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Tutorial 2 will help you to:
You have probably met with the concept ofthe glossary of the textbook defines it as: " enthalpyThe heat content of a system. in Unit 1 and in Chemistry 11. Looking it up in " Another way to think of enthalpy is as " Chemical Potential Energy ". Any change in the Potential Energy of a system means the same thing as the " Enthalpy Change ". The symbol for Enthalpy is " H ". Therefore the "change in Enthalpy" of a chemical reaction is called "' H ". As far as we’re concerned in Chemistry 12, a Potential Energy Diagram (like we looked at in the last unit) is the same thing as an "Enthalpy Diagram". Let’s look at an example:
Enthalpy
∆H
Progress of Reaction
The "EnthalpyChange"
Reactants
Products
The reaction shown on this graph is Exothermic. This means that heat isreleased or given off. What was given off as heat energy was lost as Enthalpy from the reactants. The net energy change between theproducts and the reactants is called the Enthalpy Change ( ∆H). As you can see, the enthalpy change (the case when the Products are lower on the Potential Energy (Enthalpy)∆H) in this reaction is negative. (This is always Graph.)
An exothermic reaction
So, we can make a statement here:
a rock
Well, you probably guessed correctly.doubt if no forces other than gravity are acting on the rock. Let’s assume that for now. The rock will fall down! This of course is only true without a
Let’s say someone asks youbat" a good reason would be: why the rock falls down. Even though you might not say this right "off the
" The rock falls down because of a natural tendency to achieve a position of lower gravitational potential energy****! "
It’s true, systems will tend toward a state of lower potential energy if nothing else is acting upon them. Now, in Chemistry, we are not particularly interested in gravitational potential energy (unless we are underneath the rock or skiing.) What we are interested in is chemical potential energy , otherwise known as (you guessed it) - enthalpy! Like the example of gravitational potential energy,
Chemical systems will tend toward a state of minimum enthalpy if sufficient activation energy is available and no other factors are considered.
Another way of stating this might be:
A chemical reaction will favour the side (reactants or products) with minimum enthalpy if no other factors are considered.
Thus for an exothermic reaction , if no other factors are considered:
Enthalpy
∆H
Progress of Reaction
The "EnthalpyChange"
Reactants
Products
An exothermic reaction
Here, the Products have lower enthalpy" favour the products ". In other words, if the reactants are mixed, they will tend to than the Reactants so the reaction tends to form productsreactants. spontaneously (without outside assistance) rather than remain as
Thea natural tendency here for reactants to spontaneously form products. products will be favoured because the products have minimum enthalpy. In other words, there is
Before you turn to the next page, see if you can predict what would happen in an reaction. endothermic
Fill in the blanks below, then turn to the next page and check: In an endothermic reaction , the ___________________________________ have minimum enthalpy, so the ____________________________________ will be favoured. In other words, if the reactants are mixed they will (tend to remain as reactants / spontaneously form products) ______________
(This, of course, is assuming that no other factors are affecting the system!)
5 If the reaction : CO(g) + 3H2(g) CH4(g) + H 2 O(g) + 49.3 kJ was proceeding to the right , the enthalpy would be _____________________ing. Is this a favourable change? _____________.
was proceeding to the right , the enthalpy would be ____________________ing. Is this a favourable change? _____________. Check the answers on page 1 of (^) TTuuttoorriiaall 22 - - SSoolluuttiioonnss
As you can see by looking at the exercises above, there are two ways of looking at what happens to the enthalpy :
If the reaction is exothermic formation of products (move toward the right, the products have minimum enthalpy and the) is favourable.
If the reaction is endothermic the formation of products (move toward the right) is un, the reactants have minimum enthalpy andfavourable. In this case the formation of reactants (move toward the left) is favourable.
Now, consider the simple melting of water: H 2 O(s) KHDW H 2 O(l) (the subscript (^) (s) stands for solid ) (the subscript (^) (l) stands for liquid ) If we were to look at only the enthalpy in this process, you can see that the reactant ( H 2 O(s) ) would have minimum enthalpy and would be favoured. Can you see what this statement would mean? Itwould mean that all of the water in the universe should exist only as a solid! (It would not be favourable for water to exist as a liquid!) We would all be frozen solid!!!! Obviously there is something wrong with this reasoning! We know that there is liquid water in the universe, so what gives? The answer to this problem lies in looking at another factor that governs equilibrium. That factor is
Student’s Chemistry binders are a good example. At the beginning of the semester, papers are neat and ordered. By the end of the semester, pages are torn and falling out. The thing has been overloaded andthe rings are usually bent or broken. Pages are loose and often the plastic coating is in shreds. The entropy of the binder has increased over time! This situation is not unusual, even in chemical reactions!
Another way to look at the last example is to say that:
" The side with the greater number of moles of gas has the greatest entropy. "
O
O H
O
H HO H
O
O
H^ O^ H O H
O H
O
H HO H
O
O
H^ O^ H O H
Dissolving
An Aqueous Solution
The ions in a Solid arevery ordered. They have low entropy.
When dissolved in water, the ions are surrounded by water molecules. The ions are much separated and more spread out solution is and (^) higher disordered than that of a solid. The entropy of an aqueous.
Here are few exercises for you:
d) CO(g) + Cl2(g) COCl2(g) The ___________________________________________have greater entropy. e) MgCO3(s) + 2HCl(aq) MgCl2(aq) + H 2 O(l) + CO2(g) The ___________________________________________have greater entropy. Check page 2 of TTuuttoorriiaall 22 - - SSoolluuttiioonnss for the answers to these questions. If you have any questions about these, check with your teacher!
Now, way back on page 8, we looked at the process: H 2 O(s) KHDW H 2 O(l) Remember we decided that all the H 2 O in the universe should remain as a solid because H 2 O(s) has lower enthalpy than H 2 O(l) and nature favours a state of minimum enthalpy. Well, now we can explain why there is some liquid water in the universe (lots of it):
H 2 O(l) has higher entropy than H 2 O(s)
or your binders!)^ If left alone for a long time, systems tend to get more disordered. (Like this classroom, your bedroom There is a natural tendency in nature toward maximum disorder or maximum entropy!
Chemical systems will tend toward a state of maximum entropy if no other factors are considered.
Another was of stating this might be:
A chemical reaction will favour the side (reactants or products) with maximum entropy if no other factors are considered.
Remember, the other factor which controlled reactions wasAlso remember that: enthalpy. (chemical potential energy).
Chemical systems will tend toward a state of minimum enthalpy if sufficient activation energy is available and no other factors are considered. or (see next page)......
Now, lets consider another simple process: A glass bottle is knocked down from a high shelf onto aconcrete floor and the glass shatters: Bottle on a high shelf ∆ Thousands of pieces of glass on a concrete floor The bottle falls down and not up! This happens because there is a natural tendency toward minimum gravitational potential energy (like minimum enthalpy in chemistry) In other words the tendency toward minimum gravitational potential energy favours the products (the low bottle rather than the high) (To stretch this analogy further, we could consider that the person who knocked the bottle off of theshelf was simply supplying the " activation energy ") Remember that the bottle broke into thousands of pieces when it hit the concrete. The broken pieces ofglass have more disorder ( entropy ) than the bottle, so in this process, the tendency toward maximum entropy also favours the products!
Bottle on a high shelf → Thousands of pieces of glass on a concrete floor Products haveminimum "enthalpy", so the tendencytoward minimum enthalpy favours the products.
Products havemaximum entropy, so the tendency towardmaximum entropy favours the products.
There is no "equilibrium" here when the process is finished. That bottle has completely fallen down and it is all broken. (This bottle is no longer on the shelf and it is no longer an "unbroken bottle") We can summarize what happened here:
Processes in which both the tendency toward minimum enthalpy and toward maximum entropy favour the products , will go to completion. (ie. All reactants will be converted into products. There will be no reactants left once the process is finished!)
Here’s an example of a chemical reaction in which this happens: 2K(s) + 2H 2 O(l) 2KOH(aq) + H2(g) + heat This process is exothermic (the heat term is on the right) so the products have lower enthalpy. The tendency toward minimum enthalpy favours the products. There is a mole of gas on the right ( H2(g) ) and no gases in the reactants7KHUHIRUHWKH products have greater entropy.
The tendency toward maximum entropy favours the products.
Since both tendencies favour the products , this reaction will go to completion. That is, all of the reactants (assuming you have the correct mole ratios eg. 2 moles of K to 2 moles ofH 2 O) will be converted to products. If one reactant is in excess , the limiting reactant will be completely consumed. So, if you put a little bit of potassium in a beaker of water, the reaction will keep going until all of thepotassium is used up. There will be no potassium left once the reaction is complete.
In other words, the reverse reaction does not occur!
Let’s consider one more process: 2KOH (aq) +^ H2(g) + heat^ 2K(s) +^ 2H 2 O(l) In this case, the tendency toward minimum enthalpy favours the reactants , and the tendency toward maximum entropy also favours the reactants.
Processes in which both the tendency toward minimum enthalpy and toward maximum entropy favour the reactants , will not occur at all!. (ie. None of the reactants will be converted into products. There will be no products formed!)
NOTE: bottle and jumping up onto a high shelf! This does not occur at all. (At least I’ve never seen it happen!) This would be like thousands of pieces of glass spontaneously sticking together, forming a
To summarize:
When the two tendencies oppose each other (one favours reactants, the other favours products), the reaction will reach a state of equilibrium. Processes in which both the tendency toward minimum enthalpy and toward maximum entropy favour the products , will go to completion. Processes in which both the tendency toward minimum enthalpy and toward maximum entropy favour the reactants , will not occur at all!.
_______________ < _______________ < _______________ < _______________
