AS/A-Level Chemistry- Enthalpy(iii) applications

Measuring Enthalpy Changes

Calorimetry

• involves the practical determination of enthalpy changes
• usually involves heating (or cooling) known amounts of water

🔥water is heated up: reaction is EXOTHERMIC

❄️water cools down: reaction is ENDOTHERMIC

Calculation

The energy required to change the temperature of a substance can be found as:

q = m  x  c  x  ∆Τ

where    q=heat energy(kJ);      m=mass(kg);      c=Specific Heat Capacity (kJ K ^-1 kg ^-1) [ water is 4.18 ];                                                              ∆Τ=change in temperature(K)

Graphical method

• The  temperature is taken every half minute before mixing the reactants.
• Reactants are mixed after 3 minutes.
• Further readings are taken every halfminute as the reaction mixture cools.
• Extrapolate the lines as shown and calculate the value of ∆Τ.

Example (calculating enthalpy change with given data):

225cm³ of 2.0M HCl was added to 25cm³ of 2.0M NaOH in an insulated beaker.The initial temperature of both solutions was 20°C.  The reaction mixture wasstirred to ensure mixing and the highest temperature reached by the solution was 33°C.  Calculate the Molar Enthalpy of Neutralisation.

• Temperature rise (∆Τ) =    306K - 293K=    13K
• Volume of resulting solution=    50cm³=    0.05 dm³
• Equivalent mass of water=    50g=    0.05 kg

Heat absorbed by the water (q)=    0.05  x   4.18  x  13=   2.717 kJ

Moles of HCl reacting=    2   x    25/1000=    0.05 mol

Moles of NaOH reacting=    2   x    25/1000=    0.05 mol

Equation:     NaOH    +    HCl    ——>    NaCl    +    H₂O

Moles of water produced=    0.05 mol

Enthalpy change per mol  (∆Η)=  − ( heat energy / moles of water )

=   − 2.717 / 0.05       ANS: − 54.34 kJ mol -1

Example (relating enthalpy of dissociation to bonding) in A-Level Chemistry:

Explain why the enthalpy of lattice dissociation of CaF₂is greater than the enthalpy of lattice dissociation of CaCl₂and why the enthalpy of lattice dissociation of CaO is greater than the enthalpy of lattice dissociation of BaO.

• F^-ion has a smaller size, greater charge densitythan Cl-ion.
• Stronger electrostatic forces of attraction between oppositely-charged ions, F^-and Ca²⁺ions, stronger ionic bonds between oppositely-charged ions, F^-and Ca²⁺ions.
• More energy required to break the stronger ionic bonds between oppositely-charged ions, F^-and Ca²⁺ions.
• Ca²⁺ion has a smaller size, greater charge density than Ba²⁺ion.
• Stronger electrostatic forces of attraction between oppositely-charged ions, Ca²⁺and O^2-ions, stronger ionic bonds between oppositely-charged ions, Ca²⁺and O^2-ions.
• More energy required to break the stronger ionic bonds between oppositely-charged ions,Ca²⁺and O^2-ions.

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Drafted by Eunice Wong (Chemistry)