Thermochemistry

The Flow of Energy - Heat and Work

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Chapter 17: “Thermochemistry” Read: pg. 504-534 Define Vocabulary: listed on pg. 505(10), 511(6), 520(5), 527(2)    Do:  1.  Section Assessments: pg. 510(1-11; includes Practice             Problems 17.1), pg.513-517(12-20; includes Practice             Problems 17.2, 17.3), pg. 521-526 (21-31; includes             Practice Problems 17.4, 17.5, 17.6), pg. 531-532(32-37;             includes Practice Problems 17.7) Standards: A-1, A-2, B-3, B-5, E-2, F-6 Assigned:________________      Due:_________________

2	Chapter 17:  “Thermochemistry (cont.)” Read:  pg. 504-534 Do:  1.  Chapter 17 worksheet pages         2.  Chapter 17 Standardized Test Prep: pg. 539(1-14)         3.  Lab (check schedule) CHAPTER 17 TEST Standards: A-1, A-2, B-3, B-5, E-2, F-6 Assigned:________________      Due:_________________

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (?S > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (?H < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+?H).

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen.

Heat Capacity and Specific Heat

Heat capacity or thermal capacity is a measurable physical quantity equal to the ratio of the heat added to (or removed from) an object to the resulting temperature change.[1] The SI unit of heat capacity is joule per kelvin \mathrm{\tfrac{J}{K}} and the dimensional form is L2MT-2T-1. Specific heat is the amount of heat needed to raise the temperature of a certain mass by 1 degree Celsius.

Heat capacity is an extensive property of matter, meaning it is proportional to the size of the system. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass, or volume, so that the quantity is independent of the size or extent of the sample. The molar heat capacity is the heat capacity per unit amount (SI unit: mole) of a pure substance and the specific heat capacity, often simply called specific heat, is the heat capacity per unit mass of a material. Occasionally, in engineering contexts, the volumetric heat capacity is used.

Temperature reflects the average randomized kinetic energy of constituent particles of matter (e.g. atoms or molecules) relative to the centre of mass of the system, while heat is the transfer of energy across a system boundary into the body other than by work or matter transfer. Translation, rotation, and vibration of atoms represent the degrees of freedom of motion which classically contribute to the heat capacity of gases, while only vibrations are needed to describe the heat capacities of most solids [2] , as shown by the Dulong–Petit law. Other, more exotic contributions can come from magnetic[3] and electronic [4] degrees of freedom in solids, but these rarely make substantial contributions.

For quantum mechanical reasons, at any given temperature, some of these degrees of freedom may be unavailable, or only partially available, to store thermal energy. In such cases, the specific heat capacity is a fraction of the maximum. As the temperature approaches absolute zero, the specific heat capacity of a system approaches zero, due to loss of available degrees of freedom. Quantum theory can be used to quantitatively predict the specific heat capacity of simple systems.

Enthalpy Changes

The purpose of calorimetry is to use an instrument known as a calorimeter to determine the enthalpy of a substance undergoing chemical change. In a calorimeter known as a bomb calorimeter, it is the enthalpy of combustion that is measured. This is how the caloric content of foods is determined. In both cases, since the heat absorbed or released is proportional to the amount of reactant used, molar enthalpy = DH/n is a more meaningful and characteristic quantity.

In a bomb calorimeter, the actual chamber holding the sample is known as a “bomb”. After opening its lid, we place a weighed sample in a cup at the bottom of the bomb. It is sealed, and through a valve, O2 is delivered, saturating the bomb to prepare it for ignition. The bomb is then secured within a calorimeter bucket that is filled with water (the water is the environment which will absorb the heat of combustion). A stirrer keeps the temperature of the water evenly distributed. A thermometer allows us to measure the initial temperature; the ignition wire connected to a high voltage source initiates the explosion; heat is released, and we measure the maximum temperature attained.

Q = mc DT

m = mass of the water in the calorimeter in grams (because of c ‘s units; see below), or the mass of whatever substance is acting as the environment. In reality we should assume that the material part of the calorimeter also absorbs heat. But in this course, we usually ignore that part.

c = specific heat of water or whatever is acting as the environment.

Note c is usually expressed in    , so that Q is in Joules

DT = Tf – Ti.

To get DH, remember:

DH = - Q.

We convert J to kJ by /1000, and then

Molar enthalpy = DH/n.            n = number of moles of reactant. So we convert the carefully measured mass in to moles by dividing by molar mass.

In molar heat of neutralization problems, n = CV, where

C = concentration in “M” = moles/L.

V = volume in litres.

Heating and Cooling Curve

How is it that steamboats contain so much power?

During the time of Mark Twain (real name Samuel Langhorne Clemens, 1835-1910), the steamboat was a major means of transportation on the rivers and lakes of the United States. Twain himself was a steamboat pilot on the Mississippi River for a period of time and took his pen name from the measurement of water depth (twelve feet, which was a safe depth for the boats). The boats got their power from steam – liquid water converted to a gas at high temperatures. The steam would push the pistons of the engine, causing the paddle wheels to turn and propel the boat.

Heating Curves

Imagine that you have a block of ice that is at a temperature of -30°C, well below its melting point. The ice is in a closed container. As heat is steadily added to the ice block, the water molecules will begin to vibrate faster and faster as they absorb kinetic energy. Eventually, when the ice has warmed to 0°C, the added energy will start to break apart the hydrogen bonding that keeps the water molecules in place when it is in the solid form. As the ice melts, its temperature does not rise. All of the energy that is being put into the ice goes into the melting process and not into any increase in temperature. During the melting process, the two states – solid and liquid – are in equilibrium with one another. If the system was isolated at that point and no energy was allowed to enter or leave, the ice-water mixture at 0°C would remain. Temperature is always constant during a change of state.

Continued heating of the water after the ice has completely melted will now increase the kinetic energy of the liquid molecules and the temperature will rise. Assuming that the atmospheric pressure is standard, the temperature will rise steadily until it reaches 100°C. At this point, the added energy from the heat will cause the liquid to begin to vaporize. As with the previous state change, the temperature will remain at 100°C while the water molecules are going from the liquid to the gas or vapor state. Once all the liquid has completely boiled away, continued heating of the steam (remember the container is closed) will increase its temperature above 100°C.

Heat of Fusion and Equation

It's Saturday night and you are looking forward to a fun and exciting evening with your friends. What wild and crazy events do you have planned? Maybe pizza? A movie? Some root beer floats? Nope, nope and nope.

Well then, what's this 'fun and exciting' evening? You and your friends are about to have a chemistry party. So grab your test tubes, goggles and lab coats and prepare to have a wild and crazy chemistry-filled night.

You'll start the evening by gathering some ice cubes and placing them on the kitchen table. Sure, watching ice melt doesn't sound particularly exciting, but there's actually a lot going on with the molecules inside that ice cube.

Before we talk about the molecules, let's go over some vocabulary.

• Melting or fusion is when a solid turns into a liquid.
• Freezing or solidification is when a liquid turns into a solid.

I'm sure you've heard the words 'melting' and 'freezing' before, but fusion and solidification may be new to you.

Okay, back to your ice cube. Heat energy from the table causes the molecules in the ice cube to start moving faster. They also get farther and farther apart.

In order to change a solid to a liquid, there is a certain amount of energy required. This is called theheat of fusion. Remember, fusion means melting, so you can see where it gets its name. Anyway, the heat of fusion is the amount of energy required to change a substance from a solid to a liquid at its melting point.

It's worth noting, it takes energy for something to melt, but energy is given off when something freezes. Also worth noting, there is a heat of solidification, which is the energy released when a liquid turns into a solid at its freezing point. Remember, solidification means turning a liquid into a solid, so you can see how heat of solidification gets its name. But, for now, the focus of your chemistry party is the heat of fusion and that melting ice cube, so back to that!

The heat of fusion can be measured in joules per gram, or J/g, or calories per gram, or cal/g. To keep things simple, we'll just use cal/g in this lesson.

Both joules and calories are measurements of energy. You may be familiar with Calories (with a capital C), which is also known as a kilocalorie or the calorie you see on a food label. Just so you can relate to what a calorie is, there are 1,000 calories in 1 food calorie (or kilocalorie).

So your ice cube melted and now you have a puddle of water on your kitchen table. You might be thinking your chemistry party is coming to an end, but don't worry! Things are about to get even more exciting. There is a formula you can use that involves the heat of fusion!

And here it is:

the heat of fusion = heat energy / mass

Where:

• heat of fusion is H subscript f
• mass is m
• and heat energy is q

Since it is a chemistry party, I think we should try out this formula! Let's try to figure out how much heat energy was required to turn that ice cube into a liquid puddle at its melting point! In other words, we will use the formula to solve for q.

1. We need to determine the mass of the ice cube in grams. Don't worry if you don't have a scale available. I'll help you out. Your ice cube's mass is 4.0 grams, so m = 4.0 grams.
2. Figure out the heat of fusion for water. There are tables available on the Internet and in some chemistry textbooks. Each substance has its own heat of fusion, and water's heat of fusion is 79.7 cal/g.
3. Plug these into the equation and solve for q.

Multiply by 4.0 g on both sides so you can get q by itself.

Your answer is 318.8 calories. So it takes 318.8 calories to turn a 4.0 g ice cube into a liquid, which is less than 1 food calorie! Remember, there are 1,000 calories with a little 'c' in 1 of the Calories with a big 'C' that we're familiar with from food labels.

Heat of Vaporization and Equation

Just when you thought your ice melting party couldn't get any more exciting, let's boil some water! But let's get a couple of new vocabulary words out of the way first.

• Boiling or vaporization is when a liquid is changed into a gas.
• Condensation is when a gas is changed into a liquid.

Okay, so let's put some water in a pot and watch it boil! As the water gets hotter, the molecules start bouncing around like crazy and keep getting farther and farther apart.

The heat of vaporization is the amount of energy required to turn a liquid into a gas at its boiling point. Just like with melting, it takes energy to change a liquid into a gas, but energy is lost when a gas is changed into a liquid. There is also a heat of condensation, which means the amount of energy that is released when a gas turns into a liquid at its condensation point. But our focus here is the heat of vaporization.