Octane (C8 H18) is a liquid hydrocarbon at room temperature that is the primary constituent of g

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Octane (C8 H18) is a liquid hydrocarbon at room temperature...

Key Concepts

Combustion Reactions

Combustion reactions involve a substance, typically a hydrocarbon, reacting with oxygen to produce carbon dioxide and water. These reactions are highly exothermic, meaning they release a significant amount of energy, and they serve as fundamental examples in the study of energy production and chemical kinetics.

Balancing Chemical Equations

Balancing chemical equations is a method used to ensure that the conservation of mass holds, meaning that the number of atoms for each element is the same on both sides of the reaction. This is particularly important in combustion reactions, where proper stoichiometric coefficients are necessary to accurately represent the amounts of reactants and products involved.

Thermochemistry and Reaction Energetics

Thermochemistry deals with the study of heat changes during chemical reactions and phase changes. In combustion reactions, the release of energy is characterized by a negative enthalpy change (?H). This concept is foundational in determining the energy efficiency and suitability of fuels, as well as in understanding the driving forces behind chemical processes.

Gibbs Free Energy vs. Enthalpy

Gibbs free energy (?G) is a thermodynamic potential that indicates the spontaneity of a reaction, defined by the equation ?G = ?H - T?S, where ?S is the entropy change and T is the temperature. While ?H represents the heat released or absorbed during a reaction, ?G takes into account both energy and disorder. In many exothermic reactions such as combustion, although ?H is highly negative, the Gibbs free energy is typically less negative due to the opposing effect of the T?S term when the reaction leads to a decrease in entropy.

Entropy Considerations in Combustion

In combustion reactions, the entropy change (?S) can be significant because the reaction often involves changes in the physical states of the products relative to the reactants, such as the formation of liquid water from gaseous reactants or vice versa. When the reaction produces a decrease in entropy (negative ?S), the term –T?S becomes positive, thereby making the overall ?G less negative compared to the enthalpy change, even though the reaction is spontaneous.

Transcript

00:01 It's going to be a little more interesting than we can imagine.

00:03 All right, what we're going to do is we are going to balance a hydrocarbon reaction that is combustible.

00:11 And then we're going to look at the hydrocarbon reaction to determine gives free energy.

00:17 Look at the entropy.

00:18 Look at the delta h.

00:19 Remember entropy has to do with randomness.

00:22 So if i said like as delta s is the randomness, randomness.

00:30 And then delta h is the energy.

00:37 We'll just say energy.

00:39 And then delta g, this is going to be spontaneity.

00:47 Spondaneity.

00:49 Okay? the former we're going to be looking at is that delta g is equal to delta h minus t delta s.

01:01 Now, s doesn't have a degree because s is just talking about randomness.

01:06 So, first thing we're going to look at the equation.

01:08 We're going to take, what is that, eight times two, say, okay.

01:13 We are going to take c8h18, okay? and it is a liquid.

01:23 And we're going to burn it in presence of oxygen, that's o2, which is a gas because you're burning it.

01:30 That is going to produce, if it goes to completion, which in this case it does.

01:36 Co2, which is a gas, plus h2o, which is a liquid.

01:44 All right, first thing we got to do, we got to balance that particular equation.

01:48 Now, i know there's a little jiggy that i do, so see if you can follow me.

01:53 I'm going to take the subscript of one hydrogen on the product side, and i'm going to make it the coefficient for the species that has.

02:03 Hydrogen on the reactant side.

02:06 And then i'm going to take the subscript for the hydrogen on the reactant side and i'm going to make it the coefficient for the species with hydrogen on the product side.

02:15 And my hydrogens are now balanced.

02:17 Then i'm going to look at my carbons.

02:20 Well, two times eight is going to give me 16 on the reaction side.

02:26 On the product side there's only one carbon.

02:27 So i'm going to put a 16 in front of it and now my carbons are balanced.

02:31 And then all i got to do is really look at my oxygens and make sure i have those together okay well on the reacting side i have two oxygens on the product side i obviously have more so i've got 16 times two which is 32 oxygens plus 18 oxygens and all together that's eight plus two which is 10 bring the one and that gives me 50 all right so i've got 50 oxygens on the product side i've got two oxygens on the react inside 25 times 2 will give me 50 so i'm going to turn this to a 25 and now i have a balanced equation now if there was a number that i could divide into all those coefficients i would simplify it but in this case i cannot so we have done the first thing the first thing we had to do was we needed to balance this chemical equation okay so now the chemical equation is there now here's the big question we are going to going to not look at any thermochemical data and we're going to determine whether we think delta g, which is the spontaneity for this reaction, is more or less negative than the delta h.

03:44 Well, to do that first thing i got to do, i got to look at my change.

03:49 I'm just going to do this, change in entropy, okay? we're looking for randomness.

03:53 The more randomness there is, the more positive the number is going to be...

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