10 months ago

Final NB 2016-2017 Turley

The Learning Goal for

The Learning Goal for this assignment is: The student will learn how energy is converted in a chemical or physical process and how to determine the amount of energy is absorbed or released in that process. The System and the Surroundings in Chemistry Thermochemistry The system is the part of the universe we wish to focus our attention on. In the world of chemistry, the system is the chemical reaction. For example: 2H2 + O2 ---> 2H2O The system consists of those molecules which are reacting. The surroundings are everything else; the rest of the universe. For example, say the above reaction is happening in gas phase; then the walls of the container are part of the surroundings. There are two important issues: 1. a great majority of our studies will focus on the change in the amount of energy, not the absolute amount of energy in the system or the surroundings. 2. regarding the direction of energy flow, we have a "sign convention." Two possibilities exist concerning the flow of energy between system and surroundings: 1. The system can have energy added to it, which increases its amount and lessens the energy amount in the surroundings. 2. The system can have energy removed from it, thereby lowering its amount and increasing the amount in the surroundings. We will signify an increase in energy with a positive sign and a loss of energy with a negative sign. Also, we will take the point-of-view from the system. Consequently: 1. When energy (heat or work) flow out of the system, the system decreases in its amount. This is assigned a negative sign and is called exothermic. 2. When energy (heat or work) flows into the system, the system increases its energy amount. This is assigned a positive sign and is called endothermic. We do not discuss chemical reactions from the surrounding's point-of-view. Only from the system's. Notes: - If the heat flow is going out of the system it is an exothermic reaction -If heat is going into the system it is an endothermic reaction For example an ice pack would be an endothermic reaction because it is taking heat in from the persons body 132

Specific Heat Here is the definition of specific heat: the amount of heat necessary for 1.00 gram of a substance to change 1.00 °C Note the two important factors: 1. It's 1.00 gram of a substance 2. and it changes 1.00 °C Keep in mind the fact that this is a very specific value. It is only for one gram going one degree. The specific heat is an important part of energy calculations since it tells you how much energy is needed to move each gram of the substance one degree. Every substance has its own specific heat and each phase has its own distinct value. In fact, the specific heat value of a substance changes from degree to degree, but we will ignore that. The units are often Joules per gram-degree Celsius (J/g*°C). Sometimes the unit J/kg K is also used. This last unit is technically the most correct unit to use, but since the first one is quite common, you will need to know both. I will ignore calorie-based units almost entirely. Here are the specific heat values for water: Phase J g¯1 °C¯1 J kg¯1 K¯1 Gas 2.02 2.02 x 10 3 Liquid 4.184 4.184 x 10 3 Solid 2.06 2.06 x 10 3 Notice that one set of values is simply 1000 times bigger than the other. That's to offset the influence of going from grams to kilograms in the denominator of the unit. Notice that the change from Celsius to Kelvin does not affect the value. That is because the specific heat is measured on the basis of one degree. In both scales (Celsius and Kelvin) the jump from one degree to the next are the same "distance." Sometimes a student will think that 273 must be involved somewhere. Not in this case. Specific heat values can be looked up in reference books. Typically, in the classroom, you will not be asked to memorize any specific heat values. However, you may be asked to memorize the values for the three phases of water. As you go about the Internet, you will find different values cited for specific heats of a given substance. For example, I have seen 4.186 and 4.187 used in place of 4.184 for liquid water. None of the values are wrong, it's just that specific heat values literally change from degree to degree. What happens is that an author will settle on one particular value and use it. Often, the one particular value used is what the author used as a student. Hence, 4.184. 133

Chemistry Notebook - Torres
Chemistry Notebook Lopez