Exothermic Example: Respiration

Transcription

1 Chemistry 30 notes Enthalpy Changes The enthalpy (H) of a chemical system is the total of all forms of potential energy (E p ) and kinetic energy (E k ) within a system. The enthalpy change (DH units: kj) is the change in energy resulting from a change in the bonding of the atoms within a chemical system. This can result from either: o A chemical reaction (intramolecular bonding: ionic or covalent) o A phase change (intermolecular forces: london force, dipole-dipole, hydrogen bonding) The molar enthalpy (DH units: kj/mol) is the amount of change in the enthalpy of a system when one mole of a substance undergoes a bonding change. Enthalpy cannot be measured directly. It is measured indirectly by measuring the heat (Q) transfer between the surroundings and the chemical system. If there is a net release of energy from the chemical system, the surroundings will heat up. This is an exothermic reaction. If the chemical system has a net gain of energy, the surroundings will cool down as energy is absorbed from the surroundings. This is an endothermic reaction. Methods for communicating enthalpy change. 1. Potential energy diagrams Endothermic example: Photosynthesis 6 CO 2 (g) + 6 H 2 O(l) kjò C 6 H 12 O 6 (s) + 6 O 2 (g) Exothermic Example: Respiration 6 C 6 H 12 O 11 (s) + 6 O 2 (g) Ò 6 CO 2 (g) + 6 H 2 O(l) kj Chemistry 30 Lesson 1-02 Page 1

2 2. A term in a balanced equation. a. Exothermic reactions show energy as a product. Do not use a negative sign. It is understood to be a negative value when the energy term is a product. Example: Respiration C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO H 2 O (g) kj b. Endothermic reactions require energy input and thus the energy is shown as a reactant. Example: Photosynthesis 6 CO H 2 O (g) kj C 6 H 12 O 6 (s) + 6 O 2 (g) 3. Enthalpy statement. There are two methods to do this. a. Total enthalpy of the process. 2 H 2 (g) + O 2 (g) 2 H 2 O (g) DH= kj b. Molar enthalpy of the process. Often a subscript on DH is used to identify the chemical being referenced as the value depends on the moles present in the balanced equation (it could also be indicated by the chemical name after the kj/mol units) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) D r H H2O = kj/mol or D r H= kj/mol H 2 O (g) The enthalpy change of a chemical reaction is directly proportional to the amount of substances reacting. The enthalpy change for a chemical reaction is equal to the amount (moles) of a specified chemical multiplied by the molar enthalpy change for that chemical. Formula to determine enthalpy change for a given mass of chemical: o DH = nd r H Where DH = enthalpy change (kj) n = amount of substance (moles), from either: - the balancing number from the equation. - given in problem as moles of chemical statement. - = or = D r H = molar enthalpy of reaction for specified chemical (kj/mol) Subscripts can be used to denote the reaction type, e.g. r for general reaction, c for combustion, f for formation. Notes: o the symbol D r H can be used to denote the enthalpy of reaction or molar enthalpy of reaction. You must pay attention to the units to understand which it is in each problem. o The standard enthalpy of reaction D r H o is the enthalpy change of a chemical reaction that occurs at SATP (25.0 o C and 100 kpa). The superscript o indicates SATP conditions. o Note: the term heat also means enthalpy Chemistry 30 Lesson 1-02 Page 2

3 If you need a molar enthalpy value to solve a problem, it will either: o Be given to you. o Be calculated using = o Be found on p. 4 and 5 of your data book, but only if it is a formation reaction. Example: H 2 (g) + ö O 2 (g) H 2 O (g) D f H= kj/mol Note: a decomposition reaction is just the reverse of a formation; just change the enthalpy change sign. Example: H 2 O (g) H 2 (g) + ö O 2 (g) D d H= kj/mol The data book values cannot be used for any other reaction type. Example 1 Determine the enthalpy change for the formation of 200 g of dinitrogen tetraoxide. Example 2 Consider the following thermochemical reaction equation: Fe 2 O 3 (s) + 2 Al (s) 2 Fe (l) + Al 2 O 3 (s) DH= MJ Determine the enthalpy change if 3.75 mol of Al (s) is used. Chemistry 30 Lesson 1-02 Page 3

4 Example 3 Given: 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O (g) DH= kj a. Determine the molar enthalpy of combustion of butane. b. Calculate the total energy released if 10.5 g of butane was burned. c. Determine the molar enthalpy of CO 2 (g) production. Chemistry 30 Lesson 1-02 Page 4

5 Chemistry 30 formative problems Communicating Enthalpy Change. 1. Iron(II) sulphide ore is roasted according to the following chemical equation: 4 FeS (s) + 7 O 2 (g) 2 Fe 2 O 3 (s) + 4 SO 2 (g) DH = 2456 kj (a) Rewrite this chemical equation including the energy as a term in the balanced equation. (b) What is the molar enthalpy for iron(ii) sulphide in this reaction? (c) What is the molar enthalpy for iron(iii) oxide in this reaction? 2. Boron reacts with hydrogen to form diboron hexahydride (diborane) gas. The molar enthalpy of reaction for boron is kj/mol. Write the balanced chemical equation using whole number coefficients and (a) include the energy change as a D r H statement. (b) include the energy change as a term in the balanced equation. 3. The molar enthalpy of combustion for octane, C 8 H 18 (l), is reported to be 1.3 MJ/mol. Write the balanced chemical equation using whole number coefficients and (a) include the energy change as a D r H statement (b) include the energy change as a term in the balanced equation. Chemistry 30 Lesson 1-02 Page 5

6 4. Draw potential energy diagrams to communicate the following chemical reactions. Assume SATP conditions. Enthalpies of formation (and the reverse, decomposition, by changing the sign) are found in your data book on pages 4 & 5) (a) Formation of chromium(iii) oxide (b) Simple decomposition of silver iodide (c) Formation of silicon dioxide Chemistry 30 Lesson 1-02 Page 6