Lecture 4 Chemical Periodicity: Ch 3 Dr. Harris 8/30/12 HW: Ch 3: 3, 4, 9, 27, 28,

Chemical Reactions When elements undergo a chemical reaction, the products may be quite different from the reactants The simplest reactions are those between metals and nonmetals. The product of such a reaction is an ionic compound Lets consider the reaction between sodium metal and chlorine gas

Stark Differences Between Reactants and Products Na (s) very reactive metal Cl 2 (g) toxic, reactive gas NaCl (s) table salt As you can see from the chemical equation above, products can exhibit physical characteristics that are vastly different from those of the reactants Recall the law of conservation of mass. Based on this law, can you find a problem with the equation written above?

Balancing Reactions Mass can not be created or destroyed. This means that every element involved in a reaction must be accounted for in a chemical equation. As you can see, there are two chlorine atoms on the reactant side, and only one chlorine atom one the product side. To balance the chlorine atoms, we add a coefficient of 2 to the NaCl(s) Now, we have balanced the chlorine atoms, but the sodium atoms are now unbalanced. We add a coefficient of 2 to the Na (s). The reaction is now balanced. Na (s)Cl 2 (g) NaCl (s) Na (s)Cl 2 (g) 2 NaCl (s) 2 Na (s)Cl 2 (g) 2 NaCl (s)

Coefficients vs. Subscripts 2 Na (s)Cl 2 (g)2 NaCl (s) The balanced equation above says that two Na atoms react with one chlorine gas molecule to produce two molecules of NaCl The coefficient of 2 means that there are two separate Na atoms The subscript of 2 indicates two Cl atoms bonded together in a single molecule Do not confuse coefficients and subscripts Na (s) Cl NaCl (s)

Balancing Equations Before carrying out any calculations, it is imperative that you first confirm that a given chemical equation is balanced. The rules for balancing a chemical equation are provided below. 1.First, balance those elements that appear only once on each side of the equation 2.Balance the other elements as needed. Pay attention to subscripts. 3.Include phases

Let’s balance the equation below using the rules from the previous slide. C 3 H 8 (s) + O 2 (g) CO 2 (g) + H 2 O (L) Carbon and hydrogen appear only once on each side, whereas oxygen appears twice on the right-hand side. We’ll balance C first. Now balance H. C 3 H 8 (s) + O 2 (g) 3 CO 2 (g) + H 2 O (L) C 3 H 8 (s) + O 2 (g) 3 CO 2 (g) + 4 H 2 O (L) Now balance O. C 3 H 8 (s) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (L) Balancing Equations

Group Examples Balance the following For #2, and #4, consider using a fraction if necessary.

Chemical Groups As more and more elements were discovered, chemists began to notice patterns in the chemical properties of certain elements. Consider the three metals Li, Na, and K All 3 metals are soft All 3 metals are less dense than water All 3 metals have similar appearance and low melting points The most interesting feature is that all 3 metals react with the same elements in a nearly identical manner As you see in the periodic table, these elements are all listed in the same group. Elements in a group behave similarly. Recognizing patterns allows us to predict reactions without memorizing every characteristic of every element

Group 1 - Alkali Metals The metals in group 1 are known as the alkali metals. This group is highly reactive. Li (s) K (s) Na (s) This group reacts violently with water to form hydroxides (-OH) and hydrogen gas M(s) + H 2 O(L)  MOH (aq) + H 2 (g) This group also reacts with oxygen (combustion) to yield an oxide. M (s) + O 2 (g)  M 2 O (s)

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Group 2 – Alkaline Earth Metals Metals in groups 2 (Mg, Ca, Sr, Ba) also react with water and oxygen. However, these metals are less reactive than group 1. These metals are also known to emit bright light when they are burned M(s) + H 2 O  M(OH) 2 (s) + H 2 (g) M(s) + O 2  MO(s)

Two metals, X and Y, are listed in the same group on the periodic table. X reacts with water to form a hydroxide and hydrogen gas, as shown below 2X(s) + 6H 2 O(L)  2X(OH) 3 (s) + 3H 2 (g) Predict the products that will form if Y metal reacts with water? 2Y(s) + 6H 2 O(L)  2Y(OH) 3 (s) + 3H 2 (g) Example Same as X(s)

Halogens (Group 17) These diatomic nonmetals have very similar reactivity, although Br 2 is a liquid. Halogens react with metals to form salts Na(s) + Cl 2 (g)  NaCl (s) Li (s) + Cl 2 (g)  LiCl (s) K (s) + Cl 2 (g)  KCl (s) Mg (s) + Cl 2 (g)  MgCl 2 (s) Ca(s) + Cl 2 (g)  CaCl 2 (s) Sr(s) + Cl 2 (g)  SrCl 2 (s)

Noble Gases (group 18) The noble gases are very stable, very unreactive gases. Only a few compounds have been synthesized using these elements.

Periodicity Dmitri Mendeleev created the periodic table in in 1869 by arranging the elements in order of increasing atomic mass. In doing so, he observed repetitive patterns in chemical behavior across periods This periodicity is described in the next slide.

Periodicity 1H1H Totally unreactive gas 3 Li 4 Be Highly reactive, highly conductive metal Less reactive, less conductive metal 6C6C 9F9F 10 Ne Nonconductive, nonmetallic solid Highly reactive, diatomic, nonmetallic gas Decreasing metallic character 2 He Totally unreactive gas 11 Na 12 Mg Highly reactive, highly conductive metal Less reactive, less conductive metal 14 Si 17 Cl 18 Ar Slightly conductive semi-metal Highly reactive, diatomic, nonmetallic gas Decreasing metallic character Totally unreactive gas 19 K 20 Be Highly reactive, highly conductive metal Less reactive, less conductive metal 22 C 25 F 26 Kr Nonconductive, nonmetallic solid Highly reactive, diatomic, nonmetallic liq. Decreasing metallic character

Mendeleev’s Genius At the time in which the periodic table was being constructed, not all of the elements had been discovered. Based on the observed periodicity, Mendeleev realized that gaps in the initial periodic table belonged to undiscovered elements For example, in 1869, the element following Zn on the periodic table was As. Yet, he knew to put As in group 15 rather than 13 because As behaved like P, and he knew that two undiscovered elements (Ga and Ge) would fill the gaps.

Semiconductors As you proceed from left to right across the periodic table, or up a group, the physical properties of the elements shifts from very metallic to non-metallic As stated in lecture 3, metals are conductive of heat and electricity. Nonmetals are not. Intermediates between metals and nonmetals are called semiconductors, because they are only partially conductive. Semiconductors are colorful, brittle solids.

Transition Metals Semiconductors transitions metals Transition metals span the region where the transition from metal to nonmetal occurs. Transition metals are very dense and have very high melting points.