Thermochemistry Essay Example

State Functions

• Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem.
• However, we do know that the internal energy of a system is independent of the path by which the system achieved that state. - In the system depicted in Figure 5. 9, he water could have reached room temperature from either direction.
• Therefore, internal energy is a state function.
• It depends only on the present state of the system, not on the path by which the system arrived at that state.
• And so, DE depends only on Initial and Flan.

However, q and w are not state functions. Whether the battery is shorted out or is discharged by running the fan, its DE is the same. - But q and w are different in the two cases. Work Usually in an open container the only work done is by a gas pushing on the surroundings (or by the surroundings pushing on the gas).

We can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston: w = -POP Enthalpy If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work, we can account for heat flow during the process by measuring the enthalpy of the system.

Enthalpy is the internal

energy plus the product of pressure and volume: H = E + PVC 2012 Pearson Education, Inc. Thermosetting 14 ; When the system changes at constant pressure, the change in enthalpy, OH, is OH -? + IV) ; This can be written OH = OH + pop Since DE = q + w and w = -POP, we can substitute these into the enthalpy expression: OH = DE + POP OH = (q + w) - w OH = q

So, at constant pressure, the change in enthalpy is the heat gained or lost. Thermosetting 2012 Pearson 15 Endothermic and Exothermic ; A process is endothermic when OH is positive. A process is endothermic when OH is positive. A process is exothermic when OH is negative.

Sample Exercise

Determining the Sign of OH Indicate the sign of the enthalpy change, OH, in these processes carried out under atmospheric pressure and indicate whether each process is endothermic or exothermic:

• An ice cube melts;
• 1 Goff butane (SOCIO) is combusted in sufficient oxygen to give complete combustion to CA and H2O. Analyze Our goal is to determine whether OH is positive or negative for each process.

Because each process occurs at constant pressure, the enthalpy change equals the quantity of heat absorbed or released, OH = QPS. Plan We must predict whether heat is absorbed or released by the system in each process.

Processes in which heat is absorbed are endothermic and have a positive sign for OH; those in which heat is released are exothermic and have a negative sign for OH. Solve In (a) the water that sakes up the ice cube is the system. The ice cube absorbs

heat from the surroundings as it melts, so OH is positive and the process is endothermic.

In (b) the system is the 1 Goff butane and the oxygen required to combusted it. The combustion of butane in oxygen gives off heat, so OH is negative and the process is exothermic.

Molten gold poured into a mold solidifies at atmospheric pressure. With the gold denned as the system, is the solidification an exothermic or endothermic process? Answer: In order to solidify, the gold must cool to below its melting temperature. It cools by transferring heat to its surroundings. The air around the sample would feel hot because heat is transferred to it from the molten gold, meaning the process is exothermic.

You may notice that solidification of a liquid is the reverse of the melting we analyzed in the exercise. As we will see, reversing the direction of a process changes the sign of the heat transferred. Enthalpy of Reaction The change in enthalpy, OH, is the enthalpy of the products minus the enthalpy of the reactants: OH = Hopscotch - Heartbeats Thermosetting 2012 Pearson Education, 17 This quantity, OH, is called the enthalpy of reaction, or the heat of reaction. The Truth about Enthalpy . Enthalpy is an extensive property.

OH for a reaction in the forward direction is equal in size, but opposite in sign, to OH for the reverse reaction. OH for a reaction depends on the state of the products and the state of the reactants. Thermosetting 2012 Pearson Education, Inc.

Sample Exercise

Relating OH to Quantities of Reactants and Products Continued Hydrogen peroxide can decompose to water and oxygen by the reaction 2 H2O(l)

2 H2O(l) + 02(g) OH = -196 k Calculate the quantity of heat released when 5. 00 g of H2O(l) decomposes at constant pressure. Answer: -14. 4 k Bruce E. Burette; Catherine].

Murphy; an d Patrick W ward Calorimeter Since we cannot know the exact enthalpy of the reactants and products, we measure OH through calorimeter, the measurement of heat flow.

Heat Capacity and Specific Heat

The amount of energy required to raise the temperature of a substance by 1 K (1 DC) is its heat capacity. We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g off substance by 1 K (or 1 DC).

Specific heat, then, is Specific heat = heat transferred mass 0 temperature change q Ron TO

Constant Pressure Calorimeter By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter. Thermosetting 2012 Pearson Because the specific heat for water is well known (4. 184 J/g-K), we can measure OH for the reaction with this equation: q = m Des 0 CIT

Measuring OH Using a Coffee-Cup Calorimeter

When a student mixes 50 ml of 1. 0 M HCI and 50 ml of 1.

0 M Noah in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21. DC to 27. 5 CO. Calculate the enthalpy change for the reaction in k/mol HCI, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100

ml, that its density is 1. 0 g/ml, and that its specific heat is 4. 18 J/g-K. Analyze Mixing solutions of HCI and Noah results in an acid-base reaction: HCI(aqua) + Noah(aqua) H2O(l) + NCAA(aqua) We need to calculate the heat produced per mole of HCI, given the temperature increase of the solution, the number of moles of HCI and Noah involved, and the density and specific heat of the solution.

The number of moles of HCI consumed in the reaction must be calculated from the volume and military of this substance, and this amount is then used to determine the heat produced per mol HCI.

Solve Because the total volume of the solution is 100 ml, its mass is The temperature change is Using Equation 5. 23, we have (100 ml)(1. 0 g/ml) = 100 g AT = 27. 5 CO - 21. 0 CO = 6.

5 CO = 6. 5 K corn = -CSS m TO = -(4. 18 K) = - 2. 7 0 103 J = -2. 7 k Chemistry, The Central Science, 12th Edition Theodore L. Brown; H.

Measuring OH Using a Coffee-Cup Calorimeter Continued Because the process occurs at constant pressure, To express the enthalpy change on a molar basis, we use the fact that the number of moles of HCI is given by the product of the volume (ml = 0. 050 L) and concentration.