When scientists at UC Santa Cruz directed UV radiation at an...

When scientists at UC Santa Cruz directed UV radiation at an ice crystal containing methanol, ammonia, and hydrogen cyanide, three amino acids (glycine, alanine, and serine) were detected among the products of photochemical reactions. The formation of these amino acids suggests that they may also be synthesized in comets approaching the Sun (and Earth). Determine the standard free-energy change of the hypothetical formation of glycine in comets, using standard free energies of formation of the reactants and products in this reaction $$\left[\Delta G_{\mathrm{f}}^{\circ} \text { for } \mathrm{HCN}(g)\right.$$is \right.$+125 \mathrm{kJ} / \mathrm{mol}$ and for solid glycine is $-368.4 \mathrm{kJ} / \mathrm{mol} ;$ other $\left.\Delta G_{f}^{\circ} \text { values are in Appendix } 4\right]$ $$\mathrm{CH}_{3} \mathrm{OH}(\ell)+\mathrm{HCN}(g)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}(s)+\mathrm{H}_{2}(g)$$

When scientists at UC Santa Cruz directed UV radiation at an ice crystal containing methanol, ammonia, and hydrogen cyanide, three amino acids (glycine, alanine, and serine) were detected among the products of photochemical reactions. The formation of these amino acids suggests that they may also be synthesized in comets approaching the Sun (and Earth). Determine the standard free-energy change of the hypothetical formation of glycine
in comets, using standard free energies of formation of the reactants and products in this reaction $$\left[\Delta G_{\mathrm{f}}^{\circ} \text { for } \mathrm{HCN}(g)\right.$$is \right.$+125 \mathrm{kJ} / \mathrm{mol}$ and for solid glycine is $-368.4 \mathrm{kJ} / \mathrm{mol} ;$ other $\left.\Delta G_{f}^{\circ} \text { values are in Appendix } 4\right]$
$$\mathrm{CH}_{3} \mathrm{OH}(\ell)+\mathrm{HCN}(g)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}(s)+\mathrm{H}_{2}(g)$$

Key Concepts

Standard Free Energy of Formation

This concept refers to the change in free energy when one mole of a compound is formed from its elements in their standard states. It serves as a fundamental baseline value for assessing the thermodynamic properties of substances and is crucial in determining whether a reaction will release or require energy under standard conditions.

Standard Free Energy Change of a Reaction

The standard free energy change of a reaction (?G°) is calculated by taking the difference between the sum of the standard free energies of formation of the products and that of the reactants. This value indicates the thermodynamic favorability and spontaneity of the reaction, with a negative value suggesting that the reaction is exergonic under standard conditions.

Transcript

00:04 So we have a question concerning the free energy change for the production of glycine in a comet, basically.

00:19 And we are given the chemical reaction.

00:22 And the glycine and the h2o are on the product side, and the methanol and the hcn.

00:39 And the water are on the reactant side.

00:44 So the scheme for this calculation is to add up the free energy of formation values for each of the products first.

00:58 And so the glycine and the h2 first, and then subtract from that the sum of the values for the reactants, the methanol ch -30h and the hcn, which the value was given for the hcn, and the water.

01:18 The value was also given for the glycine, so we filled that one in.

01:24 And we should really know that since these are the values for producing the substances from the what we call the elements in their standard states.

01:40 And in the case of hydrogen, that would be h2.

01:43 So in a sense, we don't really need to look it up for hydrogen.

01:51 Although we can, you can look it up in the table.

01:54 But it's always zero for an element in its standard state.

01:57 And then we go and look up the value for the methanol.

02:00 The value was given for the hcn, and we look up the value for water.

02:08 Now, it's a good idea to be careful to write in the physical state because you will find a value for water in both the liquid state and the vapor phase, the gas state.

02:25 And you do need to be very careful that the physical state, that you're looking up is the one in the question...

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