chapter16_amsr

Created by Sarah Rendulic and Alex Moran -how heat relates to chemistry __Change in Enthalpy (DH = Delta H)__ D H = H products - H reactants //Exothermic Reaction// DH = NEGATIVE A + B → C + D (DH = - #) A + B → C + D + **Heat**
 * Thermochemistry **
 * 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)

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

__ Enthalpy of Formation (DH f )__ - The amount of energy absorbed or released when a compound forms from its elements Ex: CO 2 ; C + O 2 → CO 2 DH = -393.5 kj/mol How much energy is released when 100.0 g of CO 2 forms? 100g/1 (1 mol CO 2 /44g CO 2 ) (-393.5kj/1 mol) = - 894.3

1. start with what you have (100 g CO 2 ) 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 (C 6 H)?

__Enthalpy of reaction (DHrxn)__ DHrxn = sum of DH f (products) - sum of DH f (reactants)

Ex: CH 4 + 2O 2 → CO 2 + 2H 2 0 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 (C 4 H 10 )

How much heat is released in the combustion of 4.0 mol of benzene(C 6 H)?

__ 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 C 2 H 2 + 2H 2 → C 2 H Given: 1. C 2 H 2 + H 2 → CH 4 DH = -174.4 2. C 2 H 4 + H 2 → C 2 H6 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 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
 * (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)

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