Science Buddies: "Ask an Expert"

I put one cm of magnesium ribbon into 20 mL of hydrochloric acid. As the hydrochloric acid was heated up to a higher temperature, would you expect the magnesium ribbon to dissolve faster or slower and why?

pockysticks787,

Welcome to the forum and thank you for your question. Whilst this forum is normally reserved for science fair projects, and your question seems related to homework or research, I'll try to provide you a basic answer.

Magnesium and hydrochloric acid react according to the chemical equation: Mg + 2HCl => MgCl2 + H2.

This is an exothermic reaction, meaning that the forming of the bonds MgCl2 and H2 gives off heat. While not exactly correct to do so, I'll write "heat" as a product of the reaction to read:

Mg + 2HCl => MgCl2 + H2 + heat

You can see that if you externally add heat to the left side of the equation it will shift the equilibrium of the reaction to the left in favour of the reactants, slowing the formation of bonds in the products on the right side of the equation. If you want a fast reaction, cool the HCl solution down, shifting equilibrium to the right in favour of the products. The reaction with Mg will quickly "try" to warm it up by forming chemical bonds and generating heat.

Think of it this way: it's harder to heat up a room that is already warm. Heat likes to transfer from warm to cold. The greater the temperature difference, the faster that heat transfer (or in our case the chemical reaction) occurs.

Point of note: the opposite is true for an endothermic reaction. So knowing what kind of reaction you're going to have is quite important.