So there must be something more going on to explain why endothermic reactions occur. Entropy (S) is a measure of the disorder (or chaos or randomness) of a system.
The entropy of the universe is increasing. Things become more stable when they become more disordered (Second Law of Thermodynamics for the Physicists! Think of a typical teenage bedroom: Question Can you think of other everyday examples where there is a high entropy? See power point slides Entropy measures how much the energy of motion of the particles in a substance changes per K of temperature. For each degree K the temperature rises above zero K (at zero K the substance has no entropy), a substance will gain a certain amount of energy per mole and this will differ from substance to substance. Entropy values (S) are always positive and usually quite small so we measure them in J not k].Note: The units of entropy are J moil-l K-1.
Eng. Copper has an entropy (S) of 33. 3 J moil-l K-1 1 . How much energy is transferred every time the temperature of one mole of copper goes up by 1 K? 2. How much energy is transferred every time the temperature of 0. 05 moil of copper goes up by k? 3.
How much energy is transferred every time the temperature of 6. 35 moil of copper goes up by k? 4. What will nap pen to toe entropy went copper melts* 5. What will happen to the entropy when copper evaporates? 6.Why are the entropy values of a liquid and a solid substance quite s similar but very much smaller than for the gas? 7. Why does water have an entropy value of 70.
0] moil-l K-1, of 188. 7] moil-l K-1? 8. How does the entropy of salt (NCAA (s)) compare with brine (a)? But steam has a value (An+ (a) and CLC- HTTP://wow.. Origin/a-levels/item/67-entropy for interested students.
Entropies are absolute values (unlike enthalpies) but we can comment on and calculate changes in entropy AS Entropy values increase in the order: solids < liquids/solutions < gas Question - can you explain this graph?Predicting the sign of AS for a reaction You can look at an equation (physical change or chemical reaction) and predict what wall nap pen to toe entropy t toe system; we nave to cancer two: If you increase the number of moles from the reactants to the products then you will increase the disorder. Therefore entropy will increase. As you go from solids to liquids to gases you increase the disorder. Therefore entropy will increase. Gas particles move from place to place whereas solid particles only vibrate (liquids are intermediate but much more like solids than gases).
This consideration is more significant than the first one. In this reaction: MGM(S) + 02 C)-+2 M go (s) The entropy has decreased because we have lost one mole of gas. But in this reaction: Baa coos (s) -+ Boo (s) + CIA (g) The entropy has increased because we have created one mole of gas where there was none before. A(S) + b) -+ CGI): The entropy has decreased because there are fewer moles of gas on the right hand side of the equation. For each reaction, predict, with a reason, whether the entropy increases or decreases, using a + or a - sign as in the first example. ) r(g) Sign + Reason - more moles of gas on the right b) CIA (g) + Hi (g) -+ Juju (a) + 602 (g) Reason Sign c) N (g) + HI (g) -+ NH (g) d) 02 (g) Sign e) MGM(s) + Hegel McGee (a)+ H (g) Sign f) KICK(S) g) Hew (l) K+ (a) + CLC- (a) Hew(g) Sign Before we go on to calculate changes in entropy AS we will recap on AH calculations and introduce a quicker way of doing these problems.
At AS you would have drawn a Hess' Triangle but now we can teach you a short cut for this particular kind of calculation:- AH r = (E (A HP products)) - (E (A HP reactants)) See if you get the same answer as the traditional method!Ana another - n toe eat nappy change Tort TN l's reaction using toe quack way. Calculating Entropy changes This is no different from calculating enthalpy changes, except that elements have values for entropy, unlike they do for enthalpies of formation. Try this one:- AS r = (SEES products) - (SEES reactants) 1. S c graphite= 5.
7 J moil-l K-1. S oxygen = 102. 5 J moil-l K-1. S carbon dioxide = 213. 6] moil-l K-1.
Find the entropy change when 1 moil of graphite is oxidized to carbon dioxide under standard conditions. 2.Find the change in entropy for the combustion of one mole of ethane. S ethane = 219. 5 J K-1 S liquid water = 70. 0] moil-l K-1 (also use the other values above).
3 Ha toe change In entropy Tort toe t ammonia Try mom nitrogen ala hydrogen and comment on the value S ammonia = 192. 4 J moil-l K-1 S hydrogen = 130. 6 J moil-l K-1 91. V moil-l K-1 S nitrogen = Notice that values of AS are quite small. Note the unit difference for AS and AH.
However, even an increase in entropy does not guarantee that a reaction will occur!So there is an over-arching concept which relates both enthalpy and entropy to the feasibility of a reaction. Free Energy The contributions of AS and AH to the feasibility of a reaction is given by a quantity called GAG - the Gibbs free energy change This can be calculated using the following equation: Student a activity Calculate the free energy change for the room temperature (K) decomposition of: ) Magnesium carbonate + CIA(g) guacamole's-1 AS?-k-1 b) Barium carbonate. guacamole's-1 K-1 what does Teasel mean? It m earns it is possible (but it still might not actually happen!See below for why). The relationship AH - TASK makes it clear how important the temperature is in a reaction: Use the relationship between the terms in the equation above to fill in the table below. AS GAG - always negative, always positive, sometimes negative Feasible? For the two cases that are sometimes negative.
State and explain whether temperature must be high or low in order to make GAG negative iii) In Summary If AH is positive and AS is negative, the reaction can NEVER be feasible because GAG will always be positive, whatever the temperature. TASK will always be smaller than If AH is negative and AS positive, a reaction will always be feasible because GAG will always be negative; (TASK will always be greater than AH) IT AH Is negative ala AS Is negative, a reaction wall only De Teasel IT toe temperature is low enough for AH to be less than TASK so that GAG remains negative. If AH is positive and AS also positive, a reaction will become feasible if the temperature is high enough for TASK to be greater than AH, so that GAG becomes negative.
