Use equations II and III to calculate the ΔHrxn of equation I. P4(s)+6Cl2(g)→4PCl3 ΔH=-1280 kJ

3.P4(s)+10Cl2(g)→4PCl5 ΔH= -1775 kJ

ΔHrxn of equation I = -123.5 kJ

Complete question :

Reaction 1 :

PCl₃ + Cl₂   →   PCl₅          

Reaction 2 :

P₄ + 6Cl₂     →   4PCl₃       ΔH = −1280 kj          

Reaction 3 :

P₄ + 10Cl₂   →   4PCl₅       ΔH = −1774 kj

Based on the principle of Hess's Law, the change in enthalpy of a reaction will be the same even though it is through several stages or ways

Reverse reaction 2 :

4PCl₃  ⇒ P₄ + 6Cl₂         ΔH = +1280 kj (sign change)    

Add to reaction 3 :

4PCl₃  ⇒ P₄ + 6Cl₂         ΔH = +1280 kj

P₄ + 10Cl₂  ⇒  4PCl₅       ΔH = −1774 kj  

+        

Canceled the same compound in the different side :

4PCl₃ + 4Cl₂ ⇒  4PCl₅    ΔH = -494 kJ ⇒divide by 4

PCl₃ + Cl₂ ⇒  PCl₅    ΔH = -123.5 kJ

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.