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Baking soda (powder) will react when it comes into contact with a solution of citric acid crystals dissolved in a liquid medium such as water. But why doesn't baking soda react when mixed with a solid form of an acid (citric acid crystals)? Why is water needed for a reaction to occur?

Baking Soda (Solid) + Citric Acid Solution (Liquid) = Fizzy Reaction

Baking Soda (Solid) + Citric Acid Crystals (Solid) = No Reaction (??)

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    $\begingroup$ The solids very likely do react, actually, just to a very limited extent since the contact between the substances is small. However, if you were to grind the powders in a mortar, the reaction should proceed to completion after enough effort. An indication would be that the initially dry powders would become clumpy (maybe even soggy) from the water produced in the reaction. $\endgroup$ – Nicolau Saker Neto Nov 29 '14 at 20:15
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How do you know a reaction is not occurring when you mix the two solids? The only cue we have that a reaction occurs in the solution state is the formation of bubbles of escaping carbon dioxide. If there is no liquid to displace, would we be able to tell if carbon dioxide was being produced? Yes, but not with the naked eye.

The real issue is that the solid state reaction is much, much slower.

To produce carbon dioxide, the bicarbonate ion needs to come in contact with an acidic proton on citric acid. This is unlikely in the solid state since there is very little kinetic energy.

In solution, there is more kinetic energy, so the molecules are more likely to encounter each other. Additionally, the proton transfer can propagate quickly through the hydrogen bond network in water.

$$\ce{HCO3- \bond{...} H-OH\bond{...}H-OH\bond{...}H-OH\bond{...}H-OH\bond{...}H-Cit}$$

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    $\begingroup$ Is there really more kinetic energy in the liquid state compared to the solid? I thought kinetic energy depended only on temperature, not on the phase of matter. $\endgroup$ – Nicolau Saker Neto Nov 29 '14 at 20:15
  • $\begingroup$ @NicolauSakerNeto - Total internal energy perhaps is dependent only on temperature. However, which fraction is kinetic energy is going to also depend on the identity of the substance. In a "perfect" gas-like substance with no forces between particles, almost all of the energy will be kinetic. Water is not any such thing. A lot of the energy in water is potential energy stored in the intermolecular attractions. $\endgroup$ – Ben Norris Nov 29 '14 at 20:23
  • $\begingroup$ $\ce{KE=\frac{1}{2}mv^2}$, where the velocity will depend not only on the temperature, but also on the strength of intermolecular forces. Particles with strong interactions (both attractive and repulsive) will have inelastic collisions, and thus kinetic energy and potential energy will interconvert when particles collide. $\endgroup$ – Ben Norris Nov 29 '14 at 20:27

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