CHAPTER 16
Every chemical reaction results in a change in energy. In each chemical reaction their is either an absorbtion or releasing of energy. This becomes evident through the studies of the transfers of energy as heat that accompany chemical reactions and physical changes otherwise known as thermochemistry.

Heat and Temperature


In a chemical or physical change, the energy that is released or absorbed as heat is measured in a calorimeter. An example of a calorimeter is where the known quanitities of reactants are submerged in a specific amount of water in an insulated vessel. Thus, the energy absorbed is equivalent to the energy given off or absorbed. Energy is unable to be measured directly so we use temperature changes due to its capability of being measured directly. Temperature is a measure of the average kinetic energy (measure of movement) of the particles in a sample of matter. When the kinetic energy is increases then the temperature increases making it directly proportional. In thermochemistry we generally use Celsius and Kelvin. The equation between the two is:
K= 273.15+ C
Temperature is based mainly on the energy transfer. A joule (J) is the SI unit of heat and a way to mesaure the amount of energy. Heat is not only a change in temperature it is the energy transferred between one substance to another due to a change in temperature.

Specific Heat


The amount of energy transferred is dependent on three factors:
1) The nature of the material changing temperature
2) Mass of the material changing temperature
3) The size of the temperature change
The amount of heat required to change 1 gram of a substance by 1 degree Celsius is called specific heat.
The table below gives the specific heats of numerous substances:

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The symbol Cp, stands for the specific heat measured in constant pressure conditions. The equation:
q=_mc.jpg
In the equation the q is the heat lost or gained in joules, m is the mass in grams, delta T is the change in temperature in Celsius.

Enthalpy of Reaction


Enthalpy is the amount of heat absorbed or released in a chemical reaction. it is accounts for pressure and volume and is expressed in either Joules or Calories (1 cal= 4.184 J). An enthalpy change (delta H) is the amount of energy absorbed as heat throughout the process at a constant pressure. An enthalpy change can always be expressed by products minus reactants. A thermochemical equation is an equation with the amount of energy releasd or absorbed as heat throughout the reaction.
The enthalpy of reaction or heat of reaction is the amount of energy transferred as heat during a chemical reaction.

In order to figure out the enthalpy of reaction you must:
1) First figure out the enthalpy of formation for each substance.
2) Plug the enthalpy of formation for each substance into the equation with products minus reactants.
3) Use algebra to find the enthalpy of reaction in KJ

For an exothermic reaction the change in H is always negative because it is losing energy and when the reaction is endothermic it is positive.
The course of an exothermic reaction is shows on the left and an endothermic reaction is shown on the right.
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When using thermochemical equations:
1) The coefficients are the number of moles not the molecules.
2) The physical state of the reactant or product must be put into the thermochemical reaction
3) The change in enthalpy is directly proportional to the number of moles of substance takes place in a change
4) The change in enthalpy (delta H) is not changed by a change in temperature

Calculation Enthalpies of Reaction


Hess's Law says that the overall enthalpy change in a reaction is equal to the sum of all the enthalpy changes that take place throughout the entirety of the process.

The process of applying Hess's Law takes place over multiple steps:

1. Write the equation for the reaction if it is not given.
2.. Manipulate the equations so they will add up to the overall equation. Make sure to apply the following rules:
    • If a reaction is reversed the sign of delta H must also be reversed
    • When you multiply the coefficients of the equations you must also multiply the delta H by the same factor in order to get the correct thermochemical equation.
3. Add up the equations and cancel out the common substances in reactants and products.
4. Add up the delta H's is all of the steps to get the heat of the overall reaction.

Enthalpy and Reaction Tendency


An endothermic reaction can spontaneously combust as is made evident by melting.

Entropy and Reaction Tendency


A random system that does not have a regular arrangement of parts. Entropy is a measure of the degree of randomness of particles in a substance. A solid's particles are in certain position and do not move other than their constant vibration. Due to the lack of randomness a solid has a lower entropy than that of a gas or liquid. Entropy change is simply the difference between the entropy of products and reactants. So when the entropy increased it is represented by a positive value of delta S. When a solution is formed their is also an increase in entropy due to the rise in randomness.

Free Energy


The two main directions that drive nature is the pull towards the least enthalpy and towards the largest entropy. Free energy was derived in order to relate the enthalpy and entropy at a constant pressure and specific temperature. Free energy (G) can be defined as the combination of enthalpy and entropy functions. Free- Energy change is the difference between the change in enthalpy and the product of the Kelvin temperature and the entropy change. The free- energy change stays at a constant pressure and temperature and is defined as TdeltaS. The equation below allows you to determine the free energy change. Delta G is the free energy change, delta H is the change in enthalpy, and T delta S is the enthalpy change.
free.jpg
ent.jpg
The table above shows how when the delta H is negative the delta S will be poisitve. This makes the right side of the free energy equation negative.

Practice Problems:

1. Find the enthalpy of reaction of CH4 + 202 --> CO2 + 2H20 and take into consideration that H20 is in gas form.

2. Find the energy released in the combustion of 2 mol of benzene

3. Use Hess's law to find the delta H for N204 --> 2NO2
Given- 1) N2 + O2 --> 2NO2 DeltaH= 169.5 KJ
2) N2 + O2 --> N2O4 DeltaH= 24 kj

4. How much heat is required to change the temperature of 500.2g of H2O from 10 degrees C to 25 degrees C?

5. How much energy is released in the formation of 210.3 g of Pentane?

6. What is the mass of sodium carbonate from 21.32g of sodium oxide and carbon dioxide?



Answers:
1. -802.3 KJ
You first look on your enthalpy of formation chart to figure out the amounts for each compound. You subtract the products minus reactants. Keep in mind that you multiply the enthalpy by the coefficient of each compound.

2. -6338.972 KJ
You first have to write the balanced equation and find the enthalpy of formation for each compound on the chart and subtract the products minus reactants in order to find the energy released. Your finished equation should look like this; [(-393.521 x 2) + (-241.82 x 6)] - [(82.9 x 2)] = -6338.972.

3. 145.5 KJ
You have to reverse the equation so the products become the reactants of the second given equation. Remember to also make the delta H negative. YOu then add up all of the given delta H's in order to get your final answer of 145.5 KJ.

4. 31 KJ
You simply plug the numbers into the equation as follows: q= (500.2 g)(4.184 J/G degrees C)(15 C)= 31392.552 T= 31 KJ

5. -426.9 KJ/mol
You take 210.3 grams of Pentane and divide it by the molar mass of Pentane then divide it by -146.440 (the enthalpy of formation) to get -426.8 KJ.

6. 37.48g
You first have to write out the balanced equation. You start by taking 21.32 and dividing it by 61.997 g and mulitplying that by 109 in order to get 37.48 grams.

Works Cited
-Davis, Raymond E., Regina Frey, Mickey Sarquis, and Jerry L. Sarquis. Modern Chemistry. Austin, Texas: Holt, Reneheart and Winston,2006
-Boder Reasearch. "Gibbs Free Energy." Boder Research Web. May 22, 2008.
-Blauch, David. "Calorimetry". 6/3/2009 <http://www.chm.davidson.edu/ChemistryApplets/Calorimetry/>.