THERMOCHEMISTRY

• the study of heat transfers that take place during chemical reactions.

• The heat content of a substance at a given temperature and pressure is called its enthalpy (H).

• It cannot be measured directly because there is no set starting point. The reactants start with a heat content and the products end up with a heat content.

• So we can measure how much enthalpy changes (ΔH)

• The net heat transfer of a reaction can be determined by conducting the reaction in a constant-pressure calorimeter and calculated as;

• Some reactions release energy i.e. have negative net energy transfers (ΔH is negative) and are referred to as exothermic

• Reactions in which energy is absorbed (ΔH is positive) are called endothermic.

• When a reaction occurs in one direction and its reaction enthalpy is measured, then the reaction enthalpy of the reverse reaction is found by just reversing the sign of the forward reaction

• For example the reaction enthalpy for the oxidation of CO to CO2 can be experimentally determined at 25oC and 1atm

• It follows that the enthalpy of the reverse reaction in which CO2 decomposes to give CO and O2 is just the reverse of the forward enthalpy (i.e. it becomes +283.0 kJ)

• Similarly, if the reactants and products in the equation were to be multiplied by a factor of 2, then the enthalpy change would also double because twice as many moles are then involved

• The molar amount need not to be integers, they can be fractions

Standard enthalpy changes

• Changes in enthalpy are normally reported for processes taking place under a set of standard conditions.

• In most aspects of metallurgical processes we consider the standard enthalpy change, ΔHo i.e. the change in enthalpy for a process in which the initial and final substances are in their standard states

• Standard states for various chemical substances are defined by their thermodynamic properties (mainly P and T)

• Certain values of these properties are chosen to be the standard conditions.

• Standard states for substances are therefore defined as follows; – For gases it is the gaseous phase exhibiting ideal gas

behavior at a pressure of 1 atm and a specified temperature.

– Solids and liquid: the thermodynamically stable state at a pressure of 1 atm and a specified temperature.

– For solutions: the 1-molar solution at a pressure of 1 atm at a specified temperature and exhibiting ideal solution behavior

• Therefore for all substances standard state = 1 atm , plus a specified temperature.

• Standard enthalpy for a give species can then be defined as the enthalpy of that species under standard state conditions.

• From this, the zero of the enthalpy scale can then be defined (just like sea level is the zero of altitude!)

• The following rules apply for defining the zero of enthalpy;

– Chemical elements in their standard state at 298.15oC have zero enthalpy

– For elements that can exist as allotropes (same chemical composition but different atomic structures) the form that is more stable than the other has an enthalpy of zero, e.g. oxygen gas is more stable than ozone gas and therefore O2 is the one with a zero standard state enthalpy.

Enthalpy changes accompanying

phase/physical changes

• occur when a substance changes from one physical state to another e.g. from solid to liquid.

• The standard enthalpy change that accompanies a change of physical state at 1atm is called the standard enthalpy of transition or transformation and is denoted ΔHo

trans

• The following enthalpies are therefore defined;

• Molar enthalpy of fusion: Heat absorbed at constant pressure when 1 mole of a substance is melted.

• Molar enthalpy of freezing : Heat released at constant pressure when one mole of a substance freezes

• Molar enthalpy of vaporisation: Heat absorbed to vaporise 1 mole of a substance at constant pressure and temperature.

• Molar enthalpy of condensation: Heat released when 1 mole of a substance is condensed from vapour to liquid.

• If all these changes are occurring at 1atm then they are standard enthalpies (o).

• Other enthalpy changes of physical changes include ΔHo

mixing, ΔHoatomisation etc

• There are two important aspects in enthalpies of physical change;

1) In changes involving more than one step to get to the final physical state the same value of ΔHo will be obtained regardless of the route in which the physical change proceeds.

– e.g.

– Can occur in two steps, first fusion (melting) and then vaporization of the resulting liquid:

– Overall

– i.e.

2) the standard enthalpy changes of a forward process and its reverse differ only in sign e.g. ΔHo

fus = Δhofreez

• At microstructure level in solid metals, some allotropic changes may also occur e.g. γ(austenite)→α(ferrite) in iron.

• Such changes are also accompanied by enthalpy changes i.e. ΔHo

trans

Enthalpy Changes during Chemical reactions

• Processes in extractive metallurgy involve chemical reactions in which energy is transferred during these processes.

• The enthalpy changes that take place depend on the reactions and may be classified as;

Heat of Reaction

• It is the heat that is released or absorbed in a chemical reaction i.e. ΔH

• Enthalpy of combustion: Heat absorbed or released when one mole of a substance is completely burned in excess oxygen.

– Mainly applied to fuels i.e. hydrocarbons.

– If the combustion occurs at 1atm then it becomes standard enthalpy of formation (ΔHo

c)

• Enthalpy of formation: Heat absorbed or released when 1 mole of a substance is formed from its elements.

– If the elements are in their standard states then it becomes standard enthalpy of formation (ΔHo

f)

• standard enthalpy change of a reaction

is therefore the sum of the standard enthalpies of formation of products minus the sum of the standard enthalpies of formation of the reactants. i.e.

• Elements in their standard states are not included because their

• And the of H+(aq) is also set at zero.

Hess’s Law • The enthalpy change for any reaction

depends on the products and reactants and is independent of the pathway or the number of steps between the reactant and product.

• Thus if two or more chemical equations are added to give another chemical equation, then the corresponding enthalpies of the reactions must be added

• For example the production of CO2 from graphite (C) can be analysed in 2 ways;

a)Reaction of C with O2 to give CO2 direct

b)Reaction of C with O2 to give CO followed by further oxidation of CO to give CO2.

• The reaction enthalpies;

• Can be determined using Hess’s law

• Overall

• Example:

The reduction of iron oxide in the blast furnace proceeds according to the following reactions:

3Fe2O3 + CO → 2Fe3O4 + CO2 , ΔHo298 = -53.16 kJ

Fe3O4 + CO → 3FeO + CO2 , ΔHo298 = +41.02 kJ

FeO + CO → Fe + CO2 , ΔHo298 = -18.42 kJ

• Calculate ΔHo298 for the reaction

Fe2O3 + 3CO → 2Fe + 2CO2