Content Benchmark P.12.A.7
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Students know that, in chemical reactions, elements combine in predictable ratios, and the numbers of atoms of each element do not change. I/S

Chemical reactions occur constantly in our daily lives. By understanding what the equations for these reactions represent, we can put this information to practical use such as getting energy to heat our homes, driving our cars and trucks, and cooking many of the foods we eat. Some chemical reactions, such as digesting the food we eat, are so innate that we really don’t even think about them until we want to change our diet to eat more healthy foods or to lose or gain weight.

John Dalton, an English school teacher, proposed atomic theory in 1808. Dalton’s atomic theory explained the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. Four of Dalton’s main ideas are stated below
(from http://www.chemheritage.org/EducationalServices/webquest/dalton.htm).

1. All matter is composed of tiny particles, called atoms.
2. Each element is made of a different kind of atom, and the atoms of different elements have different masses.
3. Atoms are neither created nor destroyed in chemical reactions.
4. Atoms of different elements combine in number ratios, with more than one ratio being possible for a given combination of elements.

To learn more about John Dalton and his atomic theory, go to
http://www.chemheritage.org/classroom/chemach/periodic/dalton.html

Atomic theory helps explain how elements combine in predictable ratios, and the numbers of atoms of each element do not change. For an example, consider charcoal briquettes that are burning on a grill to release heat for cooking. Charcoal is a form of carbon. When it burns, the following reaction occurs.

Carbon reacts with oxygen to produce carbon dioxide

Expressing this in a chemical equation the reaction would be expressed as

C + O₂ CO₂

In terms of what is occurring with the atoms and their bonding, both broken and formed, the reaction can be represented as shown below.

C + O=O O=C=O

The ratio of carbon to oxygen atoms to form carbon dioxide is 1 carbon atom to 2 oxygen atoms. Carbon dioxide is composed of 1 carbon atom and 2 oxygen atoms. This equation is balanced. The atoms have been rearranged in their bonds, and the total of the reacting atoms (1 carbon atom and 2 oxygen atoms) equals the total of the atoms in the product (1 carbon atom and 2 oxygen atoms).

If we look at the burning of methane (CH₄), the major component in natural gas, the following chemical reaction occurs when methane burns and produces heat.

methane burns in oxygen to produce carbon dioxide and water

This reaction is represented in the chemical equation below as

CH₄ + O₂ CO₂ + H₂O

In this case, the equation is not balanced because there are different numbers of carbon, hydrogen, and oxygen atoms on both sides of the equation. To represent the relative numbers of each substance on the reactants and products side, the equation must be balanced as

CH₄ + 2 O₂ CO₂ + 2 H₂O

When balancing chemical equations, the coefficients in front of each chemical formula must be changed. This balanced equation has 1 atom of carbon, 4 atoms of hydrogen and 4 atoms of oxygen on both the reactants and products sides. No atoms of any element were created nor destroyed. Bonds were broken and reformed. When this reaction occurs, it will always be in the molecular ratio of 1 methane to 2 oxygen to 1 carbon dioxide to 2 water. If someone burned double the molecules of methane, then 2 methane would need 4 molecules of oxygen to produce the 2 molecules of carbon dioxide and 4 molecules of water. There would be the same total number of atoms of carbon, hydrogen, and oxygen of reactants as products. This is predictable ratio.

Figure 1. Representation of atoms of carbon, hydrogen, and oxygen when methane burns in oxygen to form carbon dioxide and water. The small dots on the water molecules represent unpaired electrons on oxygen. (from: http://www.elmhurst.edu/~chm/vchembook/511natgascombust.html)

To learn more about stoichiometric relationships in combustion, go to
http://itl.chem.ufl.edu/2045/lectures/lec_4.html.

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Content Benchmark P.12.A.7

Students know that, in chemical reactions, elements combine in predictable ratios, and the numbers of atoms of each element do not change. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think that whatever the subscript is for an element on the reactant side of the equation, the same subscript must be used for that element on the product side.

However, the correct formulas for the reactants and products must be written based on the names of each substance and/or oxidation numbers rather than just “dragging” subscripts from reactant side of equation to product side.

For more explanation and practice in balancing equations, go to
http://www.unit5.org/christjs/chemical_equations.htm.

2. Students can get confused about whether to balance chemical equations by changing coefficients (which is correct) or by changing subscripts (which is incorrect) in formulas.

Once the formulas of reactants and products are written correctly, the equation must be balanced by changing coefficients in front of the chemical formula. This makes the coefficient apply to everything in the formula that comes immediately after. Balancing is often done simply by inspection and trial and error when the equations are simple.

For more information about balancing equation misconceptions, go to
http://72.14.253.104/search?q=cache:21DO1yEqv8cJ:chemed.rice.edu/
IEinCE/diagnostics/Fall98/AMT/AMTresults.html+misconceptions+in+
balancing+equations&hl=en&ct=clnk&cd=4&gl=us.

3. Students often forget that gases have mass.

If students are burning magnesium in a loosely covered crucible in lab, they are often surprised when they mass the crucible and find that the mass of the magnesium oxide ash is greater than the mass of the magnesium. It seems counterintuitive to them because they are often familiar with burning logs in a fire and having the mass of ash be much less than the mass of the logs they burned. They forget that burning logs produce gaseous carbon dioxide and water vapor, which disperses into the air. When the magnesium burns, it is combining with oxygen from the air, and thus the magnesium oxide should have a greater mass than the mass of only the magnesium. The total mass of the reactants still equals the total mass of the products, and mass is conserved.

To see a demonstration of magnesium burning in the air and an explanation of what is occurring, go to http://boyles.sdsmt.edu/magburn/magnesium_burning.htm or to http://www.angelo.edu/faculty/kboudrea/demos/
burning_magnesium/burning_magnesium.htm.

For laboratory directions, go to
http://www.sciencepages.co.uk/keystage3/resources/magnesium%20ws.pdf.

4. Students try to balance equations by placing coefficients in the middle of a formula.

Coefficients may be placed only before the entire formula. If the ratio of atoms of each element in the formula were to change, then this would violate the law of definite proportions, which states that the ratio of atoms of each element in a compound is definite and constant.

For more information on balancing equations and some practice equations, go to http://dbhs.wvusd.k12.ca.us/webdocs/Equations/Balance-Equation.html.

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Content Benchmark P.12.A.7

Students know that, in chemical reactions, elements combine in predictable ratios, and the numbers of atoms of each element do not change. I/S

Sample Test Questions

1^st Item Specification: Explain how a chemical reaction satisfies the law of conservation of mass.

Depth of Knowledge Level 1

1. The formula below represents the chemical reaction between the elements hydrogen and oxygen when the compound water is formed.

This equation supports the law of conservation of mass because

1. the total number of hydrogen and oxygen atoms in the reactants and products is twelve.
2. the mass of hydrogen and oxygen in the reactants is equal to the mass of the water in the product.
3. atoms of the elements hydrogen and oxygen are in the reactants and also in the products.
4. atoms of the elements hydrogen and oxygen react to form molecules of the compound water.

2. Natural gas is a fossil fuel that can be burned to heat our homes. The reactants in this chemical reaction are natural gas and oxygen. The products are water and carbon dioxide. The word equation below can be used to represent this reaction

natural gas + oxygen water + carbon dioxide

Which of the following is true about the mass of the reactants and the products?

1. The mass of the natural gas and oxygen will be less than the mass of the water and carbon dioxide.
2. The mass of the natural gas and oxygen will be greater than the mass of the water and carbon dioxide.
3. The mass of the natural gas and oxygen will be equal to the mass of the water and carbon dioxide.
4. The mass of the natural gas and oxygen will be destroyed to make water and carbon dioxide.

Depth of Knowledge Level 2

3. The equation representing the formation of water from oxygen and hydrogen is
   2H₂ + O₂ 2H₂O. In this reaction 32 grams of oxygen react with 4 grams of hydrogen to from two molecules of water. The law of conservation of mass is supported by the formation of water from hydrogen and oxygen because
1. 2 hydrogen atoms react with 2 oxygen atoms to form 2 water molecules.
2. 32 grams of oxygen and 4 grams of hydrogen will produce 36 grams of water.
3. the volume of the oxygen and hydrogen will equal the volume of the water.
4. the size of the hydrogen and oxygen atoms and the water molecule will be equal.

4. Analyze the table below. The following symbols each represent one atom of different elements:

Which of the above correctly represents a chemical equation showing the law of conservation of mass in a chemical reaction?

1. I
2. II
3. III
4. IV

2^nd Item Specification: Balance simple chemical reaction equations using simple whole number ratios and the conservation of mass principle.

Depth of Knowledge Level 1

5. The following equation can be used to represent the formation of carbon monoxide from carbon and oxygen

2C + O₂ 2CO

The reaction supports the conservation of mass principle because the

1. mass of the carbon and oxygen atoms are the same before
   and after the reaction.
2. the oxygen atoms are bigger than the carbon atoms before
   and after the reaction.
3. the carbon and oxygen atoms are the same shape before
   and after the reaction.
4. the carbon atoms are the same size as the oxygen atoms
   during the reaction.

6. Identify the balanced equation that supports the law of conservation of mass.
1. H₂ + O₂ 2H₂O
2. 2H₂ + 2O₂ 2H₂O
3. 2H₂ + O₂ 2H₂O
4. H₂ + 2O₂ 2H₂O

Depth of Knowledge Level 2

7. The following equation for the photosynthesis reaction is unbalanced.
   
   6CO₂ + H₂O C₆H₁₂O₆+ 6O₂

   Which equation is balanced and shows conservation of mass?

1. CO₂ + H₂O C₆H₁₂O₆+ O₂
2. CO₂ + 2H₂O 2C₆H₁₂O₆+ 2O₂
3. 6CO₂ + 6H₂O C₆H₁₂O₆+ 6O₂
4. 6CO₂ + H₂O C₆H₁₂O₆+ 6O₂

8. The following equation represents the reaction between iron (Fe) and oxygen (O) to form iron oxide (Fe₂O₃).
   

   4Fe + 3O₂2Fe₂O₃

   This equation supports the conservation of mass principle because there are

1. 10 total iron and oxygen atoms involved in the reaction.
2. 20 total iron and oxygen atoms involved in the reaction.
3. 4 iron atoms and 3 oxygen atoms before and after the reaction.
4. 4 iron atoms and 6 oxygen atoms before and after the reaction.

9. The following equation for the rusting of iron is missing the coefficients representing the correct number of iron atoms and oxygen atoms in the reactants.
   

   Fe + O₂2Fe₂O₃

   Identify the number of iron and oxygen atoms in the reactants that
   will balance the equation.

1. Four iron atoms and four oxygen atoms.
2. Four iron atoms and six oxygen atoms.
3. Two iron atoms and two oxygen atoms.
4. Two iron atoms and three oxygen atoms.

3^rd Item Specification: Explain that the law of definite proportions allows for predictions of reaction amounts.

Depth of Knowledge Level 1

10. The compound water is represented by the formula H₂O. Water is the only compound with this formula because
1. only water contains atoms of the elements hydrogen and oxygen.
2. atoms of hydrogen and oxygen are identical to each other in a reaction.
3. atoms can never be created or destroyed by physical or chemical reactions.
4. a specific compound always has the same relative numbers
   and kinds of atoms.

11. Which of the following is true about the law of definite proportions?
1. The elements hydrogen (H) and oxygen (O) will always react to form the compound water (H₂O).
2. The compound water (H₂O) will always contain the elements hydrogen (H) and oxygen (O) in the same proportions.
3. The mass of water (H₂O) will always equal the mass of the hydrogen (H) and oxygen (O) from which it is made.
4. The element hydrogen (H) will always be heavier then the element oxygen (O) in the compound water (H₂O).

Depth of Knowledge Level 2

12. A chemistry student determined the amount of carbon in 100 grams of carbon dioxide to be 27.3 percent. According the law of definite proportions, which of the following statements is true about the amount of carbon in a second 100 gram sample of carbon dioxide?
1. The amount of carbon in the second sample can be predicted
   to be 27.3 percent.
2. The amount of carbon in the second sample can be predicted
   to be 100 grams.
3. The amount of carbon in the second sample will be less than
   in the first sample.
4. The amount of carbon in the second sample will be more than
   in the first sample.

13. The mass of water is always 88.9 percent oxygen and 11.1 percent hydrogen. Which of the following is true about the amount of oxygen in 100 g of water compared to the amount of oxygen in 200 g of water?
1. 100 g of water will contain a lower percentage of oxygen than 200 g of water.
2. 100 g of water will contain 88.9 percent oxygen and 200 g of water will contain 177.8 percent oxygen.
3. 100 g of water will contain 88.9 percent oxygen and 200 g of water will contain 88.9 percent of oxygen.
4. 100 g of water will contain 88.9 g of oxygen and 200 g of water will contain 88.9 g of oxygen.

14. Calcium carbonate is a compound in the rock marble. Analyze the following table showing the composition of calcium carbonate.

Which of the following statements is true about the amount of carbon in
a 100 g sample of calcium carbonate?

1. The sample will contain 12 g of carbon and will be 12% carbon.
2. The sample will contain 12 g of carbon and will be 24% carbon.
3. The sample will contain 24 g of carbon and will be 12% carbon.
4. The sample will contain 24 g of carbon and will be 24% carbon.

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Content Benchmark P.12.A.7

Students know that, in chemical reactions, elements combine in predictable ratios, and the numbers of atoms of each element do not change. I/S

Answers to Sample Test Questions

1. B, DOK level 1
2. C, DOK level 1
3. B, DOK level 2
4. B, DOK level 2
5. A, DOK level 1
6. C, DOK level 1
7. C, DOK level 2
8. D, DOK level 2
9. B, DOK level 2
10. D, DOK level 2
11. B, DOK level 1
12. A, DOK level 2
13. C, DOK level 2
14. A, DOK level 2

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Content Benchmark P.12.A.7

Students know that, in chemical reactions, elements combine in predictable ratios, and the numbers of atoms of each element do not change. I/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. How to Balance Chemical Equations Tutorial
There are many websites that can help students understand how to balance chemical equations.

• The Illinois Institute of Technology gives directions for students to use paper squares representing atoms and balance equations by manipulating the paper “atoms”. Go to http://www.iit.edu/~smile/ch8601.html to access this activity.

• For an explanation of the logic of balancing chemical equations as well as practice, go to http://dbhs.wvusd.k12.ca.us/webdocs/Equations/Meaning-of-Equation.html.

• For several examples, explanations, and interactive practice in balancing chemical equations, go to http://richardbowles.tripod.com/chemistry/balance.htm#part1.

• For a simple explanation of conservation of mass and a diagrammatic representation of a chemical reaction, go to http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/masscons.html.

• ChemTutor has many pages of explanation and practice in balancing equations. To access this site, go to http://www.chemtutor.com/react.htm#bal.

• USC has an interactive site on which students can practice balancing equations and then check their work at http://chemmac1.usc.edu/java/balance/balance.html.

• SciLinks has activities to balance equations and represents different elements in different colors to help students visualize each element. To access, go to http://www.middleschoolscience.com/balance.html.

• The beginning of this web site gives an explanation of chemical reactions and offers many opportunities to practice balancing equations. Go to http://nobel.scas.bcit.ca/chemed2005/tradingPost/WEPM-S3-4-09_Introduction_to_POGIL.pdf.

2. Hand-On Chemical Balancing Activities
There are some websites that give directions for laboratory activities that can have students do hands on work to understand the law of conservation of mass and the predictable nature of ratios of reactants and products.

• The Illinois Institute of Technology gives a simple, yet excellent activity that will show that gases have mass. This lab involves the reaction of Alka Seltzer tablets with water and is found at http://www.iit.edu/~smile/ch9403.html.

• Another experiment from this same source uses common items of hardware (nuts, bolts, screws, etc) to help students visualize relative masses. To access this, go to http://www.iit.edu/~smile/ch8621.html.

• This website offers directions for a laboratory activity in which students react baking soda with vinegar and look at relative masses. Go to http://misterguch.brinkster.net/MLX039.doc.

• Baking soda can also be reacted with hydrochloric acid (the acid used to balance the pH of pools). For directions, go to
http://www2.ucdsb.on.ca/tiss/stretton/CHEM1/lab7.html.

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