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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Performance 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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Performance 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.

Sample Test Questions

1. The reaction equation for the decomposition of water to hydrogen and oxygen is H₂O ? H₂ + O₂. Which choice below shows the balanced equation?
a. H₂O → H₂ + O₂
b. 2 H₂O → 2 H₂ + O₂
c. 2 H₂O → 2 H₂ + 2 O
d. H₂O → H₂ + O

2. What happens to the atoms involved in a chemical reaction?
a. They change into atoms of different elements.
b. They change into energy.
c. They combine in predictable ratios.
d. The numbers of atoms of each element changes.

3. When methane burns in air to form carbon dioxide and water, the balanced equation is CH₄ + 2 O₂ → CO₂ + 2 H₂O. When 8 molecules of methane are burned, how many molecules of water are produced?
a. 8 molecules water
b. 12 molecules water
c. 16 molecules water
d. 24 molecules water

4. When potassium chlorate decomposes to potassium chloride and oxygen, the unbalanced equation for the reaction is: KClO₃ → KCl + O₂. Which set of coefficients (in order) will balance this equation?
a. 2,3,2
b. 2,4,3
c. 2,4,2
d. 2,2,3

5. Which statement is true about both physical and chemical changes?
a. Mass is conserved in both physical and chemical changes.
b. Mass is NOT conserved in either physical or chemical changes.
c. Mass is conserved in physical changes but not in chemical.
d. Mass is conserved in chemical changes but not in physical.

6. What happens when wood burns with the oxygen in air to form water and carbon dioxide?
a. The mass of the products becomes less than the mass of the reactants.
b. Most of the mass of the wood is converted into energy.
c. More energy is absorbed than given off in the reaction.
d. The mass of the wood and oxygen reacted equals the mass of the products.

7. When hydrogen and chlorine react to product hydrogen chloride, the reaction equation is H₂ + Cl₂ → HCl. What is the balanced equation for this reaction?
a. H₂ + Cl₂→ HCl
b. H₂ + Cl₂ → H₂Cl₂
c. H₂ + Cl₂ → 2H₂Cl
d. H₂ + Cl₂ → 2 HCl

8. When photosynthesis occurs, carbon dioxide and water produce glucose and oxygen. The balanced equation is 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂. How many molecules of C₆H₁₂O₆ are produced when 18 molecules O₂ are produced?
a. 1 molecule C₆H₁₂O₆
b. 2 molecules C₆H₁₂O₆
c. 3 molecules C₆H₁₂O₆
d. 6 molecules C₆H₁₂O₆

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)
2. (c)
3. (c)
4. (d)
5. (a)
6. (d)
7. (d)
8. (c)

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PerformanceBenchmark 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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