Unit 5 Energetics

Enthalpy [159] Many forms of energy are absorbed or released during chemical reactions Most energy involved in a reaction is heat Heat energy is known as enthalpy Symbol is H

System vs. surroundings [160]

Exothermic and Endothermic Reactions [160-161] Type of reaction Heat energy change Temperature change Relative enthalpies Sign of H exothermic Heat evolved or released Becomes hotter Hproducts <Hreactants negative endothermic Heat absorbed Becomes colder Hproducts >Hreactants positive Page 161 exercises 1 and 2

Exothermic energy diagram [160] Common exothermic reactions are combustion and acid base neutralization

Endothermic energy diagram [160]

http://www.youtube.com/watch?feature=endscreen&NR=1&v=VbIaK6PLrRM

Enthalpy conditions [161] Standard enthalpy changes are measured under defined conditions (standard state) 25°C or 298K 101.3 kPa or 1 atm Solution concentrations 1 mol dm-3 or 1 M Elements and compounds are in their normal physical states at these conditions

Calculation of Enthalpy Changes - stoichiometry 2 Mg (s) + O2 (g)  2 MgO (s) H = -1200 kJ What is H for the reverse reaction? How much heat is released when 4 mole of MgO is formed? How much heat is released when 10 g of O2 is consumed? How much heat is released when 25 g of MgO is formed?

Calculation of Enthalpy Changes – calorimetry [161-168] Temperature  Heat When temperature changes, heat must be absorbed or released Calorimetry is used to measure heat H or H or q = mCpT m = mass Cp = specific heat of substance (J/gK or J/g°C), amount of heat needed to raise the temperature of 1 g by 1°C T = change in temperature

Calculation of Enthalpy Changes – calorimetry [161-168] Worked example page 162 Exercises 3 and 4 page 162 Exercise 5 page 164 Exercise 6 page 165

Calculation of Enthalpy Changes – calorimetry [161-168] Enthalpy of solution example \When a 9.55 g sample of NaOH dissolves in 100.0 g of water, the temperature rises from 23.6 to 47.4C. Calculate the H (in kJ/mole NaOH) for the solution process.

Calculation of Enthalpy Changes – calorimetry [161-168] Specific heat of a metal example We are going to determine the specific heat of a metal using experimental data. In this experiment, we used a "coffee cup" calorimeter and gathered the following data: Mass of empty cup 2.31 g Mass of cup + water 180.89 g Mass of cup + water + metal 780.89 g Initial temperature of water 17.0 °C Initial temperature of metal 52.0 °C Final temperature of system 27.0 °C

Calculation of Enthalpy Changes – Hess’ Law [168-170] Hess's Law states the change in enthalpy (H), in a reaction is the same regardless whether the reaction occurs in a single step or in several steps. 

Calculation of Enthalpy Changes – Hess’ Law [168-170] Worked example page 170 Exercises 8 and 9 page 170

Calculation of Enthalpy Changes – Bond Enthalpy [170-174] Bond enthalpy is defined as the energy needed to break one mole of bonds in gaseous molecules under standard conditions Bond enthalpies measure strength of covalent bond

Calculation of Enthalpy Changes – Bond Enthalpy [170-174] Breaking bonds require energy – endothermic Forming bonds release energy – exothermic Worked example page 172 Exercises 10-12 page 172

Calculation of Enthalpy Changes – Bond Enthalpy [170-174] To calculate bond enthalpy Hreaction = Hbonds broken - Hbonds formed Worked example page 173 Exercises 13-15 page 174 If the bonds being broken are weaker that those being made, the reaction will have a _______ H and be ____________. (+ or -) (exo or endothermic)