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Enthalpy Most chemical and physical changes occur under essentially constant pressure (reactors open to the Earth’s atmosphere) very small amounts of work.

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Presentation on theme: "Enthalpy Most chemical and physical changes occur under essentially constant pressure (reactors open to the Earth’s atmosphere) very small amounts of work."— Presentation transcript:

1 Enthalpy Most chemical and physical changes occur under essentially constant pressure (reactors open to the Earth’s atmosphere) very small amounts of work are performed as system expands or contracts the change in internal energy occurs primarily, or exclusively, as heat that is gained or lost.

2 Enthalpy If a process occurs at constant pressure and the only work done is PV work, the heat flow is described by the enthalpy of the system. Enthalpy (H): a state function defined by the equation: H = E + PV (Question: Are P and V state functions?)

3 Enthalpy Although the enthalpy of a system cannot be measured, the change in enthalpy (D H) can. heat gained or lost by a system when a process occurs at constant pressure D H = Hfinal - Hinitial = qP where qP = heat gained/lost at constant pressure

4 Enthalpy Two common types of work done by chemical systems:
electrical work redox reactions incorporated into galvanic cells such as batteries mechanical work (P-V work) work done by expanding gases pressure ~ constant volume of system increases

5 Work done on surroundings
Enthalpy P-V Work: expanding gases in a cylinder of car engine P-V work done on piston eventually turns car wheels ignition Work done on surroundings

6 where D V = change in volume
Enthalpy The amount of P-V work done at a constant pressure can be found: w = - P D V where D V = change in volume P = constant pressure The negative sign indicates that work is being done by the system.

7 (at constant pressure)
Enthalpy For a process occurring at constant pressure in which the only work done is PV work, D E = qP + w D H = qP w = -P DV D E = D H - P DV (at constant pressure)

8 Enthalpy Example: If the volume of a cylinder increases from 2.00 L to 3.50 L at a constant pressure of 1.50 atm while it absorbs kJ, what is the change in internal energy of the system?

9 Enthalpy

10 Enthalpies of Reaction
The change in enthalpy always compares the final and initial enthalpies of the system: D H = Hfinal - Hinitial For a chemical reaction: Hfinal = H products Hinitial = H reactants The enthalpy change for a chemical reaction is: D H = Hproducts - Hreactants

11 Enthalpies of Reaction
The enthalpy change that accompanies a chemical reaction is called the enthalpy of reaction D Hrxn Also called heat of reaction

12 D Hrxn = Hproducts - Hreactants
Enthalpy For a chemical reaction, D Hrxn = Hproducts - Hreactants D H < 0 (negative) heat is lost by system exothermic CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)

13 Enthalpy For a chemical reaction, DHrxn = Hproducts - Hreactants
D H > 0 (positive) heat gained/absorbed by the system endothermic CO2 (g) + 2 H2O (l) CH4 (g) + 2 O2 (g)

14 Enthalpies of Reaction
If D Hrxn = positive endothermic heat must be added If D Hrxn = negative exothermic heat is given off H R time R H P time

15 Enthalpies of Reaction
DHrxn is associated with a specific chemical reaction. extensive property Depends on the amount of material Thermochemical equations are balanced chemical equations that show the associated enthalpy change balanced equation enthalpy change (DHrxn)

16 Enthalpies of Reaction
An example of a thermochemical equation: CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = kJ The coefficients in the balanced equation show the # moles of reactants and products that produced the associated DH. If the number of moles of reactant used or product formed changes, then the DH will change as well.

17 Enthalpies of Reaction
For the following reaction: CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = kJ -890. kJ kJ 1 mol CH4 2 mol O2 1 mol CO2 2 mol H2O

18 Enthalpies of Reaction
Guidelines for Using Thermochemical Equations: Enthalpy is an extensive property The magnitude of DH is directly proportional to the amount of reactant consumed or product produced

19 Enthalpies of Reaction
The thermochemical equation for burning 1 mole of CH4 (g): CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = -890.kJ When 1 mole of CH4 is burned, kJ of heat are released. When 2 moles of CH4 are burned, kJ of heat are released.

20 Enthalpies of Reaction
Example: How much heat is gained or lost when 10.0 g of butane, C4H10 (l) are burned at constant pressure? 2 C4H10 (l) + 13 O2 (g) CO2 (g) + 10 H2O (g) DH = kJ

21 Enthalpies of Reaction

22 Enthalpies of Reaction
Example: How much heat is gained or lost when 10.0 g of water vapor are formed at constant pressure in the following reaction? 2 C4H10 (l) + 13 O2 (g) CO2 (g) + 10 H2O (g) DH = kJ

23 Enthalpies of Reaction

24 Enthalpies of Reaction
Guidelines for Using Thermochemical Equations (cont). The enthalpy change for a reaction is equal in magnitude but opposite in sign to the DH for the reverse reaction. 2 H2O2 (l) H2O (l) + O2(g) DH = -196 kJ 2 H2O (l) + O2(g) H2O2 (l) DH = +196 kJ

25 Enthalpies of Reaction
Guidelines for Using Thermochemical Equations (cont): The enthalpy change for a reaction depends on the physical state of the reactants and products. CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = kJ CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) DH = -802 kJ

26 Enthalpies of Reaction
Why does the DHrxn depend on the physical state of the reactants and products? Energy is either absorbed or released when chemicals change from one physical state to another. H2O (l) H2O (g) DH = + 44 kJ

27 Calorimetry The enthalpy change associated with a chemical reaction or process can be determined experimentally. measure the heat gained or lost during a reaction (or process) at constant P Measure the change in temperature

28 Calorimetry Calorimetry:
the experimental measurement of heat gained or lost during a chemical reaction or process Calorimeter an instrument used to measure the heat gained or lost during a chemical reaction or process.

29 Calorimetry Calorimetry is used to experimentally determine:
Heat capacity or specific heat DHrxn Enthaply change for a reaction DHsoln Enthalpy change when a substance is dissolved in a solvent (dissolution) DHcombustion Enthalpy change when a substance is burned in the presence of oxygen

30 Calorimetry If you leave your keys and your chemistry book sitting in the sun on a hot summer day, which one is hotter? Why is there a difference in temperature between the two objects?

31 Calorimetry The temperature increase observed when an object absorbs a certain quantity of energy is determined by its heat capacity (C). Amount of heat required to raise the temperature of an object 1oC (or 1 K) As heat capacity increases, more heat must be added to produce a specific temperature increase.

32 Calorimetry For pure substances, heat capacity is usually given for a specified amount of substance: Molar heat capacity: amount of heat required to raise the temperature of 1 mole of a substance by 1oC Specific heat: amount of heat required to raise the temperature of 1 g of a substance by 1oC

33 Calorimetry Specific Heat = quantity of heat transferred
mass x temp change = q mass x DT Molar Heat = quantity of heat transferred Capacity moles x temp change mol x DT

34 Calorimetry Example: If 418 J is required to increase the temperature of 50.0 g of water from 24.0oC to 26.0oC, what is the specific heat of water?

35 Calorimetry Common units for specific heat are: J cal g.K g.oC

36 Calorimetry Example: What is the molar heat capacity of aluminum if it takes 9.00 J to raise the temperature of 5.00 g of aluminum from K to K?

37 Calorimetry Common units for molar heat capacity are: J cal
mol.K mol.oC

38 Calorimetry Example: If the specific heat of Al (s) is 0.90 J/g.K, how much heat is required to raise the temperature of 10.0 kg of Al from 25.0oC to 30.0oC?

39 Calorimetry

40 Calorimetry Example: If the specific heat of Fe(s) is 0.45 J/g.K, what change in temperature would be observed when 1.0 kJ of heat is added to 45 g of Fe(s)?

41 Calorimetry

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