# Reaction Rates: When Surface Area Matters!

## Summary

9th-12th
Group Size
2-4 students
Active Time
70 minutes
Total Time
70 minutes
Area of Science
Chemistry
Key Concepts
Reaction rate, chemical reaction, surface area, collision theory
Credits
Svenja Lohner, PhD, Science Buddies

## Overview

How does surface area affect the speed of a chemical reaction? Let your students find out in this sizzling lesson plan! In this project, they will explore this correlation by crushing Alka-Seltzer® tablets into different sized particles and measuring how long it takes for them to dissolve in water.

## Learning Objectives

• Understand how chemical reactions can be controlled and manipulated
• Calculate average rates of chemical reactions from experimental data
• Relate rates of chemical reactions to surface area of a reactant and frequency of collisions between reacting particles

## NGSS Alignment

This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
• HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
This lesson focuses on these aspects of NGSS Three Dimensional Learning:

 Science & Engineering Practices Disciplinary Core Ideas Crosscutting Concepts Science & Engineering Practices Planning and Carrying Out Investigations. Make directional hypotheses that specify what happens to a dependent variable when an independent variable is manipulated. Asking Questions and Defining Problems. Ask questions that can be investigated within the scope of the school laboratory, research facilities, or field (e.g., outdoor environment) with available resources and, when appropriate, frame a hypothesis based on a model or theory. Analyzing and Interpreting Data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution. Solutions. Make a quantitative and/or qualitative claim regarding the relationship between dependent and independent variables. Apply scientific ideas, principles, and/or evidence to provide an explanation of phenomena. Disciplinary Core Ideas PS1.B: Chemical Reactions. Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy. PS3.A: Definitions of Energy. These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as a combination of energy associated with the motion of particles and energy associated with the configuration (relative position of the particles). In some cases the relative position energy can be thought of as stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. Crosscutting Concepts Cause and Effect. Changes in systems may have various causes that may not have equal effects. Scale, Proportion, and Quantity. Algebraic thinking is used to examine scientific data and predict the effect of a change in one variable on another (e.g., linear growth vs. exponential growth). Stability and Change. Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible.

## Materials

Materials required for the reaction activity including: a measuring cup, tape, spoon, scale, stopwatch, paper, water, and alka-seltzer tablets.

Materials for teacher preparation and demonstration:

• Measuring cylinder (250 mL) or measuring cup
• Scale
• Optional: 8 cubes of the same size (any material works)

Materials per group of 2–4 students:

• Alka-Seltzer® tablets (or other effervescent tablets) (3)
• Timer or stopwatch
• Sheet of paper
• Heavy object to crush the tablet such as a hammer or metal spoon
• 250 mL beaker or 8 oz. cup
• Tap water
• Sink

## Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

A chemical reaction usually involves liquids, gases or solids. Some reactions happen very fast, whereas others seem to take ages. The speed of a chemical reaction is determined by its reaction rate. For many industrial applications, it is essential to be able to control reaction rates to ensure that processes happen fast enough to be economically viable, but at the same time not too quick, so as to prevent the risk of explosions. Studying chemical reaction rates allows students to investigate the factors that influence the speed of a reaction and explore reaction mechanisms in more detail.

How molecules or the reactants of a chemical reaction interact or react with each other is explained in the Collision Theory. This theory states that all reaction molecules are in constant motion and for a chemical reaction to occur they have to collide in order to form a product. A collision only leads to successful product formation if the molecules collide with sufficient energy as well as in the correct orientation. The collision frequency and the number of effective collisions determine how fast all the reactants are converted to the end product.

Any factor that affects the number of successful collisions will also change the speed of a reaction. This includes changing the number of reactant molecules (the reactant concentration) or the kinetic energy of the reactant molecules (the temperature), adding a catalyst or inhibitor to the reaction, and varying the nature of the reactants. In heterogeneous chemical reactions (whenever reactants are present in different phases), the reaction rate can also be changed by varying the surface area of the solid that reacts with the liquid or the gas. This is because molecule collisions can only happen at the surface of the solid, as all the other molecules are trapped within its body. If the same material is broken into smaller pieces, there is much more surface area exposed that is available for molecule collisions to occur (Figure 1).

Figure 1. Large particles (left) have less surface area available for reactions than smaller particles (right) of equal total volume. The total surface area increases with smaller particle sizes.

This surface area effect can be easily visualized by a simple experiment using effervescent tablets such as Alka-Seltzer® tablets. When these tablets are dropped into water, a chemical reaction happens that makes them dissolve and generate a lot of bubbles. These bubbles are carbon dioxide gas (CO2), which is produced when the Alka-Seltzer ingredients react. The main ingredients of Alka-Seltzer tablets are aspirin, citric acid, and sodium bicarbonate (NaHCO3). When sodium bicarbonate dissolves in water, it dissociates (splits apart) into sodium (Na+) and bicarbonate (HCO3-) ions. The bicarbonate reacts with hydrogen ions (H+) from the citric acid to form carbon dioxide and water as described by the following chemical equation:

Equation 1:

In this activity, students will measure how long it takes for an Alka-Seltzer tablet to completely dissolve depending on how big the pieces of the tablet are. As reaction rates are expressed as concentration change over time (Δ quantity/Δ t), and given that the Alka-Seltzer tablet dissolves throughout the reaction, students can calculate the average reaction rate (in grams of tablet dissolved over time) for each reaction according to Equation 2:

Equation 2:

where Δ indicates "delta", or "change in", t is time [s], and the squared bracket [ ] expresses the concentration of the reactant [mol/L], or the mass of the Alka-Seltzer tablet [g]. The obtained results will allow students to demonstrate the correlation between chemical reaction rates and the surface area of solid reactants.

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