Equilibrium (1 Viewer)

[independantz]

Can someone explain to me why?

When an increase in temperature occurs, the reaction will favour the endothermic side.
and
When a decrease in temperature occurs, the reaction will favour the exothermic side.

Thanks

 

[Mark576]

Le Chatelier's principle states:

"If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or total pressure, then the equilibrium shifts to partially counter-act the imposed change." (courtesy of Wikipedia )

So simply, increasing the temperature will cause the position of the equillibrium to counteract this change, and hence move accordingly so as to effect a reduction in temperature. This is why it will favour the endothermic side, as this side absorbs heat, and thus lowers the temperature.

Conversely, when a decrease in temperature occurs, the position of the equillibrium moves to counteract the reduction in temperature, hence it moves to the exothermic side, which releases heat. From this we conclude that it favours the exothermic side.

 

[Mark576]

This is interesting, explains the mathematics behind Le Chatelier's principle.

http://www.chemguide.co.uk/physical/equilibria/change.html#top

Although you need to be familiar with equillibrium constants.

 

[Lordsion]

Systems are in equilibrium.

Increase temp -> Equilibrium shifts in direction that counteracts the disturbance -> Heat absorbed (endothermic reaction) to lower temp

Decrease temp -> Equilibrium shifts in direction that counteracts the disturbance -> Heat released (exothermic reaction) to increase temp

The equilibrium always shifts in the direction that counteracts the disturbance, hence favouring a certain "side" of a reaction.

This will sound really dumb, but how do you know what side is endothermic or exothermic?

 

[WantToDoBetter]

Well i don't know any simple way to identify it without practical data...but the exothermic side is the side that will release heat energy, while the endothermic side of a reaction will absorb heat if it shifts as so.

The main endothermic reactions i know are written like they are due to their commercial applications, e.g. as in the production of ammonia.

Possibly this is something 'Physics' can explain rather than chemistry?

Lodgic! You look like a smart guy with that conical flask and all! I wonder if you can remember Enthalpy. Enthalpy can be defined as STORED ENERGY and is also represtented by the letter H. Since we can't measure the amount of sorted energy, we can only measure a CHANGE in energy which is represented by Delta=Δ. There for the change in enthapy can be represented with the symbols ΔH.

The equation to calulate enthalpy (which i'm quite sure you will learn soon) is
ΔH= -mCΔT m=mass (of water) C= is the specific heat compacity of water (which is on the chem cheat cheet in the exam, from memory 4.18) ΔT=the CHANGE (hence delta) in temperature.

In typing what i an typing it's definatly the long way, but i throught it better to explain every thing so the numbers at the end made sense.

When enthalpy is measured an insulated beaker (lol-foam cup) holds a certain amount of water, this is the m=MASS. A burner is placed under the cup, with gauze to protect it, and this burner is filled with the substance to be examined. Take the temperature before the flame and after it is extingwished. This is your ΔT=CHANGE in TEMPERATURE. This number could be a + or a -. If a - it is endothermic as it has droped in temperature, and if it's a + then it's a exothermic reaction. (FINAL-INITAL)

The equation to calulate enthalpy (which i'm quite sure you will learn soon) is
ΔH= -mCΔT m=mass (of water) C= to the specific heat compacity of water (which is on the chem cheat cheet in the exam, from memory 4.18) ΔT=the CHANGE (hence delta) in temperature.

This equation isn't the end of it and now i cbf explaining the rest but remember this confusing fact (also have to devide by molecular weight). If the final number that comes out of this equation is a MINUS it is EXOTHERMIC! PLUS is ENDOTHERMIC. Confusing i know but it's to do with the equation. NOW TO YOUR QUESTION. If your see a number in joules or kilojoules (j) (kj) under or next to your equlimbrium equation and a PLUS + OR A MINUS -, you will now beable to distingwish between endo and exothermic reactions!!!!!!!!

Now instead of conducting another class on this PM me if you have any questions or post a reply. Really hoped this helped... Thinking of a career as a science teacher, lol, how do you reckon i'd go .
Cheers.

 

[Undermyskin]

Yeah, applying enthalpy to equilibrium can help...

I've got a different explanation.

Endothermic reactions include: photosynthesis, evaporation, thermal decomposition and reduction of metallic oxides with carbon. For these reactions, energy (in this case the heat) is absorbed.

Equation:

heat + A+B -><- C +D.

When you increase the heat, due to La chatelier's theorem, this will push the reaction to the right (equilibrium shifts to the right) to counteract with this change: use the heat increased. In other words, endothermic reactions do prefer heat as the more heat the more they can eat and produce bigger quantity of products thusly.

Exothermic reactions: respiration, neutralization, precipitation and combustion of fuel.

Reactions:

C+D -><- A + B + heat. ( don't mess up it with the above. Remember we prefer to write the reactants on the left. This just shows the close relation between endothermic and exothermic reactions.)

In this case, if you increase heat. Oops, you are helping the right hand side (or more scientifically, the endothermic reaction of this close system). So, when you decrease the temperature (i.e. lower the heat/ energy), again due to La Chatelier's theorem, the equilibrium shifts to the right to counteract this change by releasing more heat.

It's like this: you get rid of the 'heat' on the right hand side, you have:

C+D -><- A+ B. Crap, this reaction requires heat to keep it constant (tendency to maintain its equilibrium). What can it do? More C and D have to react to produce A, B and more importantly 'heat'. Yes, now you have 'heat' in your equation. Woo hoo.

Remember: just simply regard 'heat' as a chemical/ particle/ ion of your reaction/ equation. This can help.

Cheers.

 

[WantToDoBetter]

Yeah, applying enthalpy to equilibrium can help...

Any equilibrium equation that you will encounter in the HSC will usually ask you what would happen if the concentration is increased or decreased(pressure for gas), or temperature is increased or decreased ect...
To be able to work out how temp ect will effect the system you firstly need to know if it is exo or endothermic. This question came out in this years HSC as a multipule choice and it was asking how pressure decrease effects the Haber process (synthesis of ammonia from nitrogen and hydrogen). This is the equation.
N<SUB>2</SUB>(g) + 3H<SUB>2</SUB>(g) çè 2NH<SUB>3</SUB>(g) ΔH=-92kJ mol<SUP>-1</SUP>
The delta H is enathpy and as i have explained above, the MINUS means that the reaction is exothermic.
The answer is that heat is absorbed. Now i can't remember why this so because i forgot! Great way to end... Anyway, maybe Undermyskin or Lodgic can answer cause i don't want 2 look it up lol.
Cheers.
AHA! I think i have it! Due to the Haber process needing high temps (increased rate of reaction, decreased yeild) and high pressure (decrease rate of reaction, increased yeild) once the pressure is lifted the yeild will significantly decrease so the reverse reaction which is endothermic(ammonia to nitrogen and hydrogen) sucks up the excess heat to maintain this new equilibrium position (which is to the left or with the reactants).
Hope it clears it up. If i'm to sketchy or didn't explain something entirely correct (quite possible) plz ask or correct me.
Cheers (again).