Chemical Equations
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C ONCEPT
Concept 1. Chemical Equations
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Chemical Equations
Lesson Objectives • • • • •
Give examples of historically significant chemical recipes and equations. Briefly describe the major milestones that took place in developing the chemical recipe of gunpowder. Understand mass relations between reactants and products for a given chemical process. Be able to use stoichiometric coefficients in chemical equations. Be able to balance chemical equations.
Lesson Vocabulary • stoichiometric coefficient: The letters a, b, c, and d where A and B are reactants, and C and D are products. The stoichiometric coefficients indicate the relative amounts of reactants and products. • balanced chemical equation: An equation where the number of atoms of each element on the reactant side is equal to the number of atoms on the product side.
Check Your Understanding 1. Which of the following are physical changes and which are chemical changes? a. b. c. d.
melting of ice a burning candle melting of candle wax sublimation of dry ice to CO2 gas.
Introduction Ever since the 9th century, humans have been fascinated with the nature of explosions. Whether to scare away evil spirits, to light up the night sky in celebration, or to be used in warfare, our understanding of gunpowder is based on our understanding of chemical recipes. Our ability to modify, share, and replicate them has allowed us to develop new recipes and to refine existing ones. Chemical reactions can be described in terms of chemical equations. They are the foundation of our modern day chemical recipes.
Origins of Chemical Recipes: Gunpowder The first people to discover gunpowder were 9th century Chinese alchemists. This discovery was made by accident while they were creating various chemical mixtures in pursuit of an elixir that would make them immortal. The 1
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FIGURE 1.1
first formulation of gunpowder was a thick toffee made from honey, saltpeter (a mixture composed primarily of potassium nitrate), and sulfur. They hoped that eating it would help them live forever. In reality, it burst into flames and burnt down their homes. Over time, Chinese alchemists refined the recipe and began to develop early pyrotechnic technology to help scare away evil spirits. A more fully developed, and more explosive, formula called for 75 percent potassium nitrate, 15 percent charcoal, and 10 percent sulfur.
FIGURE 1.2
Medieval Europe
This recipe made its way to Europe in medieval times. A Franciscan friar named Roger Bacon was particularly fascinated by the properties of gunpowder. He discovered that a key factor in the energetics of the mixture was the 2
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Concept 1. Chemical Equations
purity of the saltpeter. Bacon was responsible for developing early crystallization techniques to purify the mixture. He also discovered that the more tightly packed the powder, the larger the explosion. Bacon feared that bad things could happen if the mixture ended up in the wrong hands. He encoded the recipe in an anagram, which read (when translated from the original Latin) “And so thou wilt call up thunder and destruction if thou know the art”. The secret recipe, however, did not stay secret for long.
FIGURE 1.3
Pyrotechnics, or fireworks, used in events recorded in 14th century Italy show that the recipe was no longer a secret. During the 15th and 16th centuries, the Italians continued refining the art of pyrotechnics. Then, in 1830, a major leap forward in gunpowder technology occurred. It was discovered that replacing potassium nitrate with potassium chlorate resulted in a more energetic mixture, and so the recipe was revised once again. The modern day formulation of gun powder is called black powder. It is still commonly used today. Its formulation is still quite similar to what was used in 9th century China. Black powder is considered a low explosive. It is a mixture that burns quickly, but the resulting shock wave travels at subsonic speeds. The speed at which it burns is dependent on the accessibility of oxygen atoms to the carbon source. In contrast, high explosives like nitroglycerin detonate instead of burning, creating shock waves that are supersonic (faster than the speed of sound). 3
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FIGURE 1.4
The Chemical Equation Chemical equations describe the changes in composition that take place during a chemical reaction. Along with the identities of the starting reactants and the final products, chemical equations show the ratios in which these substances are consumed and produced. The reaction of iron with oxygen to form iron(III) oxide is shown below:
FIGURE 1.5 The sparks from a steel grinder are molten iron. The iron reacts with oxygen to form iron(III) oxide.
We can describe this reaction with a chemical equation: 4Fe(s) + 3O2(g) → 2Fe2 O3(s) This equation is said to be balanced, because the amount of each element expressed on the reactants side is equal to the amounts expressed on the products side. This is shown more explicitly in the following table:
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Concept 1. Chemical Equations
TABLE 1.1: 4Fe Reactants 4 6
Fe O
Products 4 6
Often times, the processes described by chemical equations do not represent a single reaction. For example, the following equation shows the starting materials and the products for photosynthesis: light
6CO2(g) + 6H2 O(l) → C6 H12 O6(s) + 6O2(g) This process does not occur in a single step. A sequence of many individual reactions is required to make glucose and oxygen gas out of carbon dioxide and water. Chemical equations can be used to represent individual reactions or the net change that occurs after multiple sequential chemical processes. The Balanced Chemical Equation
We can describe chemical reactions in terms of generic expressions like the following equation: aA + bB → cC + dD where A and B are reactants, and C and D are products. The letters a, b, c, and d represent stoichiometric coefficients, or the relative amount of each substance that is involved in the reaction. In this particular reaction, there are two reactants and two products, but others might have more or less. For example, the equation describing the rusting of iron had two reactants (Fe and O2 ) and one product (Fe2 O3 ). In a balanced chemical equation, the number of atoms of each element on the reactant side is equal to the number of atoms on the product side. This is necessary for all chemical equations, due to the law of conservation of mass. Atoms are neither created nor destroyed during a chemical reaction, only rearranged. Here are some examples of general expressions that will be applied to specific reactions in the next section. Example 11.1 Substance A reacts with substance B to form substance AB. Write the balanced chemical equation for this process. Answer: Write the general expression. A + B → AB Balance (it already is).
TABLE 1.2: A + B → AB A B
Reactants 1 1
Products 1 1
Example 11.2 Substance A reacts with substance B2 to form substance AB. Write the balanced chemical equation for this process. Answer: 5
www.ck12.org Write the general expression. A + B2 → AB Balance. 2A + B2 → 2AB
TABLE 1.3: A B
Reactants 2 2
Products 2 2
Example 11.3 Substance A2 reacts with substance B2 to form substance AB3 . Write the balanced chemical equation for this process. Answer: Write the general expression. A2 + B2 → AB3 Balance.
TABLE 1.4: A A B
Reactants 2 6
Products 2 6
Balancing Chemical Equations for Real Reactions
Now that we have studied the general process for describing and balancing chemical equations, we are going to apply this approach to examples that include actual chemicals. As we present the following reactions, we are going to focus only on the changes in composition from reactants to products. In later chapters, we will look at other reaction properties, such as states of matter, temperature, and the energy lost or gained by a given reaction. In the following lesson, we will look at ways to classify different types of reactions. This knowledge will allow us to make reasonable predictions about the products that might be generated from a given set of reactants. Tips for Balancing Equations Before we get started with balancing chemical equations, here are some simple tips to consider: 1. If there are polyatomic ions that exist unchanged on both sides of the equation, it is often simpler to treat them as single units than to break them down into their individual elements. 2. It is often easier to leave elements that occur in their pure elemental form (on either side of the equation) for last. 3. If a reactant or product has a coefficient of 1, this number is not explicitly written. 4. In a correctly balanced equation, all coefficients must be whole numbers. However, the use of fractions can 6
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Concept 1. Chemical Equations
be helpful as a way of finding the correct coefficients. If all atoms in an equation are balanced but some have fractional coefficients, multiply all coefficients in the entire equation (including those not explicitly written!) by the lowest common denominator to get the final balanced equation. Example 11.4 Liquid mercury is heated in the presence of oxygen to produce mercury(II) oxide. Write the balanced chemical equation for this process. Answer: Start by writing the general expression. Hg(l) + O2 (g) → HgO(s) Then, alter the coefficients to balance each element.
TABLE 1.5: 2Hg(l)+O Hg O
Reactants 2 2
Products 2 2
Notice that in this example, the formula for oxygen is the diatomic form O2 . Many pure nonmetallic elements are unstable as individual atoms and combine readily to make diatomic molecules. Hydrogen (H2 ), nitrogen (N2 ), oxygen (O2 ), and the halogens (F2 , Cl2 , Br2 , and I2 ) exist as diatomic molecules when in their pure elemental forms. Example 11.5 Hydrogen gas and fluorine gas react to form hydrogen fluoride gas. Write the balanced chemical equation for this process. Answer: Start by writing the general expression. H2 (g) + F2 (g) → HF(g) Then balance each element.
TABLE 1.6: H H F
Reactants 2 2
Products 2 2
Again, pure hydrogen and fluorine exist as diatomic gases. Example 11.6 Ammonium nitrate decomposes to form nitrogen gas, water, and oxygen gas. Write the balanced chemical equation for this process. Answer: 7
www.ck12.org Write the general expression. NH4 NO3 (s) → N2 (g) + H2 O(l) + O2 (g) Balance. Because this equation involves more than two elements, it is slightly less straightforward to balance. Since nitrogen and oxygen both occur in their pure elemental forms, we start by balancing hydrogen: NH4 NO3 (s) → N2 (g) + 2H2 O(l) + O2 (g) Hydrogen and nitrogen are now balanced, but oxygen is not. This can be fixed by changing the coefficient on its pure elemental form: NH4 NO3 (s) → N2 (g) + 2H2 O(l) + 21 O2 (g) The atoms are now balanced, but to avoid having fractional coefficients, we must multiply all coefficients in the equation by 2: 2NH4 NO3 (s) → 2N2 (g) + 4H2 O(l) + O2 (g) We can confirm that this equation is balanced by writing the following table:
TABLE 1.7: 2NH N H O
Reactants 4 8 6
Products 4 8 6
Example 11.7 Lead(II) nitrate reacts with sodium chloride to form lead(II) chloride and sodium nitrate. Write the balanced chemical equation for this process. Answer: Write the general expression. Pb(NO3 )2 + NaCl → PbCl2 + NaNO3 Balance.
TABLE 1.8: Pb(NO Pb NO3 Na Cl
Reactants 1 2 2 2
Products 1 2 2 2
By keeping the polyatomic nitrate ion intact as a single unit, balancing this equation becomes somewhat simpler. This was done because the ion exists unchanged on both sides of the equation. Note that this is in contrast to the previous example, in which the nitrate ion decomposed to form other substances. 8
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Concept 1. Chemical Equations
Lesson Summary • The composition of gunpowder gradually changed as alchemists and scientists experimented with ways to make it even more explosive. • Chemical reactions are described using chemical equations. • Stoichiometric coefficients are used in chemical equations to indicate the amounts of reactants and products. • Because of the law of conservation of mass (matter can neither be created nor destroyed through chemical reactions), chemical equations must have equal amounts of each specific atom on both sides of the equation.
Review Questions 1. Early Chinese alchemists discovered an early form of gunpowder. What was the composition of this substance? 2. What later developments were made to the gunpowder recipe that improved its pyrotechnic properties? 3. Make an argument for why the burning of a candle is consistent with the law of conservation of matter/mass. 4. Think of an experiment that you could conduct to demonstrate that mass is conserved for a given chemical change. 5. Balance the following chemical equations: a. b. c. d. e. f. g. h. i. j. k. l. m.
C + O2 → CO CO + O2 → CO2 H2 + Br2 → HBr K + H2 O → KOH + H2 O3 → O2 N2 + H2 → NH3 Zn + AgCl → ZnCl2 + Ag Cl2 + NaI → NaCl + I2 P4 O10 + H2 O → H3 PO4 Be2 C + H2 O → Be(OH)2 + CH4 S + HNO3 → H2 SO4 + NO2 + H2 O NH3 + CuO → Cu + N2 + H2 O HCl + CaCO3 → CaCl2 + H2 O + CO2
Further Reading / Supplemental Links • Youtube Video of Kaboom! The Sizzling Story of Explosions: http://www.youtube.com/watch?v=CShA5 2EKY80 • Gunpowder in Ancient China: http://www.historyforkids.org/learn/war/gunpowder.htm • Practice Balancing Chemical Equations: – http://education.jlab.org/elementbalancing/index.html – http://www.files.chem.vt.edu/RVGS/ACT/notes/scripts/bal_eq1.html – http://gregthatcher.org/Chemistry/BalanceEquation/S • Chemical Equation Balances: http://www.personal.psu.edu/jzl157/balance.htm 9
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Points to Consider • What is the relationship between chemical equations and chemical reactions? • In this chapter, an argument was made that the human fascination with fire and explosions ultimately contributed to our current understanding of chemical equations. Can you think of other aspects of nature for which further exploration has contributed to our current understanding of chemistry?
References 1. NASA. http://en.wikipedia.org/wiki/File:Chinese_rocket.gif. Public Domain 2. PericlesofAthens. http://commons.wikimedia.org/wiki/File:Chinese_Gunpowder_Formula.JPG. Public Domain 3. Joseph Wright. http://commons.wikimedia.org/wiki/File:JosephWright-Alchemist.jpg. Public Domain 4. Oliver H.. http://commons.wikimedia.org/wiki/File:Spk-RZ.jpg. Public Domain 5. Image copyright BESTWEB, 2013. http://www.shutterstock.com. Used under license from Shutterstock.com
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