Chapter 6 – Thermochemistry – Part 1 – A Review
What is the difference between thermochemistry and thermodynamics?
What are the two different ways energy can be transferred from one object to another?
Draw a picture depicting the differences between system and surroundings.
Define system.
Define surroundings.
Define universe.
What does it mean “the total energy of the universe must remain constant?
Discuss how Figure 6.2 and 6.3 are similar. Be sure to define ∆E and “q” in your discussion.
Using Figure 6.4, discuss how energy can only be transferred as work.
It is important to understand sign conventions in chemistry. Create a chart describing sign conventions for “q” and “w” and ∆E. Be to understand what each of the signs mean in the chart.
What is the First Law of Thermodynamics?
How do we represent this law in a mathematical equation?
What are the most common units of energy and their conversion factors?
Sample Problem 6.1
When gasoline burns in a car engine, the heat released causes the products CO2 and H2O to expand, which pushes the pistons outward. Excess heat is removed by the car’s radiator. If the expanding gases do 451 J of work on the pistons and the system releases 325 J to the surroundings as heat, calculate the change in energy (DE) in J, kJ, and kcal.
Enthalpy
Define enthalpy and the write the mathematical equation used to define enthalpy. Be sure to identify all variables.
Using Figure 6.6, what does it mean to be a state function?
Using Figure 6.7, what does it mean to “push back the atmosphere?”
What is ∆H? What is the mathematical representation of ∆H? Be sure to identify all variables.
Draw representations of endothermic and exothermic reactions. What does it mean to be endothermic or exothermic?
Sample Problem 6.2
A reaction taking place in a container with a piston-cylinderassembly at constant temperature produces a gas and the volume increases from 125 mL to 652 mL against an external pressure of 988 torr. Calculate the work in J (1 atmL = 101.5 J).
Sample Problem 6.3
In each of the following cases, determine the sign of ∆H, state whether the reaction is exothermic or endothermic, and draw an enthalpy diagram.
(a)H2 (g) + ½O2 (g) → H2O (l) + 285.8 kJ
(b) 40.7 kJ + H2O (l) → H2O (g)
Provide a summary of the process of calorimetry.
What is the most common mathematical equation of calorimetry? Be sure to identify all variables.
What is specific heat? What is heat capacity?
Sample Problem 6.4
A layer of copper welded to the bottom of a skillet weighs 125 g. How much heat is needed to raise the temperature of the copper layer from 25ºC to 300.ºC? The specific heat capacity (c) of Cu is 0.387 J/g∙K.
Draw and label a coffee cup calorimeter and describe how it works.
Sample Problem 6.5
A 22.05 g solid is heated in a test-tube to 100.00ºC and added to 50.00 g of water in a coffee-cup calorimeter. The water temperature changes from 25.10ºC to 28.49ºC. Find the specific heat capacity of the solid.
Sample Problem 6.6
50.0 mL of 0.500 MNaOH is placed in a coffee-cup calorimeter at 25.00ºC and 25.0 mL of 0.500 MHCl is carefully added, also at 25.00ºC. After stirring, the final temperature is 27.21ºC. Calculate qsoln (in J) and the change in enthalpy, ∆H, (in kJ/mol of H2O formed). Assume that the total volume is the sum of the individual volumes, that d = 1.00 g/mL, and that c = 4.184 J/g∙K
Describe the similarities and differences in a coffee cup calorimeter and a bomb calorimeter.
Sample Problem 6.7
A manufacturer claims that its new dietetic dessert has “fewer than 10 Calories per serving.” To test the claim, a chemist at the Department of Consumer Affairs places one serving in a bomb calorimeter and burns it in O2. The initial temperature is 21.862ºC and the temperature rises to 26.799ºC. If the heat capacity of the calorimeter is 8.151 kJ/K, is the manufacturer’s claim correct?
What are some important things to remember when doing stoichiometry with thermochemical equations?
Sample Problem 6.8
The major source of aluminum in the world is bauxite (mostly aluminum oxide). Its thermal decomposition can be represented by the equation
If aluminum is produced this way, how many grams of aluminum can form when 1.000x103 kJ of heat is transferred?
What is Hess’s Law and why is it important to chemists?
What are the “rules” for completing Hess’s Law problems?
Sample Problem 6.9
Two gaseous pollutants that form in auto exhausts are CO and NO. An environmental chemist is studying ways to convert them to less harmful gases through the following reaction:
Given the following information, calculate the unknown ∆H:
What is standard enthalpy of formation?
What are the conditions for writing an equation showing standard enthalpy of formation?
Sample Problem 6.10
Write balanced equations for the formation of 1 mol of the following compounds from their elements in their standard states and include ∆Hºf.
(a)Silver chloride, AgCl, a solid at standard conditions.
(b)Calcium carbonate, CaCO3, a solid at standard conditions.
(c)Hydrogen cyanide, HCN, a gas at standard conditions.
Sample Problem 6.11
Nitric acid, whose worldwide annual production is about 8 billion kilograms, is used to make many products, including fertilizer, dyes, and explosives. The first step in the industrial production process is the oxidation of ammonia:
4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (g)
Calculate ∆H°rxn from∆H°fvalues.