Answer to Question #179341 in General Chemistry for Matthew Smith

Question

If baking soda (NaHCO

3

) and sidewalk deicer (CaCl

2

) are mixed, there is no sign of a

chemical reaction. If these two solids are dissolved in water and mixed, however, they

rapidly react. Explain the difference in reaction rate.

Answer 1

When the two reagents are in the powdered form, many of their particles do not collide with an amount of energy high enough to form a product. This energy is the minimum amount of energy required for reactant particles to transform to product after collision (collision Theory and Arrhenius Theory). For this reason, the rate of reaction is relatively slow.

However, when both reagents are in aqueous solution, their formula units (ionic particles) move freely and collide with an amount of energy (and orientation) favourable for them to transit to product. The rate constant for this process is relatively high, hence, the rate is faster in aqueous solution.