Created by Sarah Rendulic and Alex Moran
Thermochemistry
-how heat relates to chemistry

  • Enthalpy (H)- tells the amount of heat absorbed or released in a chemical reaction
    • enthalpy accounts fo pressure and volume
    • unit is joules or calories (1 calorie = 4.184 joules)

Change in Enthalpy (DH = Delta H)
D
H = H products - H reactants

Exothermic Reactionenthalpies_reaction.gif
DH = NEGATIVE
A + B → C + D (DH = - #)
A + B → C + D + Heat


Endothermic Reaction
DH = POSSITIVE
DH = A + B → C + D (DH = #)
DH = A + B + Heat → C + D





Enthalpy of Formation (DHf)
- The amount of energy absorbed or released when a compound forms from its elements
Ex: CO2; C + O2 → CO2 DH = -393.5 kj/mol
How much energy is released when 100.0 g of CO2 forms?
100g/1 (1 mol CO2/44g CO2) (-393.5kj/1 mol) = - 894.3

1. start with what you have (100 g CO2)
2. convert to moles using the molar mass
3. put in the DH value
4. calculate

PRACTICE!
How much energy is released in the formation of 92.1g of water?


How much heat is absorbed in the formation of 87.1g of Benzene (C6H)?




Enthalpy of reaction (DHrxn)
DHrxn = sum of DHf (products) - sum of DHf (reactants)

Ex: CH4 + 2O2 → CO2 + 2H20
DH; -74.5 0 -393.52 -241.8

DHrxn = [-393.52+(-241.8 *2)] - [-74.85]
DHrxn = -1322.96 kj

PRACTICE!
How much heat is released in the combustion of 2.0 mol of butane (C4H10)


How much heat is released in the combustion of 4.0 mol of benzene(C6H)?




Hess's Law
- the DH of a reaction is equal to the sum of the DH of each step of that reaction

Find the DH for C2H2 + 2H2 → C2H
Given: 1. C2H2 + H2 → CH4 DH = -174.4
2. C2H4 + H2 → C2H6 DH = - 137 (a. decide if any need to be flipped in order to get the correct compounds on each side)
C2H2 + H2 + C2H4 + H2 → C2H4 + C2H6 (b. combine the given equations)
C2H2 + 2H2 + C2H4 → C2H4 + C2H6 (c. combine any like terms within the sides)
C2H2 + 2H2 → C2H6 = -311.4 (d. once the equation is the same as the one in question add the DH values of the givens)

***if you flip one of the givens the DH for that equation's sign also flips from - to + or vice versa.

PRACTICE!
Find the DH for S + O2 → SO2
Given: 1. 2SO2 + O2 → 2SO3 DH = -196 kj
2. 2S + 3O2 → 2SO3 DH = -790 kj


Find the DH for N2O4 → 2NO2
Given: 1. N2 + 2O2 → 2NO2 DH = 169.5
2. N2 + 2O2 → N2O4 DH = 24





Calorimtry

- Heat Capacity- the amount of heat required to change the temperature of a substance by 1*C
For example if you had a small cup of water and a big bucket of water the heat capacity of the bucket would be greater

- Specific heat (C)- the amount of energy required to change 1 gram of a substance by 1*C
For example if you had a small cup of water and a big bucket of water the specific heat of both would be the same
  • (for H2O) C = 4.184 J/g*C
  • q(heat J) = m(mass g) c(specific heat J/g*C) DT (change in temperature *C)
Ex: How much heat is required to change the temperature of 500.0g H2O from 10*C to 25*C?
q = mcDT
q = (500)(4.184)(15)
q = 31380 J

PRACTICE!
21,000 J are applied to 492 g of water at 11.0*C find the final temperature.


How much heat is required to heat 100.0 g of H2O from 35*C to 45*C


Entropy (s) - measure of the disorder or randomness of a system



Enthalpy
Entropy
Favored
exothermic
disorganized
Unfavored
endothermic
organized


Gibbs Free Energy
-used to determine if a reaction will happen based on enthapy and entropy
  • DG = DH - T DS
  • if DG is negative the reaction happens (spontaneous)
  • if DG is positive the reaction doesn't happen (non-spontaneous)

PRACTICE!
Will this reaction occur at 30*C?
C + O2 → CO2
DH = -393.5 kj
DS = 2.13 kj/mol* k

At what temperature will this reaction occur
C2H4+H2→C2H6
DH=-136.9 KJ/mol
DS=-0.1207 KJ/mol*K
DG = -172.6