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I was doing a chemistry project to see if the pH of a carbonated solution effects the time taken for it to settle after being disrupted, and I am trying to map out the reaction when the components are combined.

I was able to figure out a few, but I got stuck on carbonic acid and lemon juice. For some reason, I was getting ethane as a by product of the reaction, and I'm sure that's not right. Could someone explain what I'm doing wrong?

the formula is $\ce{H2CO3 + C6H8O7}$

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    $\begingroup$ I'm assuming $\ce{C6H8O7}$ is citric acid. Acid + acid does not necessarily do anything productive. $\endgroup$ – Ben Norris Jan 20 '14 at 12:35
  • $\begingroup$ if you're getting ethane there is something wrong here. are you sure it is lemon juice? $\endgroup$ – Xiaolei Zhu Jan 20 '14 at 17:08
  • $\begingroup$ You should show what have you done...In my opinion there are no reactions between these two acid. When you make a lemonade you mix lemon juice that has a bit of citric acid with sparkling water that has carbonic acid and you don't smell ethane. $\endgroup$ – G M Jan 20 '14 at 17:12
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    $\begingroup$ Would you mind to explain how you determined that ethane was produced? $\endgroup$ – Klaus-Dieter Warzecha Jan 20 '14 at 21:07
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These two weak acids have similar disassociation constants in water. A low Ka indicates a relatively small number of acid molecules will form an anion stabilized by water. Two weak acids with similar disassociation constants will coexist in a solution without interacting. If two weak acids have very different Ka then there can be a reaction. In this case nothing would happen. For more generalized information search for mixture of weak acids.

Carbonic Acid

$$\ce{H2CO3 + H2O <=> HCO3- + H3O+}$$ $$K_a = 4.3 \times 10^{-7} = \frac{[\ce{H3O+}][\ce{HCO3^-}]}{[\ce{H2CO3}]}$$

Citric Acid

$$\ce{H3C6H5O7 + H2O <=> H2C6H5O7- + H3O+}$$ $$K_a = 1.8 \times 10^{-6} = \frac{[\ce{H3O+}][\ce{H2C6H5O7^-}]}{[\ce{H3C6H5O7}]}$$

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By Le Chatelier's principle ,a new equilibrium will be formed for the H3O+ concentration , since H2CO3 has the lower dissociation constant.H3O+ concentration and therefore the pH will lie somewhere between the two acids

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