CALORIMETRY ENGCHEMISTRY 101 | Lecture notes Engineering Chemistry

ENGCHEM 101 Page 1

Activity No. 5

CALORIMETRY

A transfer of energy in the form of heat accompanies all chemical and physical

changes. A change that releases heat is exothermic while a change that absorbs heat is

endothermic. The flow of heat is measured in a calorimeter, an apparatus with insulating

walls, made so that essentially no heat is exchanged between the contents of the

calorimeter and the surroundings.

When heat flows into a substance, the temperature of that substance will increase. The

quantity of heat, 𝑞, required to increase the temperature of any substance is proportional

to the mass, 𝑚, of the substance and the temperature change, ∆T = Tf – Ti ,as shown in

equation 1. The proportionally constant is called the specific heat capacity, 𝑐, of that

substance.

𝑞= m x c x ∆𝑇 (1)

For chemical reaction that occurs in aqueous solution, there is an exchange of heat

between that reaction mixture and water. At constant pressure, this heat exchange is

quantitatively referred to as the enthalpy (or heat) of reaction, ∆Hrxn . The heat flow for a

reaction mixture, 𝑞rxn , is equal in magnitude but opposite in sign to that for water,𝑞H20. .

The heat flow associated with the reaction mixture is also equal to the enthalpy change

for the reaction, ∆𝐻rxn: qrxn = ∆𝐻rxn = −𝑞𝐻2𝑂 (2)

By measuring the mass and the temperature change of the water, 𝑞H20 and ∆𝐻 rxn can

be determined using Equations 1 and 2, respectively. The resulting formula for the

enthalpy of a reaction is:

∆𝐻𝑟𝑥𝑛 = −𝑚𝐻20 𝑥 𝑐𝐻20 𝑥 ∆𝑇 (3)

A rise in the temperature of water indicates that heat is given off by the reaction

mixture, so the reaction is exothermic; 𝑞H20 is positive and ∆𝐻xrn is negative. A decrease

in the temperature of water indicates that heat is absorbed by the reaction mixture, so

the reaction endothermic. In this case 𝑞H20 is negative and ∆𝐻rxn is positive.

Heat of Solution, ∆Hsoln

When a subtance is dissolved in a solvent, the resulting heat absorbed or evolved is

known as the heat of solution, which for solid solute is the sum of two factors:

∆𝐻soln= ∆𝐻LE + ∆𝐻hyd (4)

• lattice energy (∆𝐻LE ) - the energy needed to dissociate or separate the particles of

the solid ; this energy is absorbed ( ∆𝐻LE >0)

• hydration energy ( ∆𝐻hyd ) – the energy needed to surround the solid particles with

water; this energy is released (∆𝐻hyd <0 )

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ENGCHEM 101 Page 1 Activity No. 5 CALORIMETRY A transfer of energy in the form of heat accompanies all chemical and physical changes. A change that releases heat is exothermic while a change that absorbs heat is endothermic. The flow of heat is measured in a calorimeter, an apparatus with insulating walls, made so that essentially no heat is exchanged between the contents of the calorimeter and the surroundings. When heat flows into a substance, the temperature of that substance will increase. The quantity of heat, 𝑞, required to increase the temperature of any substance is proportional to the mass, 𝑚, of the substance and the temperature change, ∆T = Tf – Ti ,as shown in equation 1. The proportionally constant is called the specific heat capacity, 𝑐, of that substance. 𝑞= m x c x ∆𝑇 (1) For chemical reaction that occurs in aqueous solution, there is an exchange of heat between that reaction mixture and water. At constant pressure, this heat exchange is quantitatively referred to as the enthalpy (or heat) of reaction, ∆Hrxn. The heat flow for a reaction mixture, 𝑞rxn , is equal in magnitude but opposite in sign to that for water,𝑞H20.. The heat flow associated with the reaction mixture is also equal to the enthalpy change for the reaction, ∆𝐻rxn: qrxn = ∆𝐻rxn = −𝑞𝐻 2 𝑂 (2) By measuring the mass and the temperature change of the water, 𝑞H20 and ∆𝐻 (^) rxn can be determined using Equations 1 and 2, respectively. The resulting formula for the enthalpy of a reaction is: ∆𝐻𝑟𝑥𝑛 = −𝑚𝐻 20 𝑥 𝑐𝐻 20 𝑥 ∆𝑇 (3) A rise in the temperature of water indicates that heat is given off by the reaction mixture, so the reaction is exothermic; 𝑞H20 is positive and ∆𝐻xrn is negative. A decrease in the temperature of water indicates that heat is absorbed by the reaction mixture, so the reaction endothermic. In this case 𝑞H20 is negative and ∆𝐻rxn is positive. Heat of Solution, ∆ Hsoln When a subtance is dissolved in a solvent, the resulting heat absorbed or evolved is known as the heat of solution, which for solid solute is the sum of two factors: ∆𝐻soln= ∆𝐻LE + ∆𝐻hyd (4)

lattice energy (∆𝐻 LE ) - the energy needed to dissociate or separate the particles of the solid ; this energy is absorbed ( ∆𝐻 LE > 0 )
hydration energy ( ∆𝐻 hyd ) – the energy needed to surround the solid particles with water; this energy is released (∆𝐻 hyd <0 )

ENGCHEM 101 Page 2 Thus, is positive (endothermic) when the lattice energy is greater than the hydration energy, otherwise, the ∆𝐻soln is negative (exothermic). Heat of Neutralization, ∆𝐻 neut When a strong acid and a strong base are brought together, neutralization occurs and the heat involved is known as the heat of neutralization. The heat effect is quite large as a result of the reaction between H 3 O+^ ions from the acid and OH− ions from the base.

H 3 O+( 𝘢q) + OH-( 𝘢q)  H 2 O( l ) + heat

Heat of Fusion, ∆𝐻 fus When heat is added to a mixed-phase system that comprises a substance in its solid and liquid phases, both at the melting point of the substance, the temperature of the system does not increase until the entire solid is converted to liquid. The additional heat that is absorbed without causing a temperature increase is used to melt the solid, and is known as the heat of fusion or enthalpy of fusion. In this activity, the student will use a calorimeter simulation to determine the different heats of solution or reaction by watching a Calorimetry demonstration using this link: https://youtu.be/jv4oRVWXmAc The Calorimetry data for the activity will be obtained from this link: https://youtu.be/Tomvfu6Dl- 4

CALORIMETRY ENGCHEMISTRY 101, Lecture notes of Engineering Chemistry

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H 3 O+( 𝘢q) + OH-( 𝘢q)  H 2 O( l ) + heat