THERMODYNAMICS NOTES

Thermodynamics is a word which sounds like the concept is going to be hard. Literally "THERMO" means heat and "DYNAMICS" means changing. In this unit we are going to study the changes in heat which accompany chemical reactions. You will also be able to predict how much heat will be released or absorbed by any reaction without going to the lab for experimentation. We know that there are actually TWO "drivers" to every equation (1) enthalpy which is amount of heat trapped in the bonds of the reacting substances and (2) entropy which is the amount of disorder the individual substances display. For example, gases exhibit more entropy (disorder) than solids; 3 moles of gas exhibits more entropy than 1 mole of gas; and CCl4 is more disorderly than He due to its structure. The combination of these two drivers (and temperature) is what makes a particular reaction spontaneous (proceed by itself) or not. IF both drivers AGREE that the reaction should be spontaneous, IT WILL BE SPONTANEOUS AT ALL TEMPERATURES, and you will not be able to stop the reaction without separating the reactants. Do not confuse the words spontaneous and instantaneous. Spontaneous just simply means that it will work by itself, but it does not say anything about how fast the reaction will take place--it may take it 20 years to react, but it wilI!! IF both drivers AGREE that the reaction should NOT be spontaneous, it will NOT work at ANY temperature, no matter how much you heat it, put pressure on it or anything else. IT WILL NOT BE SPONTANEOUS AT ANY TEMPERATURE! IF the two drivers disagree on whether or not a reaction should be spontaneous or not, a third party (Gibb's free energy) is called in to act as the "judge" about what temperatures the reaction will be spontaneous and what temperatures it will not work, BUT it will work and be spontaneous at some temperature! ENTHALPY is the amount of heat which a substance contains trapped within itself (usually in the bonds). WE ARE NOT ABLE TO MEASURE THIS DIRECTLY--WE CANNOT LOOK DOWN INTO A SUBSTANCE AND "SEE" HOW MUCH HEAT IS TRAPPED IN IT. The only way we can determine the enthalpy of a substance is by observing the CHANGE in temperature when that substance reacts with another substance. Scientists over the years have spent hours reacting every element with every other element in a calorimeter and observing whether the temperature inside the calorimeter goes up or down.

(1) If the temperature inside the calorimeter goes down when two elements react together, it means that heat was absorbed from the atmosphere of the calorimeter and it therefore gets colder inside. (2) If heat is released when two elements react with each other, the temperature will go up inside the calorimeter.

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Here is a Thermodynamics Table which you will learn to use.

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For example, when pure carbon is placed in a calorimeter along with pure oxygen gas, and the two are allowed to react to form carbon dioxide, 393.5 kJ of heat are given off for every MOLE of carbon dioxide that forms. This value is recorded as the HEAT OF FORMATION of carbon dioxide in a thermodynamics table which we can then use anytime we have a reaction with carbon dioxide in it. If the heat of formation has a negative sign in front of it, it means that that much heat was GIVEN OFF when one mole of the compound was formed from its elements. If the heat of formation has a positive sign in front of it, it means that much heat had to be added to the calorimeter in order to make the elements synthesize that compound. The more negative the number for the enthalpy of formation (i.e. the farther down the number line in the negative direction) a substance is, the MORE STABLE that substance is at room temperature. This is because it gave off so much energy when it was formed and therefore has little energy left to “react”. The closer to 0 the enthalpy of formation is or the more positive an enthalpy of formation is, the more UNSTABLE a substance is at room temperature because it means that energy was absorbed at its formation and it therefore the compound is “pumped” and will react readily. Thermodynamics tables are invaluable to chemistry teachers who must store hundreds of chemicals. Stable compounds can be stored easily next to each other but unstable compounds must be separated or they will react just sitting next to each other on the shelf! Example 14-1 Look at the thermodynamics table given you. Which is a more stable compound at room temperature--carbon dioxide or carbon monoxide? (b) aluminum oxide or barium carbonate? (c) liquid water or gaseous water? You may have noticed that the pure elements listed on the table all have enthalpy of formation values of "0". THIS DOES NOT MEAN THAT FREE ELEMENTS DO NOT HAVE ANY ENERGY. They have been “assigned” a value of “0” because we cannot measure the amount of heat in an element DIRECTLY, so we have assigned the enthalpy of formation value for EVERY free element in its normal physical state at room temperature a value of "0". It means that we do not know how much heat is stored within a pure element and the only way we can determine it is by reacting an element with another element and observing whether heat was absorbed or released when the compound formed. YOU NEED TO REMEMBER THAT THE HEAT OF FORMATION OF ANY FREE ELEMENT IN ITS NORMAL PHYSICAL STATE AT ROOM TEMPERATURE IS "0 kJ/mole (whether it is listed in the table or not).

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YOU NEED TO KNOW A DEFINITION OF ΔHfo , or enthalpy of formation. It is defined

as the amount of heat absorbed or released when ONE mole of compound is formed FROM ITS CONSTITUENT ELEMENTS. By combining the enthalpy of formation values of different compounds, we can find how much energy is absorbed or released in a chemical reaction. The equation we will use is ΔHrxn = Σ nΔHf

o Products – Σ nΔHfo Reactants

Notice that what we are finding now is CHANGE OF ENTHALPY of the REACTION . If the ΔHrxn is negative, it means that number of kiloJoules of energy is released in the balanced equation, and we say that the reaction is EXOTHERMIC. If the Δ Hrxn is positive, it means that number of kiloJoules of energy must be added to the balanced equation in order to make it react and we say that the reaction is ENDOTHERMIC. Example 14-2 What is the change in enthalpy for the reaction of the combustion of methane? (b) Is this an exothermic or endothermic reaction? How do you know? In Nature things tend to go spontaneously from “high” to “low”. Reactions which are exothermic tend to be spontaneous since the reaction is going from a state of “high” heat content to a state of “low heat. If enthalpy is the only driver we are considering, that driver is saying "YES, the reaction should be spontaneous” However, remember that enthalpy is only ONE of the two drivers of every equation!!!! Example 14-3 What is the change in enthalpy for the reaction of barium oxide reacting with carbon dioxide to form barium carbonate as the only product?

(b) Based on enthalpy alone, would you expect this reaction to be spontaneous?

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Example 14-4 What is the ΔHrxn for the synthesis of ammonia (NH3) from its elements? DO NOT CONFUSE THE TERMS Δ Hf and Δ Hrxn.!!! Δ Hf is usually a book value which we use to calculate Δ Hrxn, but it is also possible to calculate the Δ Hf if the ΔHrxn is known by using the above equation. ENTROPY - is the amount of disorder and it is measured in units of J/mole-K. Notice that there are no negative values of standard entropy. In Nature, things tend to go from a state of orderliness to disorderliness spontaneously and not vice versa--your room NEVER cleans up itself (disorder to order), so we may make the assumption that equations in which disorder is increasing from reactants to products would be spontaneous,ACCORDING TO THAT DRIVER ONLY. Example 14-5 Would you expect the following equations to be spontaneous, based on entropy alone? A(s) + B(s) → C(g) H2 + O2 → H2O(l) 2 A(g) + B(g) → 2 C(g) Sometimes you cannot determine entropy change by looking at a reaction qualitatively, so we use an equation to calculate the change in disorder. It is ΔSrxn = ΣnSo Products – Σ nSo Reactants If the Δ Srxn is a + value, this means that the disorder of the products is greater than the disorder of the reactants, or in other words, the reaction is becoming more disorderly as it proceeds. According to this driver only, you would predict that the reaction is spontaneous at that temperature since things tend to go from orderly to disorderly spontaneously in Nature..

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If the Δ Srxn is a negative value, this means that the disorder of the reactants is greater than the disorder of the products, or, in other words, the reaction is becoming more orderly as it proceeds, and this would not happen spontaneously. (your room NEVER cleans up itself spontaneously!) Example 14-6 What is the change in entropy for the combustion of methane?

(b) According to this driver only, would you expect this reaction to be spontaneous in Nature?

(c) What is the change in enthalpy for the combustion of methane?

(d) According to this driver only, would you expect this reaction to be spontaneous in Nature?

(e) Do the drivers agree or disagree about spontaneity of this reaction? Reactions which are exothermic in the forward direction are endothermic to the same degree (same number, different algebraic sign) if written in the reverse direction. Reactions which are becoming more disorderly in the forward direction are becoming orderly in the same degree (same number, different algebraic sign) if written in the reverse direction. When both drivers agree that the reaction is spontaneous, IT IS SPONTANEOUS AT ALL TEMPERATURES. When both drivers agree that the reaction is not spontaneous, it will NEVER HAPPEN SPONTANEOUSLY AT ANY TEMPERATURE.

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But, when the drivers disagree on spontaneity, Gibb's free energy makes the decision. You may find the change in Gibb's free energy two different ways according to what information you are given, but either way should give you the same answer. The two equations for finding the change in Gibb's free energy in a reaction are ΔGrxn = Σ nΔ Gf Products - ΣnΔGf Reactants and Δ Grxn = ΔHrxn - T Δ Srxn The second equation is called the general thermodynamic equation, and the only thing you must be careful about is

(1) making sure that the temperature goes in in Kelvin and (2) the units match on the ΔHrxn (usually calculated in kJ) and the Δ Srxn (usually calculated in J/K) GIBB'S FREE ENERGY IS THE ONLY THING WHICH WILL DETERMINE SPONTANEITY ABSOLUTELY, NOT THE INDIVIDUAL DRIVERS!! If the Δ Grxn is negative, it means that the reaction is spontaneous at the stated temperature. If the ΔGrxn is positive, it means that the reaction is non-spontaneous at the stated temperature. Example 14-7 Would the reaction 2 H2O(l) → 2 H(g) + O2(g) be spontaneous at room temperature? Prove your answer! When the drivers disagree on spontaneity, the general thermodynamics equation can be used to find the temperature at which the reaction will become spontaneous (if it is not spontaneous at room temperature) or the temperature at which it will become non-spontaneous (if it is already spontaneous at room temperature). The only thing you must remember is that, WHEN SOLVING FOR TEMPERATURE WITH THE GENERAL THERMODYNAMICS EQUATION, YOU MUST FIRST SET THE Δ Grxn = 0. Zero is the point on the number line when positive changes to negative and this is exactly what we are trying to find mathematically—the temperature at which a reaction changes from being non-spontaneous (+) to spontaneous (-) or vice versa. In thermodynamics, the point at which the change in Gibb's free energy = 0 is known as equilibrium and you are finding the equilibrium temperature when the drivers are

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changing roles. If you do not set the Δ Grxn = 0 and instead use a calculated value of Δ Grxn instead, you will get 298 K or 25oC as your temperature. (surprise, surprise). Example 14-8 Given the equation Cdiamond + H2O(g) → H2(g) + CO(g)

(a) Does the enthalpy driver indicate that the reaction should be spontaneous?

(b) Does the entropy driver indicate that the reaction should be spontaneous?

(c) What does Gibb's say about spontaneity at room temperature?

(d)Is there a temperature at which this reaction will happen? If so, what is it?

Here is a summary table of the thermodynamics properties of reactions which you should know + - ΔHrxn

(1) endothermic rxn (2) enthalpy driver indicates it should NOT be spontaneous

(1) exothermic reaction (2) enthalpy driver indicates it should be spontaneous

Δ Srxn

(1) disorder is increasing (2) entropy driver indicates it should be spontaneous

(1) disorder is decreasing or it is becoming more orderly (2) entropy driver indicates it should NOT be spontaneous

Δ Grxn

(1) NOT SPONTANEOUS AT THE STATED TEMPERATURE

(1) SPONTANEOUS AT THE STATED TEMPERATURE

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First Law of Thermodynamics is actually the Law of Conservation of Energy. Energy is neither created nor destroyed in ordinary chemical reactions; the energy of the universe is conserved. Second Law of Thermodynamics says the disorder of the universe is increasing Third Law of Thermodynamics says the entropy of any perfect crystal at O K is O J/mole-K; there is perfect order at O K.

Now we are going to incorporate knowledge of endothermic and exothermic reactions in writing THERMOCHEMICAL equations. A thermochemical equation is nothing more than a balanced chemical equation for a given reaction which also includes the amount of heat absorbed or released on the appropriate side of the equation.

For example, if you are asked to write the thermochemical equation for the combustion of methane gas to carbon dioxide gas and water, first you must write a balanced equation for the reaction. Now you must calculate the ΔHrxn. Since the algebreaic sign of the ΔHrxn indicates that 890 kJ of heat is being given off, this number of kilojoules are added to the PRODUCT side of the equation. DO NOT INCLUDE THE NEGATIVE SIGN WHEN YOU WRITE THE NUMBER OF kJ—IT HAS SERVED ITS PURPOSE IN TELLING YOU WHETHER HEAT IS ABSORBED OR RELEASED.

CH4(g) + 2 O2(g) → CO2(g) + H2O(l) + 890 kJ

If the reaction is endothermic, the heat factor is included with the reactants:

X + Y + 560 kJ → Z

Example 14-9 Write the thermochemical equation for the synthesis of liquid water from its elements.

Example 14-10 Write the thermochemical equation for the decomposition of potassium chlorate (solid) into potassium chloride (solid) and oxygen gas.

REMEMBER THE ΔHrxn is the factor which is included in a thermochemical equation, not the ΔGrxn.

When you are given a thermochemical equation such as A + B → C + 500 kJ, this also tells you that the ΔHrxn = -500 kJ in case you need the ΔHrxn to work another problem associated with this reaction. BE SURE YOU PAY CAREFUL ATTENTION TO THE ALGEBRAIC SIGN YOU ASSIGN TO THE ENTHALPY CHANGE VALUE.

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Thermochemical equations make it possible for you to solve mass-heat problems in which you are given a mass of either a product or a reactant, and you are asked to find the amount of heat absorbed or released in the reaction as it is written. Before you can work a mass-heat problem, YOU MUST KNOW THE VALUE OF THE ΔHrxn AND YOU MUST ALSO UNDERSTAND THAT THIS ENTHALPY VALUE IS DEPENDENT ON THE SET OF BALANCING NUMBERS USED IN THE EQUATION.

Example 14-11 How much heat is released when excess methane burns with 30 grams of oxygen?

Example 14-12 Write the thermochemical equation for the decomposition of potassium chlorate into potassium chloride and oxygen gas.

(b) How many liters of oxygen gas will be produced at STP at the same time as 500 kJ of heat are released? (Be sure to use the mole-ratios of products and reactants to answer this)

(c) How many liters of oxygen gas will be collected at 800 torr and 27oC at the same time as 750 kJ of heat are released in this reaction?

Example 14-13 If it is known that 500 kJ of heat are released in the synthesis of water from its elements, how many grams of water were formed?