1. Distinguish between exothermic and endothermic reactions.

The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. It can be easily recognized by a rise in the temperature of the reaction. Change is always given a negative value, as the energy is lost from the system to the surroundings.
Examples:

1. Oxidation reactions such as combustion of fuels, respiration in plants and animals.

2. Acids with metals

3. Water with quick lime.

The chemical reactions which proceed with the absorption of heat energy is called endothermic reactions. The energy input may come from flame, electricity, sunlight or surroundings. An endothermic enthalpy change is always given a positive value, as the energy is gained by the reactants.

a. Decomposition of limestone by heating.
       

b. Photosynthesis in which energy is supplied to the reactions in the cells by sunlight.

c. Dissolving ammonium chloride in water.

2. State and explain first law of thermodynamics.

Energy is not being created by exothermic chemical reactions and it is not destroyed in endothermic reactions. Energy is transformed from the reacting surroundings or the other way around. The total energy of the whole system of reacting chemicals and surroundings remains constant. This applies to any energy transfer and is called law of conservation of energy. Energy can neither be created nor be destroyed.  The law was proposed by Helmholtz.
If a system having internal energy e, absorbs heat energy q, then internal energy is q + E. If w is the work done on the system, then internal energy changes to E₂.

    
This is the mathematical representation of first law of thermodynamics.

3. Define enthalpy with examples.

Enthalpy is the total energy content of the reacting materials. It is given by the symbol H. Enthalpy cannot be measured, but it is possible to measure enthalpy change, when energy is transferred to or from a reaction system and changes from one enthalpy to another. It is given by symbol .
                      

An exothermic enthalpy change is always a negative value. = - x KJ mol ^-1
Example:
         

When one mole of methane burns completely in oxygen, 890.3 KJ of energy are transferred to the surroundings. An endothermic enthalpy change is always a positive value,
Example: 

An input of 572 KJ of energy is needed to break down one mole of calcium carbonate to calcium oxide and carbon dioxide.

Enthalpy profiles for exothermic reaction is:

4. Define standard enthalpy changes of reactions.

The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation for the reaction, react under standard conditions to give the products in their standard states.
Example. The equation for the reaction between hydrogen and oxygen can be written in two different ways and there are different values of  in each case.

Equation (1):
       

Equation (2):
       

A standard enthalpy for a reaction takes place under standard conditions such as:

1. A pressure of l00 kilo pascals

2. A temperature of 298 K

3. Reactants and products must be in the physical states (solid, liquid or gas) that are normal for these conditions.

4. Any solutions having a concentration of 1.0 mol dm^-3

5.  Draw the enthalpy profile for the standard enthalpy change of combustion of hydrogen and sulphur.

The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.
The standard enthalpy change of combustion of hydrogen is given by the equation:
 
It is not possible to achieve complete combustion under standard conditions. Measurements are taken under experimental conditions; then a value for the enthalpy change is determined and this is corrected to standard conditions through calculation.

Combustion of sulphur gives sulphur dioxide and the equation is given by:

The enthalpy profile is drawn as follows.

6.  What are conditions satisfying the values for bond enthalpies?

The amount of energy needed to break a covalent bond is called bond enthalpy.  The values are always quoted as bond enthalpy per mole. Consider the example of oxygen gas, O₂ (g). The bond enthalpy of oxygen is the enthalpy change for the process.

E is used for bond enthalpy per mole. It is referred to as e ( x – y ), where ( x – y ) is a molecule.

The values of bond enthalpies per mole satisfy the following four conditions.

1. They are all positive, as the changes during breaking of bonds are endothermic. The same quantities of energy would be released in an exothermic change when the bonds form.

2. They are average values. The actual value of the bond enthalpy for a particular bond depends upon which molecule the bond is in.

3. They are very difficult to measure directly. They are usually calculated using data from measurements of enthalpy changes of combustion of several compounds.

7. State and explain Hess’s law.

Hess's law stats that the total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided the initial and final conditions are the same.

A  → D;     ΔΗ  = y  KJ mol^-1

If allowed to takes place in three steps:

A  → B;     ΔΗ  = X₁  KJ mol^-1

B  → C;     ΔΗ  = X₂  KJ mo^l-1

C  → D;     ΔΗ  = X₃  KJ mol^-1

Hess's law can be regarded either as an expansion of the law of conservation of energy or as a consequence of the fact that the enthalpy,  H is a state function. 

Hess’s law is used to:

(a) Determine enthalpy of reactions

(b) Determine enthalpy of formation

(c) Determine standard enthalpy of reactions

(d) Determines enthalpy of transition of allotropic forms

(e) Calculate bond energies

8. The enthalpy of formation of HBr is – 36.2KJ. If the bond energies of H – H and Br – Br are 431 KJ and 188 KJ, calculate the bond energy of H – Br bond.

Enthalpy of formation of HBr is:

9. Calculate the enthalpy of formation of ethyne from the following data.  The enthalpies of combustion of C_(s), H_2(g)  and C₂ H₂  are  -393.5, -285.8 and -1297 KJ mol ^-1 respectively. 

10. Calculate the enthalpy of formation of ethane using the following data.  Enthalpies  of combustion of carbon, hydrogen and ethane are  -393.5 KJ mol ^-1, -285. 8 KJ mol ^-1 and - 890.5 KJ mol^-1.