Heat changes accompanying physical changes and Explanation of the source of the heat of solution

The calculation of the change in heat content is of a great importance where , the calculation of the change in heat content accompanying the burning of different fuels , helps us during designing the engines to know which type of fuel is more suitable .

The calculation of the change in heat content accompanying the burning of different materials , helps the firemen in identifying and choosing the most suitable method to put off the fire .

The change in the heat content differs according to the type of change ( physical or chemical ) .

Heat changes accompanying physical changes

Heat changes accompanying physical changes

Heat changes accompanying physical changes

Examples of heat changes accompanying physical changes are Standard heat of solution and Standard heat of dilution .

Standard heat of solution Δ H° sol

On dissolving a solid substance in a liquid , this process is accompanied by an increase or a decrease in the temperature of the resulted solution .

By dissolving Ammonium nitrate ( NH4NO3 ) in water , the temperature of the solution decreases due to absorbing an amount of energy .

The solution is called Endothermic solution .

NH4NO3 + H2O ( l ) → NH4+ ( aq ) + NO3 ( aq )

Δ H° sol = + 25.7 kJ / mol               

On dissolving Sodium hydroxide ( NaOH ) in water , the temperature of the solution increases , due to releasing an amount of energy .

 The solution is called Exothermic solution .

NaOH ( s ) + H2O ( l ) → Na+ ( aq ) + OH ( aq )

Δ H° sol = – 51 kJ / mol       

Standard heat of solution

The quantity of heat absorbed or released on dissolving one mole of the solute in a certain amount of the solvent to obtain a saturated solution under standard conditions .

The heat of solution can be calculated by the following relation :

qp  = m . c . Δ T

Where , In diluted solutions , the solution mass ( m ) can be expressed by the volume , because the density of water in normal conditions equals 1 g / cm3 .

The specific heat of the diluted solution can be expressed by the specific heat of water 4.18 J/ g . ° C .

If the volume of the produced solution is ( 1 L ) and the amount of solute is ( 1 mol ) , the heat change in this case is called Molar heat of solution .

Molar heat of solution

The heat change resulting from dissolving one mole from the solute to form one litre of the solution .

If the amount of solute is greater than 1 mole , we can calculate the molar heat of solution as the following :

Molar heat of solution ( Δ H sol ) = the amount of absorbed or released heat during dissolving ( qp ) ÷ number of moles of solute ( n )

Δ H sol = qp /  n

Explanation of the source of the heat of solution

Dissolving process is affected by three forces which are :

Attraction force between molecules ( particles ) of solvent  , Attraction force between molecules ( particles ) of solute and Attraction force between particles of solvent and solute .

So , the dissolving process takes place in three steps which are :

Separating solvent’s molecules from each other

Endothermic process , where an amount of energy ( Δ H1 ) is absorbed to overcome the attraction force between the molecules of the solvent with each other .

Separating solute’s particles from each other

Endothermic process , where an amount of energy ( Δ H2 ) is absorbed to overcome the attraction force between the particles of the solute with each other .

Dissolving process

Exothermic process , where , an amount of energy  ( Δ H3 ) is released when solvent molecules are combined with the particles of solute .

The value of heat of solution Δ H° sol equals to the sum of these three energies :

Δ H° sol = Δ H1 + Δ H2 + Δ H2

If the solvent is water : Dissolving process is called Hydration , The energy released from dissolving process is called Hydration energy .

Hydration energy is the attaching of the dissociated ions with water molecules .

The hydration energy of silver ions equals – 510 kJ/mol means that the amount of energy released from attaching 1 mol of silver ions with water molecules equals – 510 kJ .

The type of dissolving process ( exothermic or endothermic ) is determined by the value of Δ H° sol .

If ( Δ H1 + Δ H2 ) < Δ H3   , So , Δ H° sol < 0 ( Negative sign ) and the dissolving is Exothermic .

If ( Δ H1 + Δ H2 ) > Δ H3   , So , Δ H° sol > 0 ( positive sign ) and the dissolving is Endothermic      

The standard heat of solution of lithium bromine equals – 49 kJ/mol means that the heat released from dissolving 1 mol of lithium bromine in an amount of solvent to get a saturated solution of it ( in standard conditions ) equals – 49 kJ .

The molar heat of solution of silver iodide equals + 84.4 kJ/mol means that the heat absorbed from dissolving 1 mol of silver iodide in an amount of solvent to form 1 L of a solution equals 84.4 kJ .

Standard heat of dilution Δ H° dil

When 1 mol of sodium hydroxide NaOH ( s ) dissolved in two different amounts of water H2O ( l ) , the heat of solution differs as the amount of water differs , as the following equations :

 NaOH ( s ) + 5H2O ( l ) → NaOH ( aq ) ,  Δ H1 = – 37.8 kJ/mol

NaOH ( s ) + 200H2O ( l ) → NaOH ( aq ) ,  Δ H1 = – 42.3 kJ/mol

It is observed that the amount of released energy increases by adding another amount of water .

The released or absorbed energy that resulted from adding another amount of solvent ( dilution ) is called Standard heat of dilution Δ H° dil

Standard heat of dilution Δ H° dil is the quantity of released or absorbed heat for each one mole of solute when diluting the solution from a high concentration to another lower concentration with the condition of being in its standard state .

The standard heat of dilution of sodium hydroxide solution is – 4.5 kJ/mol means that the heat released from each 1 mol of sodium hydroxide solution , when it is diluted from a higher concentration to lower concentration at standard conditions equals 4.5 kJ .

Dilution process takes place in two opposite steps , according to the energy :

Separation energy ( Endothermic process )

Because during dilution , the number of molecules of water increases and separate the ions or molecules of the solute from each in the concentrated solution , which needs an amount of energy .

Attaching energy ( Exothermic process )

Because the ions or molecules of the solute are attached to a greater number of molecules of the solvent , which leads to releasing an amount of energy .

The heat of dilution is the sum of those two energies ( separation and attaching ) .

Heat Changes accompanying Chemical changes and Hess’s law of constant heat summation

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