You are given the following data: H2(g) ⟶ 2 H(g) Δ H^∘=436.4 kJ / mol Br2(g) ⟶ 2 Br(

You are given the following data: H2(g) ⟶ 2 H(g) Δ...

You are given the following data: $$\begin{array}{ll}\mathrm{H}_{2}(g) \longrightarrow 2 \mathrm{H}(g) & \Delta H^{\circ}=436.4 \mathrm{kJ} / \mathrm{mol} \\ \mathrm{Br}_{2}(g) \longrightarrow 2 \mathrm{Br}(g) & \Delta H^{\circ}=192.5 \mathrm{kJ} / \mathrm{mol} \\\mathrm{H}_{2}(g)+\mathrm{Br}_{2}(g) \longrightarrow & 2 \mathrm{HBr}(g) \\& \Delta H^{\circ}=-72.4 \mathrm{kJ} / \mathrm{mol}\end{array}$$ Calculate $\Delta H^{\circ}$ for the reaction $$\mathrm{H}(g)+\mathrm{Br}(g) \longrightarrow \mathrm{HBr}(g)$$

You are given the following data: $$\begin{array}{ll}\mathrm{H}_{2}(g) \longrightarrow 2 \mathrm{H}(g) & \Delta H^{\circ}=436.4 \mathrm{kJ} / \mathrm{mol} \\
\mathrm{Br}_{2}(g) \longrightarrow 2 \mathrm{Br}(g) & \Delta H^{\circ}=192.5 \mathrm{kJ} / \mathrm{mol} \\\mathrm{H}_{2}(g)+\mathrm{Br}_{2}(g) \longrightarrow & 2 \mathrm{HBr}(g) \\& \Delta H^{\circ}=-72.4 \mathrm{kJ} / \mathrm{mol}\end{array}$$
Calculate $\Delta H^{\circ}$ for the reaction $$\mathrm{H}(g)+\mathrm{Br}(g) \longrightarrow \mathrm{HBr}(g)$$

Key Concepts

Reaction Enthalpy Calculation

Reaction enthalpy calculation involves determining the net heat absorbed or released during a reaction by comparing the energies associated with breaking bonds in the reactants to those formed in the products. This process, often facilitated by Hess's Law, allows for the determination of the enthalpy change for complex reactions by using known enthalpy changes of related simpler reactions.

Hess's Law

Hess's Law states that the total enthalpy change of a chemical reaction is the same regardless of the number of steps in the reaction or the pathway taken, because enthalpy is a state function. This principle allows one to calculate the overall reaction enthalpy by summing the enthalpy changes of individual steps or intermediate reactions.

Bond Dissociation Energy

Bond dissociation energy is the energy required to break a specific chemical bond in a molecule in the gas phase. It is a measure of bond strength and is crucial in thermochemistry for calculating the energy needed to break bonds in reactants and the energy released upon forming new bonds in products.

Transcript

00:01 So for this problem, we're given three thermochemical equations, and we want to find the delta h for this desired equation.

00:16 The three equations that we are given are the following.

00:26 This should be that.

00:29 And the delta h for this reaction is 436 .4 kilojoules per mole.

00:37 Second reaction we are given is this.

00:49 Delta -h for this reaction is 192 .5 kilojoules per wall.

00:55 And the final reaction is, and the heat of this reaction is going to be negative 72 .4 kilojoules per wall.

01:16 So the first thing that we're going to do is we're going to look at each of these reactions to find which reactants correspond to which reaction.

01:24 So the first one that we want is this singularly atomic hydrogen at this gas molecule.

01:32 And the only equation in which that appears is the first one.

01:38 Now there's a couple of problems here.

01:40 The first is that there's two moles of h while we only want one mole and it's on the wrong side.

01:45 Instead of being on the reactants, it's on the products.

01:48 So to correct for that, we're going to multiply this entire equation by negative one -half, which will correct both of those things for us.

01:57 The second thing that we want is we want this singularly atomic br.

02:04 Once again, the only equation that has that is the second one.

02:10 And we have the exact same problem here.

02:12 There's two moles instead of one mole and it's on the left side instead of the right side...

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