Coupled Reactions

Topics: Chemical reaction, Magnesium, Exergonic Pages: 6 (1286 words) Published: February 7, 2013
Coupled Reactions


A way to drive a non-spontaneous reaction forward is by coupling it with another reaction that is highly spontaneous, resulting to a spontaneous overall reaction. In this study, since the extraction of C(s) from CO2(s) is a non-spontaneous process, it was coupled with the oxide formation reaction of Mg, a reaction with high spontaneity, so that the process of the extraction of C(s) would proceed. The reactions and products which were a mix of black and white particles were keenly observed. The resulting overall reaction of the coupled reactions shows that the products are MgO(s) and C(s).


Preparation of the dry ice chamber

As a note, proper cryogenic gloves and a face mask should be worn. The first step is to gouge a hole, 3-4 cm in diameter, in the middle of the 2 slabs of dry ice, and 4 canals from the hole to the center of the four edges. After this, polish both slabs’ surface until they fit perfectly together.

Reaction between Mg and CO2

Place the dry ice on a towel and fill the cavity of one slab with 2g Mg ribbon. Then, light the Mg using a barbecue lighter. When it starts burning, quickly place the other slab on top. After that, step back, turn the lights off, and observe the reactions. Do not stare directly at the setup and inhale the produced vapor. When the glowing has stopped, remove the top slab and observe the products formed. If a white ball is attained, crack the ice open to observe the materials inside.

Proper waste disposal

Wrap all solid particles in a piece of paper and put them in the solid waste jar. Also, let the dry ice sublimate in an area with good ventilation.



1. It took a long time to light the Mg ribbons because in order for it to burn, the Mg ribbons need a certain amount of energy (or activation energy) given off by the flame. When the activation energy is reached, it will then start to burn. But the Mg is exposed to a cold environment, which increases the heat/energy needed to ignite it. Another factor is that the Mg ribbons are surrounded by a high amount of CO2(g) compared to the O2(g) which is needed to burn the Mg.

(Ophardt, 2010)

2. It is important to immediately cover the Mg ribbon with the other slab of dry ice once it starts burning because if not, since this reaction is spontaneous, it will continue to react with the O2(g) around, forming MgO(s), therefore lessening the number of moles of Mg(s) that would react together with the CO2(g) to form the desired product.

3. 2Mg(s) + CO2(g) → 2MgO(s) + C(s)

ΔGorxn = ΣnΔGproducts – ΣnΔGreactants

ΔGorxn = [2(–569.4) + 0] – [2(0) + –394.4]

ΔGorxn = –744.4 kJ/mol

A negative ΔGorxn implies a spontaneous reaction, and thus the products should be seen at the end of the reaction. As what the experimental observations show, after the glowing of the dry ice, the top slab was removed, and the products which was a mix of black and white matter were present; this proves the negative value of ΔGorxn.

4. Two other reactions that can be coupled with Mg

a. 2Mg(s) + O2(g) → 2MgO(s) ΔGo = [2(–569.4)] – [2(0) + 0] = –1139 kJ/mol

2ZnO(s) → 2Zn(s) + O2(g) ΔGo = [2(0) + 0] – [2(–318.3)] = +636.6 kJ/mol

2Mg(s) + 2ZnO(s) →2MgO(s) + 2Zn(s) ΔGo = –502.4 kJ/mol

b. 2Mg(s) + O2(g) → 2MgO(s) ΔGo = –1139 kJ/mol

2/3Cr2O3(s) → 4/3Cr(s) + O2(g) ΔGo = [(4/3)(0) + 0] – [(2/3)( –1058)] = +705.3 kJ/mol 2Mg(s) +2/3Cr2O3(s) →2MgO(s) +4/3Cr(s)ΔGo = –433.7 kJ/mol

5. ATP + H2O → ADP + Pi ΔGo = –31 kJ/mol => exergonic reaction

Pi + glucose → glucose-6-P + H2O ΔGo = +14 kJ/mol => endergonic reaction...
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