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As chemistry thermodynamics

Referring to an energy profile diagram, in an exothermic reaction are reactants high in energy because they're more unstable?

basically after revisiting AS chemistry i began to wonder if the reason why reactants had a higher energy in an energy-profile diagram is due to lots of potential energy stored in the bonds which made them relatively energetically unstable thus only needing a small amount of activation energy to break their unstable energized bonds to form energetically stable products with a lower bond energy.

BASICALLY: what i want to know is: the reason why reactants are at a higher energy level in an exothermic reaction?
do they have more potetnial energy stored in their bonds?

if so does that mean theyre unstable? if thats the case then what makes them so unstable?

feel like im getting really confused :frown:
(edited 8 years ago)
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if something is energetically unstable (like reactants in an exothermic reaction) does that mean that they have a higher energy ( higher bond energy?) what does higher energy in terms of an energy-profile diagram mean?
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Original post by mercuryman
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i give up
I hope you find answer. I want to know as well!!!
Post in the chemistry section perhaps?
(edited 8 years ago)
Basically breaking bonds requires energy whereas making them releases energy. If a reaction is exothermic it means that the amount of energy to break the bonds in the reactants is lower than the amount of energy released by forming new bonds.
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Original post by samb1234
Basically breaking bonds requires energy whereas making them releases energy. If a reaction is exothermic it means that the amount of energy to break the bonds in the reactants is lower than the amount of energy released by forming new bonds.


tysm for getting back! :smile:

yes i get that, but what i wanted to know is: the reason why reactants are at a higher energy level in an exothermic reaction, is it because theyre unstable? if so what makes them so unstable?
Original post by Pentaquark
I hope you find answer. I want to know as well!!!
Post in the chemistry section perhaps?


idk how you do that :frown:
Original post by mercuryman
idk how you do that :frown:


http://www.thestudentroom.co.uk/forumdisplay.php?f=130 ?
:smile:
Original post by mercuryman
tysm for getting back! :smile:

yes i get that, but what i wanted to know is: the reason why reactants are at a higher energy level in an exothermic reaction, is it because theyre unstable? if so what makes them so unstable?


Do you know anything about entropy? Basically the easiest way to explain is with entropy. Imagine you have say 100 molecules of gas which move randomly and can either be in container 1 or container 2. Statistically the probability that at any one time all 100 are in one container is very small, so you can say that the average will be nearer the 50/50 mark. This is what you know as diffusion but is an example of entropy, i.e. the disorder of the system increases. What this essentially means is that for a chemical reaction to be spontaneous, it's entropy not enthalpy that matters. If the overall entropy change is positive (i.e the system becomes more disordered) then the reaction will take place.

Qualitatively, the reactant has a certain number of 'quanta' (basically amount) of energy. However assuming it's a solid the amount of disorder will be fairly low, as the structure is fairly rigid. When it reacts it loses energy to the surroundings which increases the kinetic energy of the surrounding gas molecules which hugely increases the disorder of the overall environment so the reaction takes place (and is why the initial compound is unstable)

Quantitatively, the formula for the entropy change is given by (sum of entropy of products - sum of entropy of reactants) - (the enthalpy change/Temp in kelvin). If a reaction is exothermic then delta h is negative, and for a typical exothermic reaction the increases in entropy of the surroundings (the -delta h/t bit) is much greater than the decrease in entropy of the new product, and hence the reaction takes place spontaneously.
Original post by samb1234
Do you know anything about entropy? Basically the easiest way to explain is with entropy. Imagine you have say 100 molecules of gas which move randomly and can either be in container 1 or container 2. Statistically the probability that at any one time all 100 are in one container is very small, so you can say that the average will be nearer the 50/50 mark. This is what you know as diffusion but is an example of entropy, i.e. the disorder of the system increases. What this essentially means is that for a chemical reaction to be spontaneous, it's entropy not enthalpy that matters. If the overall entropy change is positive (i.e the system becomes more disordered) then the reaction will take place.

Qualitatively, the reactant has a certain number of 'quanta' (basically amount) of energy. However assuming it's a solid the amount of disorder will be fairly low, as the structure is fairly rigid. When it reacts it loses energy to the surroundings which increases the kinetic energy of the surrounding gas molecules which hugely increases the disorder of the overall environment so the reaction takes place (and is why the initial compound is unstable)

Quantitatively, the formula for the entropy change is given by (sum of entropy of products - sum of entropy of reactants) - (the enthalpy change/Temp in kelvin). If a reaction is exothermic then delta h is negative, and for a typical exothermic reaction the increases in entropy of the surroundings (the -delta h/t bit) is much greater than the decrease in entropy of the new product, and hence the reaction takes place spontaneously.


Ok, I understand that entropy is a measure of disorder and that the higher the entropy the greater the KE the particles have to become disordered. But i still don't get why reactants have a higher energy level in exothermic reactions? Are they naturally meant to be more disordered?

my inintial understanding is that:

Reactants are at a high energy level because they have a lot of potential energy in their bonds that could be released by breaking THOSE
bonds.

However, reactants such as Methane and oxygen have weaker bonds than CO2 and H20, making them unstable?

in short, reactants in an exothermic reaction like methane for instance, have weak bonds with high potential energy?

would you think this is correct too?
Original post by mercuryman
Ok, I understand that entropy is a measure of disorder and that the higher the entropy the greater the KE the particles have to become disordered. But i still don't get why reactants have a higher energy level in exothermic reactions? Are they naturally meant to be more disordered?

my inintial understanding is that:

Reactants are at a high energy level because they have a lot of potential energy in their bonds that could be released by breaking THOSE
bonds.

However, reactants such as Methane and oxygen have weaker bonds than CO2 and H20, making them unstable?

in short, reactants in an exothermic reaction like methane for instance, have weak bonds with high potential energy?

would you think this is correct too?


Sort of. It's not really potential energy as such, it's more similar to say a car. When the car is moving at 50ms-1 it has a higher kinetic energy and when it brakes and drops down to say 30ms-1 it gives out the difference in energy to the environment, which is analogous to what happens in a reaction.

To get into why say methane has the amount of energy it does is well beyond the realms of this, afaik you would need to get into MO theory and do some pretty complex maths to show that methane has that amount of energy.

As a simplified thing if you just considered the energy of everything relative to methane and you said that methane has an energy of X. When it reacts, it takes energy to break the c-h bonds and then energy is released when new bonds are formed. If it takes a -ve amount of energy to do this (i.e. more energy is given out than you put in) then it makes sense that the energy of the products must be lower (as energy is a conserved quantity so the energy of the products plus the energy given out must be identical). Therefore you can see that, relative to the reactant, the product must have a lower amount of energy if the reaction is exothermic.

Spoiler


hmm, so essentially bonds in reactants have a greater Potential Energy, and that during bond formation this inner potential energy is reduced through it being converted into thermal energy which is given off to the environment. Thereby reducing the Potential energy of bonds that make up the product(s)?
Original post by mercuryman
hmm, so essentially bonds in reactants have a greater Potential Energy, and that during bond formation this inner potential energy is reduced through it being converted into thermal energy which is given off to the environment. Thereby reducing the Potential energy of bonds that make up the product(s)?


correct. Thats how i understand it.

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