Enthalpy

Defining the term  Enthalpy Is defined as the heat content contained by a substance. The word is derived from a Greek word meaning “heat inside”. In Chemistry, it is noted by H The problem is that you can’t directly measure H, only changes in H. So we usually talk about Enthalpy as ΔH.

The units of Enthalpy H(g) + H(g)  H 2 (g)H 2 (g)  H(g) + H(g) What units are used to measure Enthalpies?

Exothermic vs. Endothermic Reactions & their Enthalpies H(g) + H(g)  H 2 (g)H 2 (g)  H(g) + H(g) Which reaction was Exothermic? Why? 5.1.3Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.

Enthalpy and stability H(g) + H(g)  H 2 (g)H 2 (g)  H(g) + H(g) Which reaction do you think has the most stable products?

Discuss the following graph 5.1.4Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products and the sign of the enthalpy change for the reaction.

Defining the term  Standard Enthalpy Change of Reaction (ΔH˚) Is defined as the heat gained or lost by a system during a specified chemical reaction when: The temperature is 298K (25˚C) The pressure is 100 kPa (1 atm) All concentrations are 1 ml dm -3 (1 M) All substances are in their standard state (the state that they would normally be in at these conditions) These values can be looked up in a book (or online) 5.1.1Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction (ΔH ɵ )

Calculation of energy changes  Accomplished using calorimeters and is based on the premise that  Heat change of reaction = - heat change of water in the calorimeter = - m H2O ΔT H2O c H2O  As the water has gained the heat produced by the reaction, the heat change of the reaction is negative when the temperature of the water increases.  If you measure the change at constant pressure (like an open contaner), ΔH is also known as q p (so q= - mΔTc as well)

A look at the variables  ΔH = - m H2O ΔT H2O c H2O  ΔH = Enthalpy of the reaction (measured in J)  m H2O = the mass of the water in the calorimeter (measured in grams)  ΔT H2O = the change of temperature of the water in the calorimeter (˚C)  c H2O = the specific heat capacity of water (=4.18 J/g or 1 cal/g for liquid water)

Heat Capacity vs. Specific Heat Capacity  Heat Capacity = the amount of heat energy needed to change the temperature of an entire system 1 K The system could be a calorimeter, a room, a building, a planet…  Specific Heat Capacity = the amount of heat energy needed to change the temperature of 1 g of a substance by 1 K It is more specific than Heat Capacity

Calorimeters Cup Calorimeter * Constant P Bomb Calorimeter * Constant V

Enthalpy Problem  Calculate the enthalpy of combustion from the following data. Assume all the heat from the reaction is absorbed by the water. Compare your value with the IB Data Booklet value and suggest reasons for any differences. Mass of water in copper calorimeter/g= Temperature increase in water/˚C=13.00 Mass of ethanol burned/g=0.45  Q – Should your answer be > or < zero?  Group up and try it!

Answer  Step 1 – Determine the ΔH of the reaction. Mass of water in copper calorimeter/g= Temperature increase in water/˚C=13.00 Mass of ethanol burned/g=0.45  ΔH = - m H2O ΔT H2O c H2O  ΔH = - (200.00)(13.00)(4.18)  ΔH = - 10,900 J or kJ

Answer  Step 2 – Convert the ΔH of the reaction to a standard ΔH˚(ΔH /moles = ΔH˚) Mass of water in copper calorimeter/g= Temperature increase in water/˚C=13.00 Mass of ethanol burned/g=0.45  Moles = mass/M = 0.45/46.08 = 9.8E-3  ΔH˚ = ΔH /moles = -10.9/9.8E-3  ΔH˚ = kJ mol Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water.

Answer  Compare your value with the IB Data Booklet value and suggest reasons for any differences. IB Data Booklet Value = kJ mol -1 Lab value for ΔH˚ = kJ mol -1  The booklet value suggests that more heat should have been gained by the calorimeter. The lab data suggests that not all of the heat lost by the reaction was gained by the calorimeter Evaluate the results of experiments to determine enthalpy changes.

Homework (Due Tomorrow)  Exercise 5.1 Pages #1-5  Exercise 5.2 Page 140 #1-6