Classify each of the following reactions as one of the four...

Classify each of the following reactions as one of the four possible types summarized in Table 19.3 : (a) $\mathrm{N}_{2}(g)+3 \mathrm{~F}_{2}(g) \longrightarrow 2 \mathrm{NF}_{3}(g)$ $$ \Delta H^{\circ}=-249 \mathrm{~kJ} ; \Delta S^{\circ}=-278 \mathrm{~J} / \mathrm{K} $$ (b) $\mathrm{N}_{2}(g)+3 \mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NCl}_{3}(g)$ $$ \Delta H^{\circ}=460 \mathrm{~kJ} ; \Delta S^{\circ}=-275 \mathrm{~J} / \mathrm{K} $$ (c) $\mathrm{N}_{2} \mathrm{~F}_{4}(g) \longrightarrow 2 \mathrm{NF}_{2}(g)$ $$ \Delta H^{\circ}=85 \mathrm{~kJ} ; \Delta S^{\circ}=198 \mathrm{~J} / \mathrm{K} $$

Classify each of the following reactions as one of the four possible types summarized in Table 19.3 :
(a) $\mathrm{N}_{2}(g)+3 \mathrm{~F}_{2}(g) \longrightarrow 2 \mathrm{NF}_{3}(g)$
$$
\Delta H^{\circ}=-249 \mathrm{~kJ} ; \Delta S^{\circ}=-278 \mathrm{~J} / \mathrm{K}
$$
(b) $\mathrm{N}_{2}(g)+3 \mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{NCl}_{3}(g)$
$$
\Delta H^{\circ}=460 \mathrm{~kJ} ; \Delta S^{\circ}=-275 \mathrm{~J} / \mathrm{K}
$$
(c) $\mathrm{N}_{2} \mathrm{~F}_{4}(g) \longrightarrow 2 \mathrm{NF}_{2}(g)$
$$
\Delta H^{\circ}=85 \mathrm{~kJ} ; \Delta S^{\circ}=198 \mathrm{~J} / \mathrm{K}
$$

Key Concepts

Enthalpy Change (?H)

Enthalpy change refers to the heat absorbed or released during a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releasing heat, negative ?H) or endothermic (absorbing heat, positive ?H), which is fundamental in understanding the energy profile of a reaction.

Entropy Change (?S)

Entropy change measures the change in disorder or randomness in a system during a chemical reaction. A positive ?S indicates an increase in disorder (more dispersed energy), while a negative ?S signals a decrease in disorder, both of which influence the reaction’s spontaneity.

Gibbs Free Energy (?G) and Reaction Spontaneity

Gibbs free energy integrates both enthalpy and entropy changes to determine the spontaneity of a reaction. The equation ?G = ?H - T?S reveals that a negative ?G indicates a spontaneous process. This concept is crucial because it explains how both energy and disorder changes impact whether a reaction will proceed.

Thermodynamic Reaction Classification

This concept involves categorizing reactions based on the signs of their enthalpy and entropy changes, as summarized in classification tables such as Table 19.3. By knowing whether the reaction is exothermic or endothermic and whether it leads to an increase or decrease in entropy, one can classify the reaction type and predict the temperature dependence of its spontaneity.

Transcript

00:01 Let's see if we can determine the conditions under which reactions will or won't be spontaneous if we know their change in enthalpy and their change in entropy.

00:12 We're going to need to use the gibbs free energy equation, delta g0 equals delta h0 minus t delta s not, to be able to figure this out.

00:23 And so if we're given values, the values themselves right here are very important.

00:28 What we have to know is whether these values are positive or negative.

00:32 Remember, for a reaction to be spontaneous, delta g0 has to be negative.

00:38 Now, if we have a negative value for delta h0 and we have a negative value for delta s not, what that really means, because we have to look at the entire term here, that negative t delta s not, because temperature in temperature, kelvin is always positive, that negative times a negative means that this whole term is going to wind up being positive.

01:10 And so what does that mean for us? we need the whole thing to be negative and we have competing things here.

01:14 We have a negative term in enthalpy and a positive term with t delta s.

01:19 And so what that means is we need a low value of t and a low value of t would wind up giving a positive that's less positive than this negative term, giving an overall negative delta g.

01:35 So to make this work, this is going to be spontaneous, but it's only going to be spontaneous with low temperature.

01:48 Low enough temperature will make this positive term low enough to make the overall reaction negative.

01:57 Let's take a look at a couple more examples of that.

02:01 And so we'll take a look at nitrogen again, but this time we'll add chlorine and we'll make ncl3.

02:18 We have to take a look at these values.

02:20 Now, if you notice the delta h for this one has a different sign.

02:25 It's going to be positive for 60 kilojoules.

02:29 And the delta s is still negative...