Born haber cycle of MgO

Hess's Law Example 2

Hess's Law Background and Example 1

Calorimeter

Calorimeter
- device used to measure the heat released (or absorbed) by a  
  physical or chemical process
- there are two types of calorimeter :
     + bomb calorimeter (constant pressure)
     + coffee-cup calorimeter (constant volume)

Bomb Calorimeter



- to measure heat released in combustion reaction

How a bomb calorimeter works?

1. With continual stirring, the initial temperature of preweighed water bath is noted.

2. The sample is ignited electrically

3. Heat released from the combustion react is transferred to the rest of the calorimeter.

4.The maximum temperature is measured.

Coffee-cup Calorimeter

- to measure heat released in non-combustion reaction such as heat of neutralization and heat of solution.


How a coffee-cup calorimeter works?

1.The calorimeter consist of a known mass of water (or solution) in an insulated container equipped with a thermometer and stirrer.
2. Initial temperature of the water is measured.
3. The process take place (addition of acid and base of soluble salt)
4. The contents are stirred.
5. Final temperature of water is measured.



Calculation of ΔHrxn

qsys = qwater + qcal + qrxn

no heat enters or leave the system

qsys = 0

qrxn = -(qcal + qwater)

qwater = mcΔT

qcal = CcalΔT



Capacity

Specific Heat Capacity, (c)

Amount of heat (q) required to raise temperature of 1 gram of a substance by 1k (1°C).


Eg :  H₂O_(l)  =  4.18 J/g·°C meaning it takes 4.18 Joules of
                        energy to heat 1.0 grams of water 1.0 °C.


Heat Capacity, (C)

Amount of heat (q) required to raise temperature of a given quantity of substance by 1k (1°C).

Every object has its own capacity.

Endothermic and Exothermic Reaction

               Endothermic and Exothermic are two terms used to describe what happens to the energy of a system undergoing a change.

               Endothermic processes require energy to take place. The system has to absorb energy from its surroundings.  Eg : ice melting

              

               Exothermic processes give off energy. The system releases energy to the surroundings. Eg : water freezing



Types of Enthalphy

• Enthalpy of Formation, ΔHf

           Heat change when 1 mole of a compound is formed from

its elements at standard condition.

          

     H2 (g) + ½ O2 (g) → H2O (l)                  H= −286 kJmol−1

 

•  Enthalpy of Combustion, ΔHcomb

            Heat released when 1 mole of substance is

burn completely in excess oxygen at standard conditions.

                
      C (s) + O (g) → CO (g)                           H= −393 kJmol−1

•  Enthalpy of Neutralisation, ΔHneut

            Heat released when 1 mole of H2O is formed from

 the neutralisation of acid and base at standard conditions.

          
   HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (aq)    H = kJmol−1

• Enthalpy of Atomisation, ΔHatom

            Heat absorbed when i mole of gaseous atom is formed

 from its element at standard conditions.

             
                 Na (s) → Na (g)                   H= 109 kJ mol−1

• Enthalpy of Sublimation, ΔHsublim

           Heat change when one mole of a substance sublimes

(solid into gas).

    
              
                I (s) → I (g)                       H= +106 kJ mol−1

• Enthalpy of Hydration, ΔHhyd

          Heat released when 1 mole of gaseous ion is hydrated in

water.

          
            Na+ (g) → Na + (aq)            H= − 406 kJ mol−1

• Enthalpy of Solution, ΔHsoln

          Heat change when 1 mole of a solute dissolves in water to

 form a very dilute solution.

          
        KCl (s) → K+ (aq) + Cl− (aq)        H= + 690 kJ mol−1