Calculate ΔH^∘ for each reaction.

Name: Problem 36, Chapter 6: Organic Chemistry
Uploaded: 2022-08-27T19:39:40-08:00
Duration: 5 min 4 s
Channel: Henry R
Description: Calculate ΔH^∘ for each reaction.

Key Concepts

Standard Enthalpy Change (?H°)

This is the heat change that occurs under standard conditions (usually 1 atm pressure and 298 K temperature) when a chemical reaction takes place. It indicates whether a reaction is exothermic or endothermic and is a crucial measure in thermodynamics for predicting reaction energy profiles.

Enthalpy of Formation

This concept refers to the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states. It is often used as a reference in calculating the overall reaction enthalpy through the formula ?H°reaction = ??H°f(products) - ??H°f(reactants).

Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is independent of the pathway between the initial and final states. This principle allows the calculation of reaction enthalpies by combining known enthalpy changes from different steps or reactions.

Thermochemical Equations

These are balanced chemical equations that include the enthalpy change for the reaction. They provide a framework for understanding how heat is absorbed or released during a reaction and are essential in performing and tracking energy calculations in chemistry.

Transcript

00:01 Okay, so we need to calculate delta h for the following reactions.

00:06 We've got hydroxide radical and methane going to a ch3 radical and water.

00:21 So what it appears is happening to me is that we're forming an oh bond and we're losing a ch bond.

00:32 Hopefully that is fairly apparent we lost a ch bond from there to there and we gained an oh bond from there to there so what if we lose is the h bond that's what we have to pay so we look at our table bond association energies how much energy does it cost to break a ch3 to hydrogen bond let's see that is 104 k -cals per mole okay, what about creating? well, how much is it a cost to break an oh bond in water? to break costs 119 k -cals per mole.

01:39 Okay.

01:41 So what we're going to do is take every bond that we broke and add that up.

01:48 In this case, there's just the one, 104.

01:51 Then we're going to subtract the bond association energy for every bond we made, which in this case is the hoh bond, which is one.

01:58 119, and that's our delta h.

02:04 And so in total, this is going to be negative 15 k kals per mole, which means this is favorable, and that this is exothermic.

02:16 We put off 15k cows per mole, right? because the bond we formed is harder to break than the bond we actually broke now, which means we get more energy out than we put in.

02:29 Okay.

02:30 Now we have ethanol, or methanol, i should say, plus hbr, going to methyl bromide plus h2o.

02:49 Okay, so what did we gain and lose here? here we lost a c -h bond, right? we broke, sorry, not a c -o bond.

03:08 Okay, we also broke an h -b -r bond.

03:11 Right because this proton ends up with this water.

03:15 All right, so we need to break the hbr bond.

03:19 First, we broke an hbr bond.

03:24 All right.

03:25 What bonds did we form? we formed a carbon, a ch3 bromine bond.

03:36 And we also formed an h -o -h bond...