Question

1. Flammable substances will burn more rapidly in pure oxygen than in air. Explain why this happens

2. in terms of energy and bonds, explain what causes a reaction to be exothermic or endothermic. Do not simply tell what it means for a reaction to be exothermic or endothermic.

Answer 1

1.) AS we know air consist of contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases and in pure oxygen has 100% oxygen shares.Here we have common flammable that have autoignition temperatures(temperature at which combustion can start) can be 300°C (572°F) to 550°C (1022°F).

Now we know that combustion can only takes place in presence of oxygen. so, first lets compare the oxygen present in air vs oxygen present in pure oxygen gas.

AS from above Statement we get to know that air contains only :- 20.95% oxygen

and pure oxygen has :- 100% oxygen.

Now there is simple rule regarding the combustion of flammable object is more oxygen supplied per sec more lighter the fire and vice versa . so  in short we can say that more combustion is directly proportional to amount of oxygen present per sec.

SO ,Flammable substances will burn more rapidly in pure oxygen than in air.

2)in terms of energy and bonds we can say that Suppose a reaction taking place like AB + CD --------> AC + BD

Now you can see that here bond between A and B , C and D get break and formation of bond between takes place in A and C , B and D , so if the energy require to break the bond the between AB and CD is less than energy required to form bond between AC and BD .then we can say that reaction is exothermic otherwise endothermic.

Now if terms of energy:-

You know that delta H formation is calculated as = sum of delta h in product - sum of delta h in reactant

therefore delta h = delta h(AC) +delta h(BD) -{delta h(AB) + delta h(CD)}

IF delta h is positive than endothermic and if negative than exothermic.

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