  Compare the your value using Hess Law with the value you determined through calorimetry.
 
Take the first 2 reactions and manipulate /cancel to get the overall reaction to be equal to the third reaction.
Use your Enthalpies to calculate the 3rd reaction (just like the worksheet.)  Compare it to the value of the Enthalpy that you directly obtained through calorimetry.
Show all work/ calculations/ Hess Law / Percent error and Hand in.
2. Heat of formation – (intro and with demo)
a) Discussion of reaction of Demo to discuss what Heat of formation (Hf0of NaCl.
b) Thermo tables, elemental states, thermodynamic standard conditions.
c) Compared Hf0 of Al2O3 and NaCl, and discussed strength of Bonds or lattice energy.
d) Drew diagram of the Free atom Zone (FAZ), which is the reference point to show Hf0 of reactants and products of another reaction.
e) Used Hf0 to determine the Hrxn of NaCl ionizing into water and determining whether the water will increase or decrease in temperature.
F) Used Hfto solve for the theoretical value of the third reaction.
thermo AP Tables.pdf
View Download
 
thermo AP Tables additions.pdf
View Download
                                Period 3/4 – 
 
 
 
1) complete data collection for the Hess law calorimetry lab
 
 
2) Verify Hess LAW:  (We complete this complete this once we learn about Hess Law)
 
Using the Hrxn of  reaction 1 and reaction 2 determine the Hrxn for reaction 3 using Hess Law.  Compare the your value using Hess Law with the value you determined through calorimetry.
 
Take the first 2 reactions and manipulate /cancel to get the overall reaction to be equal to the third reaction.
Use your Enthalpies to calculate the 3rd reaction (just like the worksheet.)  Compare it to the value of the Enthalpy that you directly obtained through calorimetry.
Show all work/ calculations/ Hess Law / Percent error and Hand in.
 
2) Heat of formation – new concept
 
a) Discussion of reaction of Demo to discuss what Heat of formation (Hf0of NaCl.
b) Thermo tables, elemental states, thermodynamic standard conditions.
c) Compared Hfof Al2O3 and NaCl, and discussed strength of Bonds or lattice energy.
d) Drew diagram of the Free atom layer, which is the reference point to show Hfof reactants and products of another reaction.
3) Reviewed the North Riverhead Digram and used metaphors with Chemical Reactions/stability and introduced Heats of Formation.
3/11  – Wednesday Homework:  
Please pay particular attention to the questions I am asking you to complete.  I have a reason for every problem that I assign.  Please complete them in the correct order.
 
1. READ MY NOTES posted at the start of today to it says END OF NOTES…
 
2. Watch the Hess law lecture with Heats of FORMATION
 
Hess Law with Heats of Formation:
 
 
3: Lab 21 – Hess Law Lab Activity is due Friday 3/13 in the purple crate “of fun” .
     Non-formal lab requirements posted below.
4.  Complete questions 1 – 4 of the Thermo 3 heat of formation. pdf worksheet and review with the key.
 
Look at the highlighted reference table above!
5:  Please complete question 5 on the  Thermo 1 calorimetry.pdf worksheet ( you have it already) and question 5 
       and 7 from the thermo 4 – Hess Law ditto.pdf worksheet.
* in question 5 in thermo 4 – Hess Law – the overall reaction is the heat of reaction of Mg(OH)2
Thermo 1 calorimetry.pdf
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thermo 1 calorimetry key 0809.pdf
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thermo 4 – Hess Law ditto.pdf
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thermo 4 – Hess Law ditto key.pdf
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thermo 3 heat of formation.pdf
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thermo 3 heat of formation key.pdf
View Download
 
LAB 21  requirements:
 
Objectives:
 
Part 1: 
Show all calculations clearly and neatly.  If you have to rewrite them then do so. I will take off for a lack of neatness. Show all units and sig figs correctly. There are 4 separate parts to your calculations.
 
Part 2: (3 parts)
a) On a separate piece of paper write the three reactions and their H/mole values.
Please use reaction 1 and reaction 2 and manipulate them to cancel out to get reaction 3.
Just like you did with Monday’s homework in thermo 4 – Hess Law ditto.
 
Thermo 4 – Hess Law ditto key.pdf
View Download
b) Calculate what the H/mole of the third reaction should be using Hess’s Law (reaction manipulation)  Compare it with the H/mole of that you obtained through calorimetry in lab for reaction 3
 
c) Calculate the known by using Heat of Formation Tablature for the 3rd reaction. Complete a percent error based on this value as your known or theoretical and run a percent error on both of your experimental values (value of H/mole using reactions and then with H/mole from calorimetry.
 
You should notice that the reaction manipulation H/mole value of reaction 3 will be closer to H/mole value obtained through calculations with heat of formations.
 
Error Analysis:  There is one major error with all forms of calorimetry.
 
Staple calculations and reactions. Hand in tomorrow.
Today’s Demo: Heat of Formation of NaCl
 
End of Wednesday…

 
3/12  – Thursday – Period 2/3
 
1. Complete comparison of ∆Hof Al2O3 and NaCl with thermite reaction (outside).
 
– Modeled the calculation of the ∆Hrxn for the thermite reaction using heats of formation
– Wrote a a potential energy diagram for the Thermite reaction and used the idea of potential      
  energy to describe the formula of :    
 
                ∆Hrxn   =  Sum of the ∆H Products – Sum of the ∆H Reactants
 
                 ∆Hrxn =    Potential Energy of the Products – Potential Energy of the Reactants
I am focused on you being able to visualize the FAZ diagram that illustrates the ∆H values that determine stability in the potential energy (stored energy of chemical bonds)  by how much energy is released to form individual compounds in a chemical reaction. 
 
Hess Law works because all compounds are tied to the same Baseline = F.A.Z.  (Free Atom Zone).  Every chemical in every chemical reaction has a unique potential energy because of how much energy is released in its formation from the same Baseline (FAZ)!   THAT is how ∆H’s are interconnected in reactions!!
 
To obtain a ∆Hrxn all you need to do sum all of the ∆Hf of the reactants you will obtain the total energy lost to form the reactants.  This will tell what potential Energy is REMAINING!!!!!!
 
Do the same for the products in the and compare remaining potential energy from the products and reactants AND YOU HAVE THE ∆H of the entire reaction.
 
THIS IS HESS LAW!
 
Today we will learn that this idea allows us to apply Hess law to many all values of ∆H, which has many names
including:
 
Heat of dissolution, Heat of decomposition, Heat of Fusion, Heat of vaporization, 
Heat of sublimation, Heat of combustion, Heat of …..
These are all ∆H values!!!!
2. Started the Phase change enthalpy.pdf
     
* I really linked the idea of potential energy changes in the heating curve at the phase changes so that we better understand the Heat of Fusion and Heat of Vaporization that is used in Regents Chemistry.
 
This is an important concept as ∆H’s can also be used to determine the energy changes if you have a certain amount of the reactant and you know it’s ∆H . For instance if we calculate the ∆H fusion of water or Heat of Fusion (remember ∆H has a lot of names) from ∆H values for the physical process of H2O (s) —> H2O (l) we get the amount of Energy needed to be consumed (or absorbed) per mole of H2O (s).   We can use this value to determine calculate amount needed to melt (fusion) of a certain amount of ice.  We do this for chemical reactions as well.
 
We can determine the ∆Hvap (Heat of Vaporization) the same way with ∆H0  values for the physical process
  H2O (l) —> H2O (g) and use it value to measure the energy it takes to vaporize the water with given amount of water in the liquid phase.
 
Also it was good to review why kinetic energy gets converted to potential energy at the phase changes.
Phase change enthalpy.pdf
View Download
 
 
Today’s Lesson videos:
                                  
                                        Period 4-
 
1. Complete comparison of ∆Hof Al2O3 and NaCl with NaCl synthesis and thermite reaction 
 
 
       
1: started the Phase change enthalpy.pdf
 
Phase change enthaply worksheet review: 
 
Phase change enthalpy.pdf
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  Concepts: heating and cooling curve, heat of fusion,heat of vaporizat
     
* I really linked the idea of potential energy changes in the heating curve at the phase changes so that we better understand the Heat of Fusion and Heat of Vaporization that is used in Regents Chemistry.
 
This is an important concept as ∆H’s can also be used to determine the energy changes if you have a certain amount of the reactant and you know it’s ∆H . For instance if we calculate the ∆H fusion of water or Heat of Fusion (remember ∆H has a lot of names) from ∆H0  values for the physical process of H2O (s) —> H2O (l) we get the amount of Energy needed to be consumed (or absorbed) per mole of H2O (s).   We can use this value to determine calculate amount needed to melt (fusion) of a certain amount of ice.  We do this for chemical reactions as well.
 
We can determine the ∆Hvap (Heat of Vaporization) the same way with ∆H0  values for the physical process
  H2O (l) —> H2O (g) and use it value to measure the energy it takes to vaporize the water with given amount of water in the liquid phase.
 
Also it was good to review why kinetic energy gets converted to potential energy at the phase changes.
2:  Table I – Heat of dissolution of NaCl demo – temperature fell (H dissolution = +4.2 KJ/mol)
     Complete discussion of heat of formation into stability of bonds and H dissolution.
∆H = -55.9 kJ/mol
 
Today’s demo:  No one actually hit record on the ipad today so there no video of today 🙁
 
3/12 Thursday Homework:  Mock Part 2 question of an old AP question – The question I am giving you represents a previous released part 2 question. Remember that the first hour an a half of the AP Chemistry Exam will be the multiple choice. After a 10 minute break (after the completion of the MC) you will begin the part 2 portion which is 1 hour and 45 minutes. In this section you will have 7 part 2 free response questions.  Questions 1- 3 are usually 10 point questions, while questions 4 – 7 are 4 point questions.  Tonights homework represents a question from part 2 that is a 10 point question.
 
1. Complete Free Response AP question. Please give yourself about 20 -25 minutes and try to entirely complete this question before looking at the key. 
 
2.  Please review your work with the key posted below and grade your work based on the AP Central Rubric posted below.
 
Free Response Thermo, Hess, Calorimetry AP problem.pdf
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This is the key to the above problem
2013 Part 2 – AP Key p.pdf
View Download
 
Optional
Old Class lecture on Heats of formation: 
End of THursday..

3/13  – Friday –  
 
Today was about linking the concepts of heat of formation (Hf)  with heat of dissolution (Hsol)
  Table I in the Regents Reference Tables lists ionization reactions and their Hrxn.  Salts that dissolve water are endothermic (lower temperature of solution) or exothermic (increase temperature of solution).
1:  Table I – Heat of dissolution of NaCl demo – temperature fell (H dissolution = +4.2 KJ/mol)
     Complete discussion of heat of formation into stability of bonds and H dissolution.
*Connections – We can use heat of formations from our tables to calculate the Hrxn . The Hrxn has many names tha describe the reaction.  In this case we are looking at a ionization reaction that occurs in water (aq) and thus we call this change in enthalpy (H) the Hdissolution or Hsol (solution).
 
The Hdissolution for CaCl2 (s) is exothermic (negative H) and thus the solution temperature rises as heat flows from the chemical (CaCl2 (s) ) to the water.  This means that the combined ions that result from the dissolving of CaCl2 (s) ARE MORE STABLE in water than water H-bonding to itself.  This results in a release of energy!
 
Today we observed NaCl dissolving and it was endothermic (positive H) and thus the solution temperature decreases as heat flows water the chemical (CaCl2 (s) ) to the water. (this results in an absorbtion of energy or I like to say consumption of energy).  This means that the combined ions that result from the dissolving of NaCl (s) ARE LESS STABLE in water than water H-bonding to itself.  This results in a absorption or consumption of energy!
 
 
 

Period 2 – 

 
1.  Review of homework Mock AP Question:
 
     2013 Part 2 – AP Key p.pdf
   View Download
2. Hess law truth – ∆H0  values are the GOAT’s in HESS LAW!
    slide 30 – slide 32 in Thermo presentation- 
    (canceling reactants in multiple chemical reaction Hess law problem is 
     really cancelling ∆H values! )
3. Heat of dissolution/heat of formation review with CaCl2
       Table I – 
 
    Hot Packs/Cold packs
4:  Bond Enthalpies – Demo – “bonds broken” – bonds formed“.
 
Bond Enthalpy – 2H2O2  -> O2 +  2H2O
      determined Hrxn with heat of formation then with bond enthalpies
       modeled question 1 on Bonding 6 – Bond energies.pdf worksheet.
 
Bonding 6 – Bond energies Table.pdf
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Bonding 6 – Bond energies.pdf
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Bonding 6 – Bond energies key.pdf
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                                            Period 3/4
 
-Completed the heat of formation of NaCl demo
-Completed the thermite reaction
 
1. Review of homework Mock AP Question: – did not get to do this!!
 
     2013 Part 2 – AP Key p.pdf
   View Download
 
2. Hess law truth – ∆H0  values are the GOAT’s in HESS LAW!
    slide 30 – slide 32 in Thermo presentation- 
    (canceling reactants in multiple chemical reaction Hess law problem is 
     really cancelling ∆H0  values! )
 
3. Heat of dissolution/heat of formation review with CaCl2 and NaCl
    Table I – did not get to do this!
 
    Hot Packs/Cold packs – Enthalpy change in the dissolution is due to the 
    ENERGY NEEDed to breaking of the ionic bonds in the salt and the attractive forces in water
    and the LOSS of Energy needed in forming new bonds or attractive forces in the products.
 
4. Bond Enthalpy with demo – – 2H2O2  -> O2 +  2H2O   Started this
      determined Hrxn with heat of formation then with bond enthalpies
       modeled question 1 on Bonding 6 – Bond energies.pdf worksheet.
 
  ∆H rxn = “bonds broken” – bonds formed“.
Thermodymamics Presentation: SLIDE 30 – 32!!!! Who’s the GOAT???????
Bond energy Notes:
 
Today’s Notes:
 
Yesterday I introduced a new way to obtain H of a chemical reaction using Average Bond Energy values.  Through careful experimentation of many chemists we have a comprehensive list of the bond energy or bond enthalpy’s of all covalent (nonmetal to nonmetal bonds).   
 
If we know the energy it takes to break a bond then we know the energy it takes to form a bond because:
                                                      qsys    =     – qsurr
                         Energy Needed to break a bond     =       Energy needed to form a bond
                                                       Energy consumed  (+)     =        Energy released (-)
 
So bond energy values are always positive unlike heats of Formation values (Hf
 

In this reaction the Hcomb of propane (C3H8) the reaction is exothermic (-H ) because the energy needed to form the bonds of the STABLE products is greater than the energy needed to break the bonds of the reactants.

 

Remember that these values are Averages because bond lengths are constantly changing as they absorb and release infrared radiation. 

Notice that all bond enthalpies (bond energy) are positive as it ALWAYS requires energy to break bonds which are the “covalent sharing electron conditions” that provide STABLE electron configurations for all atoms in the molecule!

Notice the C=C bond energy is NOT double the bond energy of C-C because a double bond has sigma and a pi bond. If you remember a pi bond is weaker than the direct overlap of a sigma bond.

Notice the *C-C bond energy is somewhere in between a single C-C bond and a C=C bond because of RESONANCE in Benzene (which is sp2 hybridized).

Its Bond order is 1.5!
 
 
Lets not forget that the internuclear graph is plotted with potential energy! The potential energy drops as bonds are made because the attraction for the each other atoms valence electrons is provides stability as available orbitals are filled. In the case of Hydrogen below they will attain the stability of He when they bond diatonically.  Notice the potential energy drops because the ∆H bond (bond enthalpy or bond Energy) is Negative as heat is released from the atoms when they bond.  Can you identify the Free atom zone?
 

What is the bond enthalpy of H2?

Bond Enthalpies will be given in small tables in AP questions when needed but remember that Bond Enthalpy is always positive to describe the Energy needed to be absorbed
or consumed or BREAK an existing bond.

Bonds being Broken always = +∆H

Potential Energy increasing

Bond being FORMED always = -∆H

Potential Energy decreasing

∆H rxn = bonds broken” – “bonds formed“.

SO if we have Bond Enthalpy:  
 
                     ∆H rxn =        ∑ bonds broken               –         ∑ bonds formed
                                              [ ∑ bond enthalpy of reactants]  –  [ ∑ bond enthalpy of products ]  
 
This formula is NOT provided in your reference tables.        
        
And if we have Heats of Formation (∆H):
                   ∆H rxn =   ∑  ∆Hof the Products ]  –  [ ∑  ∆Hof the Products ]
 
This formula IS provided in your reference tables.        
Both of these equations ARE HESS LAW!!!
Today’s Video’s:

 

 
 
NaCl formation Demo:
 
Today’s demo:  I dream of genie reaction
 
 
 
Today’s Thermite demo actual video from period 3/4 class:
 

 

3/13  Friday Homework: PLease Refresh as this is going to change.
1.  Please view my Bond Enthalpy Lecture below: 
     I will model the 1st reaction in the Bonding 6 – Bond energies key.pdf worksheet.

 

Bond Enthalpy Lecture:   (period 2 should start at              – we did most of this in class)
                                                   (period 3/4 – NOT OPTIONAL!)
 
 
2. Please complete the front side of the Bonding 6 – Bond energies.pdf worksheet and review with the key.  You will need a table of Average Bond Energies!  
We started question 1 in class with the soda bottles demo (I dream of genie demo).
 
Bonding 6 – Bond energies Table.pdf
View Download
Bonding 6 – Bond energies.pdf
View Download
 
Bonding 6 – Bond energies key.pdf
View Download
3. Please view the Heat of dissolution Lecture – (period 2 optional – we did this in class)
                                                                                          (period 3/4 – NOT OPTIONAL!)
 
Heat of dissolution Lecture – 
4. Please complete a another Free Response part 2 question. Give yourself 20 – 25 minutes and complete it in its entirety before scoring with key below.
AP Chemistry – 2016 Free Reponse question 1.pdf
View Download
New key that is correct!
AP Chem 2016 ques 1 AP Key.pdf
View Download
5.  Complete the Form below which will be based 1 – 4 above.
Heat of dissolution, Bond enthapy, Hess law Form – 
 

Heat of Dissolution, Bond Enthalpy Form

End of week 6!

 
 
 
 
 
 
NOT UPDATED BEYOND HERE!