Materials Included In Kit

Additional Materials Required

Prelab Preparation

Cutting Iron Sheets: Using the provided heavy-duty scissors, cut each galvanized iron sheet into four 3 x 3 cm squares. Each lab group will get 10 squares.

Safety Precautions

Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Sulfuric acid is severely corrosive to eyes, skin and other tissue. Avoid contact with skin and eyes. The pieces of galvanized metal may have sharp edges that can cut skin. Handle the metal pieces with forceps. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess acids may be neutralized with base and washed down the drain with excess water to Flinn Suggested Disposal Method #24b. Flush all neutralized solutions down the drain with an excess of water according to Flinn Suggested Disposal Method #26b.The used metal pieces may be disposed of in the solid waste (trash) according to Flinn Suggested Disposal Method #26a.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The laboratory work for this experiment can be completed in one 50-minute class period. The pre-laboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• The higher concentrations of acids will have some splattering as the coating reacts. Make sure students have their materials clear of the immediate area.
• A slight bend in the metal will allow the bottom of the sheet to react more completely with the acid. The bend must be slight so that is does not go above the 20 mL of liquid. You may want to bend one square to show students the bend.
• Toward the end of the reaction, the solution in the beaker will be very hazy. Students will need to slightly agitate the beaker by swirling very gently to clear the solution and verify that the reaction is complete.
• It is strongly recommended that the students observe one sheet at a time. Carefully timing each sheet as its zinc is removed is crucial for accurate graphs.
• If you are worried about splattering from the reaction, students can place a watch glass over the top of the beaker. The lip of the beaker will allow enough of the gas to escape.

Teacher Tips

• Check with your technical education department. They may be able to cut the galvanized sheets for you.
• Galvanized iron was first introduced in France in the early 1800s. Modern methods for the preparation of galvanized iron utilize the electrochemical plating of zinc onto iron. Iron is placed into a solution of a zinc salt, and electricity is passed through the solution to reduce the zinc ions to metallic zinc. The term “galvanized” iron honors Luigi Galvani, a pioneer in the study of electricity.
• A nice extension of this lab is to have students collect the hydrogen gas produced by the reaction in a syringe. With this, they can measure the change in volume of the gas as the reaction proceeds. You can make this an inquiry-based lab by having the students design the apparatus to collect and measure the gas.

Answers to Prelab Questions

1. Hydrochloric acid and sulfuric acid are strong acids. What hazards are associated with the use of these acids, and what safety precautions must be followed?

Hydrochloric acid and sulfuric acid are toxic and corrosive and can cause severe skin burns. Wear chemical splash goggles, chemical resistant gloves and a chemical-resistant apron or lab coat to prevent contact of the acids with skin, eyes and clothing.

2. What reaction observations will be used to determine when the zinc coating has completely reacted?

Zinc reacts with HCl, and one of the products is hydrogen gas, which will bubble out of the solution. Once the bubbles stop, the zinc has been completely used up.

3. Sacrificial anodes have a similar purpose to galvanization. Define the term sacrificial anode and how it works. In what applications are zinc anodes commonly used?

A sacrificial anode is made from a metal alloy with a more negative electrochemical potential than the metal of the structure it is protecting. The difference in potential between the two metals means the sacrificial anode material will corrode instead, stopping oxidation reactions on the metal structure that is being protected. Zinc is typically used for structures that are exposed to saltwater like boat hulls and off-shore pipelines.

4. Use the rate law shown here to answer the questions that follow.

Rate = k [A][B]²

1. What is the order of each reactant and the overall order?

   Reactant A is first order. Reactant B is second order. The reaction is third order overall.

2. If the concentration of reactant B is doubled, what affect will this have on the rate of the reaction?

   Since B is second order, the rate of the reaction will increase by a factor of four if the concentration of B is doubled.

Sample Data

A. Dilution Preparations

Show all work.
M₁V₁ = M₂V₂
(6 M)V₁ = (2 M)(20 mL)      V₁ = 6.7 mL acid
(6 M)V₁ = (3 M)(20 mL)      V₁ = 10 mL acid
(6 M)V₁ = (4 M)(20 mL)      V₁ = 13.3 mL acid
(6 M)V₁ = (5 M)(20 mL)      V₁ = 16.7 mL acid

B. Zinc Coating Removal

Reaction Observations

C. Determining Reaction Order 

Reactant Order
HCl: 2nd order
H₂SO₄: 2nd order 

Rate Constant (k)
HCl: 1.03 x 10^–3 M^–1s^–1
H₂SO₄: 2.16 x 10^–3 M^–1s^–1

Graphs

Answers to Questions

1. Not all galvanized materials are coated with the same thickness of zinc. What is the purpose behind this? (Hint: Think about environmental factors).

The thicker the coating, the longer it will take the HCl to remove it. Objects that have a thicker coating are typically used in applications in which they are more exposed to the outside environment. Items that are protected from these effects will have a thinner coating.

2. Answer the questions that follow for each acid.

1. Write the complete rate law, including the rate constant and order.

   HCl: Rate = k[HCl]² = (1.03 x 10^–3 M^–1s^–1) [HCl]²
   H₂SO₄: Rate = k[H₂SO₄]² = (2.16 x 10^–3 M^–1s^–1) [H₂SO₄]²

2. If the concentration of each acid is 0.5 M, calculate the rate of the reaction.

   HCl: Rate = (1.03 x 10^–3 M^–1s^–1) [0.5 M]² = 2.58 x 10^–4 M•s^–1
   H₂SO₄: Rate = (2.16 x 10^–3 M^–1s^–1) [0.5 M]² = 5.4 x 10^–4 M•s^–1

3. A student conducted the experiment performed in this lab using hydrobromic and oxalic acid instead of hydrochloric and sulfuric acid. The data is shown in the following table. Provide a reasonable explanation for why the coating removal timing differs between the two acids.

Even though oxalic acid is a diprotic acid and might be expected to react faster, both of its protons are weak. This is in contrast to the hydrobromic acid, which is strong, leading to a higher concentration of hydronium ions in solution and a faster reaction rate.

Introduction

You may have come across the term “galvanized” at the hardware store. It can be seen on everything from nails to washers to roofing sheets. But what does it mean to galvanize something? In this experiment, we will take a closer look at galvanized coating and the kinetics of removing it.

The video will help you better understand collision theory and how different experimental conditions can affect the rate of a reaction.

Concepts

• Kinetics
• Reaction rates
• Reaction order
• Single replacement reaction

Background

Galvanized iron is produced by coating iron with a very thin layer of metallic zinc. The zinc coating protects the underlying iron metal against rusting or corrosion. Metals have different levels of reactivity and are sorted according to the substance with which they react. Reactions fall under three categories: reacts with water, reacts with acids and unreactive. These rankings are used to determine the products in a single replacement reaction. One metal will replace another if it has a higher reactivity. Looking at Table 1, zinc is more reactive than iron and thus reacts with oxygen in the air and with water before the iron does.

The zinc coating prevents oxygen from reaching the iron by forming a zinc oxide passivation layer, preventing further corrosion. The greater reactivity of zinc continues to protect iron even after the surface of the zinc has been broken or breached. Galvanized iron has many applications, including rain gutters, heating ducts, nails and screws. Zinc reacts with hydrochloric acid according to Equation 1.

The products of the reaction are zinc chloride, which dissolves in the hydrochloric acid solution, and hydrogen gas, which bubbles out of the solution. A prolonged exposure of zinc with hydrochloric acid will cause the coating to degrade, exposing the metal beneath. While galvanized coatings help prevent rusting and corrosion, they are not impervious to environmental factors.

One prominent factor that affects the coating is acid rain. While acid rain is not concentrated hydrochloric acid, over time it can wear down the coating. Depending on the usage, the galvanized coating can be applied at different thicknesses. This will alter the rate of coating removal due to environmental factors.

The rate of a chemical reaction is a measure of how fast the reaction occurs. Some chemical reactions occur as soon as the reactants come in contact with each other, such as acid–base reactions. Other chemical reactions can take years to occur, such as the oxidation and corrosion of iron. All rates are measured in terms of the time it takes to complete an event. In a chemical reaction, the event that is completed is the conversion of reactants to products. A chemical reaction rate is measured in terms of the rate of disappearance of reactants and appearance of products. The reaction is complete when all of one or more reactants has been consumed and converted to products. Since the reaction produces hydrogen gas as a product, we can measure the amount of time it takes for the hydrogen bubbles to cease. This will let us know that no more hydrogen gas is being produced and the reactants have run out.

It is important to remember that only substances in the aqueous or gas phase are included when writing rate laws. Pure solids and pure liquids are omitted because their concentrations will not change. If you know the rate constant, order can be determined by the units of that constant.

The overall rate of a chemical reaction may depend on the concentrations of one or more of the reactants, or it may be independent of the reactant concentrations. Exactly how the rate depends on reactant concentration is expressed in an equation called a rate law. For a general chemical equation, such as

the general rate law would be written as

where k is the rate constant, [A] and [B] are the molar concentrations of each of the reactants and n and m are exponents that determine how the rate depends on the reactant concentrations. The rate constant and the exponents n and m must be determined experimentally—they cannot be determined simply by looking at the balanced chemical equation. The rate constant for a reaction does not depend on the reactant concentrations, but it does depend on temperature. The exponents n and m give the order of the reaction. The reaction in Equation 3 is said to be nth order with respect to A and mth order with respect to B. The overall reaction order is obtained by taking the sum of n + m. Generally, the exponents n and m are positive whole numbers; however, they may contain fractions or even be negative numbers. Because the exponents n and m vary from reaction to reaction, rate laws for different reactions take on different forms, see Table 2.

To find the order and rate constant for a given reactant, the linear plot information for each order can be graphed. Whichever produces a straight line indicates the order with respect to that reactant. The rate constant can be determined from the slope of the line.

Experiment Overview

In this experiment you will measure the rate at which galvanized coating is removed from steel sheets using two different types of acids, then determine the reaction order of each.

Materials

Prelab Questions

1. Hydrochloric acid and sulfuric acid are strong acids. What hazards are associated with the use of these acids, and what safety precautions must be followed?
2. What reaction observations will be used to determine when the zinc coating has completely reacted?
3. Sacrificial anodes have a similar purpose to galvanization. Define the term sacrificial anode and explain how it works. In what applications are zinc anodes commonly used?
4. Use the rate law shown here to answer the questions that follow.

Rate = k [A][B]²

1. What is the order of each reactant and the overall order?
2. If the concentration of reactant B is doubled, what affect will this have on the rate of the reaction?

Safety Precautions

Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Sulfuric acid is severely corrosive to eyes, skin and other tissue. Avoid contact with skin and eyes. The pieces of galvanized metal may have sharp edges that can cut skin, handle with forceps. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part A. Dilution Preparation

1. Obtain 10 galvanized iron squares from your teacher.
2. Being careful not to touch the sharp edges, give each square a slight bend. Don’t bend it too far or it will not be completely covered when placed in the acid.

3. With a starting concentration of 6 M, calculate the dilutions you need to produce 20 mL each of the acid concentrations listed in your data table. This will be the same for both hydrochloric and sulfuric acids since you are using the same concentrations. Have your teacher verify your calculations before moving forward.
4. Accurately measure 66 mL of 6 M HCl into a 150 mL beaker.
5. Prepare each of the HCl solutions according to your calculations in separate 150 mL beakers. Note: When diluting acids, it is important to always add acid to water, not the other way around.

Part B. Zinc Coating Removal

6. Carefully place a metal square in the 2 M HCl beaker using forceps while your partner simultaneously starts the timer. Avoid dropping in the metal in to prevent splashing acid.
7. Record how long it takes for the square to cease bubbling, and record this time on your data sheet. Also record your observations of the reactions.
8. Once the metal stops reacting (vigorously bubbling), carefully add about 100 mL of cold tap water to the reaction beaker. This will dilute the acid solution and fully stop the reaction.
9. Pour off the diluted acid into a waste beaker as directed by your teacher.
10. Remove the metal from the beaker with forceps. Holding the metal with the forceps, rinse the metal thoroughly with tap water.
11. Return the used metal pieces to your teacher for disposal.
12. Repeat steps 6–11 with each of the HCl concentrations.
13. Repeat steps 2–12 for the sulfuric acid dilutions.

C. Determining Reactant Order

14. Using the line information in Table 2, calculate the concentrations of HCl and H₂SO₄ to be graphed for each order. (These will be the same since the concentrations for each were identical).
15. Convert your time values into seconds.
16. Graph each acid versus time for zero, first and second order. You will have three graphs for each acid, six graphs in total.
17. The order is determined by whichever graph shows the most linear trend. Add a best-fit line and have the R-squared value displayed. You want this number to be close to 1, indicating the most linear graph.
18. Display the line equation on the graph to obtain the slope for the linear graph.
19. Record this information, including units when necessary, on your data sheet.
20. (Optional) Check out the lab video to learn how to coat pennies in zinc to turn them silver, then heating them to a golden shine.