CHEMISTRY NOW: Chemistry of Leavening Agents in Bread-Making (Grades 9-12) Print

Objective:

Students will draw conclusions about leavening agents and their implications for the food industry. Students will balance equations relating to leaving agents used to make bread. Students will identify the acids and bases in a chemical reaction. Students will collaboratively design an experiment that collects data about leavening agents.


Introduction Notes:

 

CHEMISTRY NOW: Chemistry of Leavening Agents in Bread-Making (Grades 9-12)

 

 

Subject Area:           Chemistry

Grade Level:            High School Chemistry

Lesson Title:            Chemistry of Leavening Agents in Bread-Making

 

National Science Education Standards:

  • Science as Inquiry: Abilities to Do Scientific Inquiry, 9–12

  • Science as Inquiry: Understanding About Scientific Inquiry, 9–12

  • Structure and Properties of Matter: 912
  • Chemical Reactions: 912

 

Suggested Prior Knowledge: acidbase reactions, identification of acids and bases, moles, balanced equations, formulas

 

Purpose: To enrich student concept knowledge of chemical reactions in a comparative study of leavening agents used in bread-making

 

Key Vocabulary: 

 

acida chemical compound that dissociates in solution, releasing hydrogen ions and lowering the solution pH (a proton donor)

 

basea chemical species that donates electrons or hydroxide ions or that accepts protons

 

decomposition reaction—the breakdown of a compound into its corresponding elements or smaller compounds, e.g., electrolysis of water 2H2O(I) → 2H2 + O2

 

      leavening agentincorporates gas bubbles into doughs and batters; usually carbon dioxide produced by biological agents, or by chemical agents reacting with moisture, heat, acidity, or other triggers.

 

neutralization reactionfor purposes of this lesson plan, we will refer to the point at which 1 mole of hydrogen ions reacts with one mole of hydroxide ions, or the neutralization of an acid and a base to produce water.

 

 

Objectives:

  1. Students will draw conclusions about leavening agents and their implications for the food industry.
  2. Students will balance equations relating to leaving agents used to make bread.
  3. Students will identify the acids and bases in a chemical reaction.
  4. Students will collaboratively design an experiment that collects data about leavening agents.

 

Materials (each group should have access to the following):

- goggles and formal safety procedures

- student worksheet

- 20 g of sodium bicarbonate (baking soda)

- 20 g of potassium hydrogen tartrate (cream of tartar)

- 20 g of baking powder containing sodium aluminum sulfate and sodium bicarbonate

- 20 g of yeast

- 20 g of glucose or sucrose (sugar)

- 20 ml of acetic acid (vinegar)

- digital scale

- scoopula

- clock with second hand (stopwatch)

- hot and cold tap water

- bucket or trough

- 2 one-hole rubber stoppers

- 2 short pieces of glass tubing to fit stoppers

- 250 ml Erlenmeyer flask

- approx. 100 cm of plastic tubing to fit over glass tubing

- filter paper or paper

- 100 ml beaker

 

Procedure:

 

1. Introduce students to what they will be doing. They will be conducting an experiment that tests and compares three leavening agents used to make bread. Refresh student concept knowledge with a discussion about molarity, products of acid–base reactions, key vocabulary, or additional concepts relating to this lab. If possible, have students conduct some prior research about leavening agents used to make bread or other products; if not, use the following question to get the class focused:

 

  • Has anybody ever noticed that bread has small air pockets inside (holes)? How do you think those holes get there? Leavening agents produce carbon dioxide that is trapped inside the dough when bread is cooked. Yeast, baking powder, and baking soda are leavening agents that produce carbon dioxide.

 

 

2. Show the chemical reactions for each of the three leavening agents and discuss the stoichiometry and the chemical reactions in each. Familiarize students with the reactions and what is happening in each. Balance the equations, identify the stoichiometric ratios, and identify the acids and bases in the following reactions. (A complete breakdown of these reactions is included in this lesson plan.)

 

 

__?__NaHCO3 + __?__KHC4H4O6 __?__KNaC4H4O6 + __?__H2O + __?__CO2

(already balanced; see background information)

 

__?__NaAl(SO4)2 + __?__NaHCO3__?__Al(OH)3 + __?__Na2SO4 + __?__CO2

(see background information for answer)

 

__?__C6H12O6 (aq) → __?__C2H5OH(aq) + __?__CO2(g)

(see background information for answer)

 

 

Questions students should be able to answer after these analyses include the following:

  • What is a common product in all of these reactions? (carbon dioxide)
  • Looking at these reactions, which do you predict would produce the most carbon dioxide or the fluffiest bread?
  • How many moles of each reactant are reactant are required in each reaction? (See the background to various reactions in this lesson plan.)
  • Looking at the reaction, is there a base or an acid involved? Can we identify them? (See Background to Identifying Acids or Bases in this lesson plan.)

 

Background to Identifying Acids or Bases:

The names of most acids end in acid. Acid formulas have one of these forms: HX(aq) or HaXbOc. Ionic compounds that contain hydroxide (OH), carbonate (CO32−), or hydrogen carbonate (HCO3) anions are basic. Ammonia, NH3, is also a base.

 

Background to Baking Soda and Cream of Tartar Reactions:

 

Baking soda (a base), also known as sodium bicarbonate, has the chemical formula NaHCO3. Cream of tartar (an acid), also known as tartrate salt, has the formula KHC4H4O6. The reaction is given by the following:

 

NaHCO3 + KHC4H4O6 KNaC4H4O6 + H2O + CO2

 

Baking soda is the source of the carbon dioxide, and the acid–base reaction in this case is more accurately described as an acid-activated decomposition of baking soda, which can be generically represented as:   NaHCO3 + H+ Na+ + CO2 + H2O

 

When dissolved in water baking soda separates into sodium (Na+) and bicarbonate ions (HCO3):

NaHCO3 Na+(aq) + HCO3(aq)

 

 

Potassium hydrogen tartrate (cream of tartar) solution in water partially dissociates into hydrogen (H+) and hydrogen tartrate ions (HC4H4O6):

KHC4H4O6 H+(aq) + HC4H4O6(aq)

 

The reaction between baking soda and cream of tartar is actually two reactions, an acid–base reaction followed by a decomposition reaction.

 

When the two ingredients are mixed, hydrogen ions (H+) from the cream of tartar react with the bicarbonate ions (HCO3) from the baking soda to form a new chemical called carbonic acid (H2CO3): H+ + HCO3 → H2CO3

 

The carbonic acid thus formed then immediately decomposes into carbon dioxide gas (CO2) and water (H2O): H2CO3 → H2O + CO2

 

 

 

Background to Baking Soda and Sodium Aluminum Sulfate Reaction:

Baking powders contain sodium aluminum sulfate: NaAl(SO4)2. You will notice that the generic formula for baking powder is the same as for baking soda and cream of tartar.

The reaction for baking powder is:

NaAl(SO4)2 + 3 NaHCO3 → Al(OH)3 + 2 Na2SO4 + 3 CO2

Baking soda and aluminum sulfate dissociate in water and, just as in baking soda with cream of tartar, the reaction is a two-step one. Sodium aluminum sulfate dissociates into Na+, Al+3, and SO4−2 ions during the reaction.

 

Background to Yeast, Glucose, and Water Reaction:

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)

 

The chemical equation summarizes the fermentation of glucose. One moleof glucose is converted into two moles of ethanoland two moles of carbondioxide:

sucrose +H2O + invertase (yeast enzyme) → glucose

glucose + zymase (yeast enzyme) → ethanol + carbon dioxide

C12H22O11 +H2O + invertase → C6H12O6

C6H12O6 + zymase → 2C2H5OH + 2CO2

 

Note: Before fermentation takes place glycolysis occurs, a biological process involving 10 intermediate steps producing high-energy compounds necessary in both aerobic and anaerobic respiration. The intermediate steps have been excluded in the chemical equation above. Glycolysis is summarized by the chemical equation C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO + 2 ATP + 2 NADH + 2 H2O + 2H+, and can be discussed further with students in an extension or enrichment activity to demonstrate the biological and chemical complexity of living things.

 

3. Facilitate a conversation with students to design an experiment that allows us to understand the pros and cons of the different leavening agents in bread making. The following questions may help:

 

  •  Leavening agents produce carbon dioxide. How can we design an experiment to test leavening agents to see how well they work? The experiment in this lab uses the collection of products (mainly carbon dioxide).
  •  What data should we collect?
  • What variables should we account for in our experimental design?

 

4. Once a class procedure has been designed, the experiments should resemble the following two examples:

 

Example 1: In a small beaker students test the varying leavening agents:

  • with or without water
  • with or without acid
  • at cool or warm temperatures

 

Example 2: Set up a gas collection apparatus to collect the carbon dioxide produced from the varying reactants as follows:

 

 

5. During the course of student experimentation, reinforce the concepts associated with each reaction. As students experiment, ask questions that refer to the chemical reactions discussed earlier. Asking these questions will prompt students’ thinking about what to include in their conclusions.

  • Why is that bubbling?
  • What has happened to the reaction rate with an increase in temperature?
  • Why doesn't baking soda react in water but baking powder does?
  • What happens to sodium bicarbonate when placed in water?
  • Are there ions in the solution you created?

 

6. Finally, have students make decisions as to where error may have been introduced into the lab.

 

Additional Resource:

http://www.5min.com/Video/How-Leavening-Works-83227000

 

 

 

 

 

Student Worksheet for Chemistry of Leavening Agents in Bread Making

 (Part 1: Reactivity and Rates of Reaction of Leavening Agents)

 

 

Experiment Title: ____________________Date: ______ Name: ____________________

 

Student Hypothesis: _______________________________________________________

Materials:

______________________________________________________________________

Procedure:

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

 

Data:

Sample ID

Sample mass

Sample mass

Observations:

Does a reaction occur?

Other

baking soda + water

 

 

 

 

baking soda + vinegar

 

 

 

 

baking soda + cream of tartar + water

 

 

 

 

yeast + water

 

 

 

 

yeast + vinegar

 

 

 

 

yeast + water + sugar

 

 

 

 

baking powder + water

 

 

 

 

baking powder + vinegar

 

 

 

 

other

 

 

 

 

 

 

Conclusions (Part 1) (use the back if necessary):

 

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________


 

 

 

Student Worksheet for Chemistry of Leavening Agents in Bread Making

 

(Part 2: Comparing Carbon Dioxide Produced From Leavening Agents)

 

 

Experiment Title: ____________________Date: ______ Name: ___________________

 

Student Hypothesis: _____________________________________________________________

Materials: _____________________________________________________________________

 

Procedure:

 

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________

______________________________________________________________________


Data:

Sample ID

Sample Mass

Sample Mass

Sample Mass

Ti

Tf

Observations:

Volume CO2

baking soda + water

 

 

 

 

 

 

 

baking soda + vinegar

 

 

 

 

 

 

 

baking soda + cream of tartar + water

 

 

 

 

 

 

 

yeast + water

 

 

 

 

 

 

 

yeast + vinegar

 

 

 

 

 

 

 

yeast + water + sugar

 

 

 

 

 

 

 

baking powder + water

 

 

 

 

 

 

 

baking powder + vinegar

 

 

 

 

 

 

 

other

 

 

 

 

 

 

 

other

 

 

 

 

 

 

 

 

 

Conclusions (Part 2):

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

_______________________________________________________________________

 

 

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