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
Cl2 (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) Cl2 (g) NaCl (s)
Na (s) Cl2 (g) 2 NaCl (s)
2 Na (s) Cl2 (g) 2 NaCl (s)
Coefficients vs. Subscripts
2 Na (s) Cl2 (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)
Na (s)Cl Cl NaCl (s)
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.
C3H8 (s) + O2 (g) CO2 (g) + H2O (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.
C3H8 (s) + O2 (g) 3 CO2 (g) + H2O (L)
C3H8 (s) + O2 (g) 3 CO2 (g) + 4 H2O (L)
• Now balance O.
C3H8 (s) + 5 O2 (g) 3 CO2 (g) + 4 H2O (L)
Balancing Equations
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) + H2O(L) MOH (aq) + H2(g)
• This group also reacts with oxygen (combustion) to yield an oxide.
M (s) + O2(g) M2O (s)
http://www.youtube.com/watch?v=MTcgo46nxNE
http://www.youtube.com/watch?v=qRmNPKVEGeQ&feature=related (small)
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
• http://www.youtube.com/watch?v=dX52Obmzh1k
M(s) + H2O M(OH)2(s) + H2(g)
M(s) + O2 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) + 6H2O(L) 2X(OH)3 (s) + 3H2 (g)
• Predict the products that will form if Y metal reacts with water?
2Y(s) + 6H2O(L) 2Y(OH)3 (s) + 3H2 (g)
Example
Same as X(s)
Halogens (Group 17)
• These diatomic nonmetals have very similar reactivity, although Br2 is a liquid.
• Halogens react with metals to form salts
Na(s) + Cl2(g) NaCl (s) Li (s) + Cl2(g) LiCl (s)
K (s) + Cl2(g) KCl (s)
Mg (s) + Cl2(g) MgCl2 (s) Ca(s) + Cl2(g) CaCl2 (s)
Sr(s) + Cl2(g) SrCl2 (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.
Periodicity1H
Totally unreactive gas
3Li
4Be
Highly reactive, highly conductive metal
Less reactive, less conductive metal
6C
9F
10Ne
Nonconductive, nonmetallic solid
Highly reactive, diatomic, nonmetallic gas
Decreasing metallic character
2He
Totally unreactive gas
Totally unreactive gas
11Na
12Mg
Highly reactive, highly conductive metal
Less reactive, less conductive metal
14Si
17Cl
18Ar
Slightly conductive semi-metal
Highly reactive, diatomic, nonmetallic gas
Decreasing metallic character
Totally unreactive gas
19K
20Be
Highly reactive, highly conductive metal
Less reactive, less conductive metal
22C
25F
26Kr
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.
Top Related