Friday, November 28, 2014

Sulfuric Acid and Sugar

November 28, 2014, The Canberra Times reported that, "Students were looking on as a teacher conducted an experiment involving sugar and sulfuric acid inside a cabinet when the glass container holding the acid exploded."
The experiment referred to is probably similar to the one shown in the YouTube video below in which concentrated sulfuric acid from a container is poured over sugar in a beaker.

Safety Notes
Sulfuric acid will cause permanent damage if it comes into contact with the eyes or skin.
Concentrated solutions of sulfuric acid are extremely corrosive. When sulfuric acid is dissolved in water enough heat is released to make water boil!
Carbon monoxide and carbon dioxide are both toxic gases.
Sulfur dioxide gas is toxic in high concentration and is a severe respiratory irritant at lower concentration.Some people, especially those prone to asthma, may be especially sensitive to sulfur dioxide. In the presence of moisture, sulfur dioxide forms an acidic, corrosive solution, which in contact with the skin or eyes may lead to burns.

You will notice that the reaction seems to proceed slowly at first. The reaction mixture turns yellow as the reaction begins. This reaction releases heat, it is said to be an exothermic reaction. The heat produced by the reaction then speeds up the rate of further reactions, and, in the video, this is also accelerated by stirring the mixture.

Table sugar is made up sucrose, molecular formula C12H22O11 and structural formula as shown below:

The reaction between sucrose and sulfuric acid in which solid carbon, water vapour and heat are produced is known as a dehydration reaction  :
C12H22O11(s) → 12C(s) + 11H2O(g)     ΔH = -918.9 kJ mol-1
Since the enthalpy change for this reaction is negative, the reaction is exothermic, the reaction gives off heat.
Sulfuric acid molecules have a great affinity for water, that is, sulfuric acid will readily and spontaneously dissolve in water. The water produced by the dehydration of sucrose will then be used to dilute the sulfuric acid that is present. This reaction is also exothermic.
H2SO4(l) → H2SO4(aq)     ΔH = - kJ mol-1
The heat produced by this reaction also speeds up the rate of the dehydration reaction and subsequent dilution reactions.
Solid carbon is black, so the "black snake" is just carbon.
But what causes the carbon to "rise up" out of the beaker? This must be the result of evolving gases forcing their way through the mixture as the reaction proceeds. The gases that have been identified as products of this reaction  are:
  • carbon monoxide (CO) 66% of the dry gas volume (ie, water has been condensed out)
  • carbon dioxide (CO2) 17% of the dry gas volume
  • sulfur dioxide (SO2) 17% of the dry gas volume
Since sulfuric acid does not oxidize carbon, it is most unlikely that the carbon monoxide and carbon dioxide gases are the result of a reaction between the black snake carbon and sulfuric acid. It is much more reasonable to assume that some of the sucrose undergoes dehydration by the sulfuric acid while some this sucrose (and/or some of the resulting intermediate organic products) is oxidized by the sulfuric acid to produce carbon monoxide gas and carbon dioxide gas. During this process, the sulfuric acid will itself be reduced, resulting in the formation of sulfur dioxide gas.

Further Reading:

Suggested Study Questions:
  1.  Describe two hazards in the YouTube Video.
  2.  Describe the safety precautions you would take to minimize the risk of the hazards identified in question 1 above.  
  3.  Why do you think the concentrated sulfuric acid is added to the sugar rather than adding the sugar to the container of sulfuric acid?
  4.  In a typical experiment, 25 mL of 18 mol L-1 sulfuric acid is added to 50 g of granulated sugar (sucrose). Calculate the amount in moles of :
    • sucrose used
    • sulfuric acid used
  5. Calculate the mass of carbon that could be produced in the typical experiment given in question 4.
  6. What assumptions have you made in order to calculate the mass of carbon in question 5?
  7. Assume that all 50 g of the sucrose is now oxidized at 25oC to produce carbon dioxide gas and liquid water. What is the maximum volume of carbon dioxide gas, in litres, that could be produced?
  8.  Predict what you think might happen if a 50 g of granulated sugar were quickly added to 25 mL of concentrated sulfuric acid in a 100 mL conical flask that was being swirled continuously.
  9. Sucrose is a disaccharide, made up of the monosaccharide glucose and the monosaccharide fructose. Starch and cellulose are both polysaccharides, that is, they are made of repeating glucose monomer units. Predict what would happen if you spilled concentrated sulfuric acid on:
    • a paper cup (cellulose)
    • a piece of raw potato (starch)
    • a cotton shirt (cellulose)
  10. Design an experiment that could safely be performed in the laboratory to test your predictions in question 9.
  11. Do you think that concentrated sulfuric acid will react with the monosaccharide glucose? Explain your answer using a chemical equation.
  12.  Design an experiment that could safely be performed in the laboratory to test your prediction in question 11.

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