Engineering Chemistry CHM 406 Thermochemistry Engineering Chemistry CHM 406

Definitions Thermochemistry: the study of heat absorbed or given off in chemical reactions. Thermodynamics: the study og the relationships between heat and other forms of energy (“work”). Heat: energy that flows into or out of a system because of temperature differences between system and surroundings. Positive if heat is absorbed Negative if heat is evolved.

Forms of Energy Heat (kinetic energy of molecular motions, such as vibrations & rotations). Mechanical energy Electrical energy Work Light, sound, etc. Kinetic energy Potential energy Chemical energy (a kind of potential energy – sometimes called ”free energy”)

Law of Conservation of Energy Energy can neither be created nor destroyed. Matter is a form of energy. The total amount of energy in the universe is constant.

Enthalpy The amount of heat given off or absorbed in a chemical reaction or physical process depends on The conditions (temperature (T), pressure (P), etc) The amount of substances reacting and their physical state. Heat given off or absorbed at constant pressure is known as ENTHALPY (DH).

Enthalpy (contd) Although every reactant and product has an enthalpy (H) associated with it, we cannot determine H, since we don’t know where to start. We can only measure the difference in H values. E.g., A + B → C + D DH = (HC + HD) – (HA + HB)

Standard enthalpies Enthalpies measured under standard conditions are called standard enthalpies (DHo) Standard conditions usually are T = 25oC = 298 K P = 1 atmosphere = 1.01 X 105 Pa. DHo values for a large number of reactions have been tabulated.

Thermochemical equations Balanced chemical equations in which the coefficients refer to the number of moles of reactants / products; accompanied by a DH value for those numbers of moles indicated. Physical state of each reactant / product (s, l, g, aq, etc) should be indicated, DH may depend on this. Fractional coefficients are permissible! E.g., 2 H2 (g) + O2 (g) → 2 H2O (g) : DH° = - 483.7 kJ 2 H2 (g) + O2 (g) → 2 H2O (l) : DH° = - 571.7 kJ

Manipulating thermochemical equations You can multiply or divide the entire equation (i.e., each coefficient) by any number; DH must be treated the same way. You can reverse the direction of the equation; the sign of DH changes. E.g. 2 H2 (g) + O2 (g) → 2 H2O (g) : DH° = - 483.7 kJ H2 (g) + ½ O2 (g) → H2O (g) : DH° = - 241.9 kJ 2 H2O (g) → 2 H2 (g) + O2 (g) : DH° = + 483.7 kJ

Hess’s Law of Heat Summation For a chemical equation which is the sum of two or more steps, the DH for the overall equation is the sum of the DH’s of the individual steps. Can be used to calculate DH for a process where it cannot be measured directly. A → B : DH1 B → C : DH2 Overall: A → C : DH = DH1 + DH2

Example W (s) + C (s) → WC (s) : DH = ? DH is difficult to measure because the reaction takes place at 1400oC. However, the following are given. 2 W (s) + 3 O2 (g) → 2 WO3 (s) : DH = - 1686 kJ C (s) + O2 (g) → CO2 (g) : DH = - 394 kJ 2 WC (s) + 5 O2 (g) → 2 WO3 (s) + 2 CO2 (g) : DH = - 2392 kJ

Solution We can obtain the desired equation by manipulating the other three: ½ eq. 1 + eq. 2 - ½ eq. 3 ½ eq. 1 : W + 3/2 O2 → WO3 : DH = - 843 kJ eq. 2 : C + O2 → CO2 : DH = - 394 kJ – ½ eq. 3 : WO3 + CO2 → WC + 5/2 O2 : DH = + 1196 kJ Overall: W + C → WC DH = – 843 – 394 + 1196 = - 41 kJ

Standard enthalpy of formation This is the enthalpy for the formation of one mole of a compound in its standard state, from its elements in their normal (reference) forms in their standard states. Denoted DHfo E.g., C (graphite) + 2 Cl2 (g) → CCl4 (l) DHfo = –135.4 kJ mol-1 Note: C(s) can be either graphite or diamond. These are called allotropes. Graphite is the reference state.

Standard enthalpies of formation (contd) DHfo values have been tabulated and can be used to calculate other DHo values, using Hess’s Law. By definition, DHfo values of elements in their standard (reference) states are zero. More examples: ½ N2 (g) + 3/2 H2 (g) → NH3 (g) (ammonia) DHfo = –45.9 kJ mol-1 2 C (graphite) + 3 H2 (g) + ½ O2 (g) → C2H6O (l) (ethanol) DHfo = –277.7 kJ mol-1

Relationship between heat and temperature If heat is supplied to an object (system) its temperature increases. For a given amount of heat, the increase in temperature depends on the “heat capacity” of the object. This is a property of the material(s) of which the object is made. q = C DT change in temperature (K) Heat (J) heat capacity (J K-1)

Heat capacity Heat capacity depends on The material; water has a high heat capacity, while metals are lower. The amount of material It can be expressed per gram of substance (specific heat, s) q = m s DT (C = m s) mass (g) specific heat (J g-1 K-1)

Fuels Thermochemistry can be used to determine how much energy can be obtained by the complete combustion with O2 of different fuels. Coal (mostly C): 30.6 kJ g-1 Natural gas (CH4): 50.1 kJ g-1 Petrol (C8H18): 44.4 kJ g-1 A similar analysis can be done for foods (proteins, carbohydrates, fats).