Chem 121 G. Thermochemistry

Transcription

1 Chem 121 G. Thermochemistry Energy 1 st law Enthalpy, enthalpy of combustion, fuels Calorimetry Enthalpy of reaction Hess's Law, calculations Energy Energy: capacity to do work or transfer heat Matter possesses energy in the form of: 1

2 Units of energy Joules e.g. a 4 kg mass moving at 1 m/s has a kinetic energy E K = ½ m v 2 Calories Nutritional calorie (You may also come across the Kilowatt-hour, a common non-si unit of energy.) System and surroundings Open system: Closed system: Isolated system: 2

3 Transferring energy: work and heat First Law of Thermodynamics Energy measure System 10 Surroundings 2 transfer System 8 Surroundings for a chemical reaction: C(s) + O 2 (g) CO 2 (g) 3

4 Reverse reaction: CO 2 (g) C(s) + O 2 (g) Change in internal energy depends on heat and work q: w: A reminder: 4

5 e.g. Altitude change depends only on initial and final altitude work done when a force moves an object pressure = force / area, rearrange to get substitute F into the eqn. for work If ΔV is positive, system does work on the surroundings If external pressure is zero (expansion against a vacuum) 5

6 Enthalpy Chemical changes typically take place at constant pressure, heat flow easy to measure Thermodynamic quantity for the heat change at constant pressure, enthalpy change ΔH so, when P is constant: Enthalpy is a state function (this follows because U, P and V are all state functions) Recall that: ΔE = q + w Enthalpy of reaction ΔH = H final H initial so in chemical reactions ΔH is called the enthalpy of reaction, Thermochemical equation is the balanced chemical reaction and the value of ΔH 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) ΔH = 196 kj 2H 2 O 2 (l) ΔH = 196 kj ΔH = +196 kj 2H 2 O 2 (l) 2H 2 O(g) + O 2 (g) ΔH = 108 kj 2H 2 O(l) + O 2 (g) 6

7 Calorimetry Heat capacity and specific heat Size of the temperature change per unit of heat depends on the nature of the substance Molar heat capacity, C m Specific heat, C s Specific heat can be determined experimentally amount of heat transferred specific heat (grams of substance) x (change in temperature) An example: determining the specific heat of lead: (a) Heat 150g lead to 100 C (b) Add the hot lead to 50g water at 22 C (c) Measure the final temperature 7

8 Calorimetry is the measurement of heat flow Constant-pressure calorimetry Heat flows between reaction and solution Constant-volume calorimetry Heat from chemical reaction absorbed by water and all components of calorimeter calorimeter plus water is an object - Hess s Law We can use tabulated ΔH values to calculate the enthalpy of reactions Example: N 2 (g) + 3H 2 (g) 2 NH 3 (g) H rxn =? N 2 (g) + 3H 2 (g) N 2 H 4 (g) + H 2 (g) H = 95.4 kj N 2 H 4 (g) + H 2 (g) 2 NH 3 (g) ΔH = kj 8

9 Hess s Law : The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps Procedure: Arrange reactions so (final) reactants appear on the left and products appear on the right Any reaction that is reversed must have the sign of its ΔH changed All intermediates must occur on both the right and the left so they cancel e.g. what is ΔH for ½N 2 (g) + O 2 (g) NO 2 (g) given: N 2 (g) + O 2 (g) 2NO( g) ΔH = kj NO 2 (g) NO(g) + ½ O 2 (g) ΔH = kj - A reaction can be multiplied by a coefficient, but ΔH for that reaction must be multiplied by the same coefficient. Hess s Law We can use tabulated ΔH values to calculate the enthalpy of reactions ΔH depends on amounts and initial and final states of reactants and products ΔH is a state function, so does not depend upon how we get from reactants to products Example: N 2 (g) + 3H 2 (g) 2 NH 3 (g) H rxn =? N 2 (g) + 2H 2 (g) N 2 H 4 (g) NH 3 (g) ½ N 2 H 4 (g) + ½ H 2 (g) H = 95.4 kj ΔH = kj 9

10 Enthalpies of formation Hess s law can be used with tabulated ΔH values for both chemical and physical changes Physical changes: vapourization ΔH vap fusion ΔH fus Reactions: combustion reactions, ΔH comb formation reactions: ΔH f Magnitude of ΔH depends on T, P and physical state of reactants and products Thermodynamic Standard state (provides a reference point, like sea level) Standard enthalpy change Standard enthalpy of formation e.g. for ethanol: C 2 H 5 OH(l) For pure elements in their most stable form at standard conditions 10

11 Using enthalpies of formation to calculate enthalpies of reaction steps: Foods and Fuels Fuel value is the energy released when one gram of substance is combusted Foods differ in fuel value and body s ability to store Fuel value: Bio-availability: Fuels Combustion produces CO 2 and H 2 O; Both have large negative ΔH f fossil fuels: coal: solid, mined, abundant Energy use, USA data, 2008 petroleum: liquid, refined natural gas: gas, 85% methane, 10% ethane, 3% propane 11