Enthalpy5.1.15.1.35.1.45.2.35.2.4

Defining the term

EnthalpyIs 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 HThe 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) H2(g) H2(g) H(g) + H(g)

What units are used to measure Enthalpies?

Exothermic vs. Endothermic Reactions & their Enthalpies

H(g) + H(g) H2(g) H2(g) H(g) + H(g)

Which reaction was Exothermic? Why?

5.1.3 Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.

Enthalpy and stability

H(g) + H(g) H2(g) H2(g) H(g) + H(g)

Which reaction do you think has the most stable products?

Discuss the following graph

5.1.4 Deduce, 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)

5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction (ΔHɵ)

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 = - mH2OΔTH2OcH2O

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 qp (so q= - mΔTc as well)

A look at the variables ΔH = - mH2OΔTH2OcH2O

ΔH = Enthalpy of the reaction (measured in J)

mH2O = the mass of the water in the calorimeter (measured in grams)

ΔTH2O = the change of temperature of the water in the calorimeter (˚C)

cH2O = 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 KThe 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 KIt 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=200.00

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=200.00 Temperature increase in water/˚C=13.00 Mass of ethanol burned/g=0.45 ΔH = - mH2OΔTH2OcH2O

ΔH = - (200.00)(13.00)(4.18) ΔH = - 10,900 J or -10.9 kJ

Answer Step 2 – Convert the ΔH of the reaction

to a standard ΔH˚(ΔH /moles = ΔH˚) Mass of water in copper

calorimeter/g=200.00 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˚ = -1100 kJ mol -1

5.2.3 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 = -1367 kJ mol -1

Lab value for ΔH˚ = - 1100 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.

5.2.4 Evaluate the results of experiments to determine enthalpy changes.

Homework (Due Tomorrow) Exercise 5.1

Pages 137-138#1-5

Exercise 5.2Page 140#1-6