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Copy of Chapter 6: Thermochemistry
Transcript of Copy of Chapter 6: Thermochemistry
Practice Problems: The Book
Consider the following reaction:
2H2 (g) + O2 (g) -> 2H20 (l) H = -572 kJ
a. How much heat is evolved for the production of 1.00 mol of H2O?
b. How much heat is evolved when 4.03 g of hydrogen is reacted with excess oxygen?
c. How much heat is evolved when 186 g of oxygen is reacted with excess hydrogen?
d. The total volume of hydrogen gas needed to fill the Hindenburg was 2.0 x 108 L at 1.0 atm and 25 C. How much heat was evolved when the Hindenburg exploded, assuming all of the hydrogen reacted?
In a coffee-cup calorimeter, 1.60 g of NH4NO3 is mixed with 75.0 g of water at an initial temperature of 25.00 C. After dissolution of the salt, the final temperature of the calorimeter is 23.34 C. Assuming the solution has a heat capacity of 4.184 J/(C x g) and assuming no heat loss to the calorimeter, calculate the enthalpy change for the dissolution of NH4NO3, in units of kJ/mol.
Practice Problems: AP
Heat and Work
A balloon filled with 39.1 mol helium has a volume of 876 L at 0.0 C and 1.00 atm pressure. The temperature of the balloon is increased to 38.0 C as it expands to a volume of 998 L, the pressure remaining constant. Calculate q, w, and the change in enthalpy for the helium in the balloon. (The molar heat capacity for helium gas is 20.8 J/C x mol.
The standard enthalpy of combustion of ethene gas, C2H4 is -1411.1 kJ/mol at 298 K. Given the following enthalpies of formation, calculation the initial heat of formation for C2H4.
CO2 (g)= -393.5 kJ/mol
H2O = -285.8 kJ/mol
Potential and Kinetic Energy
Which has a greater kinetic energy,
A ball with a mass of 1.0kg and velocity of 2.5 m/s or an apple with a mass of 2.5kg and velocity of 1.0 m/s?
KE = .5(mass(kg))x(velocity(m/s))^2
ball KE = .5(1.0 kg)x(2.5 m/s)^2 = 3.125J
apple KE = .5(2.5 kg)x(1.0 m/s)^2 = 1.25J
The ball has greater kinetic energy.
Heat is a form of energy (Unit: Joules)
Chemicals store energy in their bonds
Law of Conservation of Energy
Energy is neither created nor destroyed - only transferred (temperature change)
Reactions that give off energy as they progress
Some of the potential energy stored in the chemical bonds is converted to thermal energy through heat
Products are generally more stable (stronger bonds) than reactants
Reactions in which energy is absorbed from the surroundings
Energy flows into the system to increase the potential energy of the system.
Products are generally less stable (weaker bonds) than reactants
Enthalpy: change in heat (∆Delate H = H of products - H of reactants)
Calorimetry: Study of transfer of heat
Exothermic vs. Endothermic
Change in heat = (mass)(specific heat)(change in temperature)
Using Hess's Law:
1. Work backward from the final reaction
2. Reverse reactions as needed, being sure to also reverse the delta H
3. Remember that identical substances found on both sides of the summed equation cancel each other
This, also known as 𝐻_𝑓^𝑜 refers to the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in standard states.