THERMORCHEMISTRY

FIRST TIPS TO MASTER IN THIS TOPIC:

1. You need to have a calculator.
2. Good in conversion of mol to mass, mol to no. of particles, mol to volume of gas in room condition and STP.
3. Balance the equation.
4. Label the unit with the respective symbols.

 WHAT YOU NEED TO KNOW FROM THIS TOPIC:

1. Differentiate between the bond breaks and the bond reforms.
2. If the reactants breaks a bond = the heat is absorbed.
3. If the product reforms ( forming a bond ) = the heat is released.
4. When the total energy absorbed is more than the total energy released = Endothermic reaction.( with a positive sign, + ).
5. When the total energy absorbed is less than the total energy released = Exothermic reaction. ( with a negative sign, - )
6. List the examples of the endothermic reactions and exothermic reaction. Make sure you enable to write the equation correctly and state the observation.
7. Enable to describe the endothermic reaction / exothermic reaction.

For example:

 CH₄ + 2O₂ à CO₂ + 2H₂O     ΔH = -890 kJ mol^-1

a)     The combustion of methane is exothermic reaction.

b)     1 mol of methane burns completely in excess oxygen to form 1 mol of carbon dioxide gas and 2 mol of water releases 890kJ heat energy.

c)      The total energy absorbed when methane breaks the bond is lower than the total energy released when carbon dioxide and water were formed.

d)     The surrounding temperature is rise.

8. Draw the energy level diagram. ( EXXO =  Z, energy content in reactants is higher , ENDO = S, energy content in reactants is lower. )

    9. EXOTHERMIC AND ENDOTHERMIC REACTION IN DAILY LIFE

 a)     coal is burnt heat is given off. The heat given off is used for cooking or to keep the house warm

b)     Cold packs and Hot packs are often used in sports. They are used to lessen pain for injuries. Salts used in cold packs – NH₄Cl , NH₄NO₃, Na₂S₂O₃, KNO₃ packed separately with water. Salts used in hot packs – AlCl₃, CaO₃ , CaCl_.2H₂O, CuSO₄.2H₂O used to dissolves in water. When the pack is needed, the membrane is broken.

    10. TO CALCULATE THE ENERGY CHANGE:

                                       ΔH = mcθ

m = the mass of water used in the reaction(for combustion, water is used to  

       absorbed the heat release from the burning of ethanol).

c = the water heat capacity , 4.2 Jg^-1oC^-1

θ = the changes in the temperature of water used.