Having a problem with an Enthalpy Change Question. Please Help

[Zainab Sultan]

Hey Guys!
I came across this question in past paper June 2011 p23 chemistry that says " Hydrazine does not burn spontaneously in oxygen. Suggest a reason for this."
I went to the mark scheme and it said " because Ea is too high". What does that exactly mean?
If anyone knows the clear explanation for this I will really appreciate your concern to answer.
Thanks!

 

[Dark Destination]

Hey Guys!
I came across this question in past paper June 2011 p23 chemistry that says " Hydrazine does not burn spontaneously in oxygen. Suggest a reason for this."
I went to the mark scheme and it said " because Ea is too high". What does that exactly mean?
If anyone knows the clear explanation for this I will really appreciate your concern to answer.
Thanks!

You should know that the Activation Energy is the minimum amount of energy needed to kick-start a reaction.
A reaction involves bond-breaking and bond-formation, energy release and energy absorption.

An Exothermic reaction is then a reaction in which the energy released is GREATER than the energy absorbed, so the net change is that energy is released due to which its enthalpy change is negative.

When you construct a energy profile diagram, you see that in an exothermic reaction the product energy level is below that of the reactants, but before that is achieved, the energy of reactants increases. First activation energy is provided to break off some of the bonds, and then bond formation and energy release takes place resulting in the formation of products.

Whenever a gas burns, bonds need to be broken.

You must have studied the test for Hydrogen in IGCSE/O-Level.
You have to insert a burning splint into a test tube containing Hydrogen. Ever wondered why that is so?
Basically, Hydrogen is present inside the test tube, along with Oxygen (Air). But Hydrogen is not reacting with Oxygen.
A certain amount of energy (The activation energy) needs to be provided to the Hydrogen, to break its H-H bonds, and only then it will react with Oxygen to form water.

So the burning splint, is the heat energy you are providing to the mixture, it is the activation energy of the reaction after which the reaction takes place:

2H2 + O2 --> 2H2O

Water droplets are also seem in that test tube, and a ''pop'' sound is observed, which tells us Hydrogen was present. Because Hydrogen ''burns'' with a pop sound.

Now that you understand that a certain activation energy is needed for a gas to burn, that can be applied to other concepts.
In a cooker thingy in kitchens, methane or Natural gas is supplied to the stove (is that what its called?), but a lighted splint is taken near it. That is again the activation energy required to burn the Methane, and for the exothermic reaction to take place, to release heat energy that will be used to cook the food.

If you were in a house, and you opened a container containing a flammable gas. The flammable gas would quickly diffuse and spread evenly throughout the room. But it would not cause an explosion, until you light a splint. The moment you light a splint... BOOM! There will be an explosion.
The gas is called ''Flammable'' because it can easily catch fire, it can easily react with Oxygen, because it requires minimum amount of activation energy to kick-start its reaction with Oxygen.

HYDRAZINE is N2H4. It's a gaseous compound of Nitrogen. Why doesn't it react spontaneously with Oxygen? There can only be one reason, it has a very high activation energy. A spontaneous reaction is one that takes place quickly with minimum input from the user, that means a reaction that requires less activation energy.

I am not saying Hydrazine can not react with Oxygen. It can. If you provide the necessary energy, the bonds in N2H4 would be broken, and it would react with the available oxygen. But, Hydrazine, due to some chemical reasons has an activation energy that is much higher than other compounds, and due to this it does not burn spontaneously in oxygen.

I hope you understand.

 

[Zainab Sultan]

You should know that the Activation Energy is the minimum amount of energy needed to kick-start a reaction.
A reaction involves bond-breaking and bond-formation, energy release and energy absorption.

An Exothermic reaction is then a reaction in which the energy released is GREATER than the energy absorbed, so the net change is that energy is released due to which its enthalpy change is negative.

When you construct a energy profile diagram, you see that in an exothermic reaction the product energy level is below that of the reactants, but before that is achieved, the energy of reactants increases. First activation energy is provided to break off some of the bonds, and then bond formation and energy release takes place resulting in the formation of products.

Whenever a gas burns, bonds need to be broken.

You must have studied the test for Hydrogen in IGCSE/O-Level.
You have to insert a burning splint into a test tube containing Hydrogen. Ever wondered why that is so?
Basically, Hydrogen is present inside the test tube, along with Oxygen (Air). But Hydrogen is not reacting with Oxygen.
A certain amount of energy (The activation energy) needs to be provided to the Hydrogen, to break its H-H bonds, and only then it will react with Oxygen to form water.

So the burning splint, is the heat energy you are providing to the mixture, it is the activation energy of the reaction after which the reaction takes place:

2H2 + O2 --> 2H2O

Water droplets are also seem in that test tube, and a ''pop'' sound is observed, which tells us Hydrogen was present. Because Hydrogen ''burns'' with a pop sound.

Now that you understand that a certain activation energy is needed for a gas to burn, that can be applied to other concepts.
In a cooker thingy in kitchens, methane or Natural gas is supplied to the stove (is that what its called?), but a lighted splint is taken near it. That is again the activation energy required to burn the Methane, and for the exothermic reaction to take place, to release heat energy that will be used to cook the food.

If you were in a house, and you opened a container containing a flammable gas. The flammable gas would quickly diffuse and spread evenly throughout the room. But it would not cause an explosion, until you light a splint. The moment you light a splint... BOOM! There will be an explosion.
The gas is called ''Flammable'' because it can easily catch fire, it can easily react with Oxygen, because it requires minimum amount of activation energy to kick-start its reaction with Oxygen.

HYDRAZINE is N2H4. It's a gaseous compound of Nitrogen. Why doesn't it react spontaneously with Oxygen? There can only be one reason, it has a very high activation energy. A spontaneous reaction is one that takes place quickly with minimum input from the user, that means a reaction that requires less activation energy.

I am not saying Hydrazine can not react with Oxygen. It can. If you provide the necessary energy, the bonds in N2H4 would be broken, and it would react with the available oxygen. But, Hydrazine, due to some chemical reasons has an activation energy that is much higher than other compounds, and due to this it does not burn spontaneously in oxygen.

I hope you understand.

That was really helpful. Thank you very much!