How would tearing the magnesium ribbon into smaller pieces affect the reaction rate?

Rate of reaction is the change in concentration of a reactant or a product with change in time. It is a measure of how fast a reaction takes place. The rate of reactant is dependant on nature of reactants, temperature, surface area of the reactants and pressure.

Greater the surface area of a reactant, more will be number of collisions that can take place between the reactant molecules. The number of reactant molecules exposed for the reaction to take place would increase with an increase in surface area. So, the rate of reaction would increase with an increase in surface area. Therefore, when Mg ribbon is torn into small pieces its surface area increases and so the rate of reaction will increase too.