Unit 3: Chemical Reactions

Transcription

1 Unit 3: Chemical Reactions Covalent and Ionic Bonds In order to name binary compounds, or compounds between 2 ions, it is important to differentiate between covalent and ionic bonds. Being able to identify the difference between an ionic compound and a covalent molecule will allow you to name your compounds accurately. Before diving into the differences between covalent and ionic bond, knowing exactly what a bond is, is very important to your understanding. Whenever you see a bond in chemistry, the bond involves electrons. All the talk about valence electrons and how an atom is structured is really talking about how the electrons can move from one place to another to form a bond, or shared between two atoms. It is really all about the electrons! Keep in mind that electrons are negatively charged. Therefore, bonds formed by electrons will most likely occur with a positively charged ion. What is a covalent bond? A covalent bond is simply a bond that involves the sharing of electrons between the two bonding elements. The Sharing of electrons has always been the definition of covalent bonds, but what does that really mean? When atoms bond, they either give away, take or share electrons. Think of a game of tug-o-war and the electrons are what are being pulled. In covalent bonds, both atoms are pulling the electrons equally, such that the electrons are equally shared between the two pulling atoms. One atom pulls one way, while the other pulls in the opposite direction but both are pulling with the same amount of force that electrons are shared. In later lessons, you will learn why some elements have a stronger pull than others, and that is called electronegativity. How can you identify a covalent bond? Covalent molecules are formed between a non-metal and non-metal together. A non-metal is basically any element on the right side of the staircase of the periodic table. Examples include Nitrogen, Oxygen, Chlorine, Bromine and etc If these non-metals are bonded with another non-metal, the bond would be a covalent bond.

2 Naming covalent bonds: Naming covalent bonds requires Latin prefixes to tell the reader the number of atoms you have per element. In other words, because of how covalent bonds are formed, the chemical formula name must include the number of atoms that are bonded. 1 st Step: Using the Latin Prefixes The Latin Prefixes: 1 mono 2 di 3 tri 4 tetra 5 penta 6 hexa 7 hepta 8 octa 9 nona 10 deca 2 nd key step: If you have 5 oxygen atoms such as O 5, you have to say penta-oxygen If you have 10 chlorine atoms such as Cl 10, you have to say deca-chlorine In addition to using Latin Prefixes, the suffix (ending) of the second element must be changed. Drop the last 3 letters (usually) and add the ending ide. Should you change the ending of the first element? Nope! Leave it the same way. N 2 O 5 There are 2 Nitrogens There are 5 Oxygens Di Nitrogen Penta Oxide Dinitrogen pentaoxide What is an ionic bond? In an ionic bond, electrons are pulled away from an element and transferred to the next. In the tug-o-war example above, one team will be so much stronger that it will pull the entire rope to their side, leaving the losing side without the rope. This is similar to ionic bonds where one element grabs the electrons from the other, while the other atom loses the electrons. Ionic bonds

3 How can you identify an ionic bond? Ionic bonds are formed between a non-metal element and a metal element. A non-metal element as you recall from above are elements found on the right of the staircase of the periodic table. Elements such as oxygen, chlorine, bromine and etc are non-metals. Metals on the other hand are found on the left side of the staircase of the Periodic table. Metals would include a lot larger range of elements from your Alkali metals such as Sodium, Cesium, Potassium etc to transition metals in the middle to elements such as Gallium. Any metal bonded to a non-metal would be an ionic bond. Metal Non-metal Ionic Bond! Naming an ionic bond Naming an ionic bond is slightly easier than covalent bonds because you don t need prefixes but there are several types. First off, to name an ionic bond such as CaCl 2 you should be able to recognize that Ca (left side!) is a metal and Cl (right side) is a non-metal. Second, you should be able to recognize the ion charges for each of them. Calcium has a 2+ charge meaning, will lose or has lost 2 electrons, while chlorine has a 1- charge meaning will gain or has gained 1 electron. To name CaCl 2 the first element will exactly the same, Calcium. The second element will have the suffix ide. Calcium Chloride Going backwards to chemical formula: Many students struggle with this step because in order to go backwards into the chemical formula, you must understand and balance the charges. Remember, ionic bonds involve a positive cation (+) and a negative anion (-) and they exchange electrons to form the bond. Going back to the Calcium chloride example, separate the two elements: Calcium and Chlorine Identify the charges for each ion: Ca 2+ and Cl 1- The total of the two charges (+ and -) must equal to zero for the compound to be neutral.

4 So far we have = +1 left over. We need to add another -1 charge of chlorine making it CaCl 2 with 1 calcium atom and 2 chlorine atoms. Another example: Magnesium bromide Mg 2+ and Br 1- So far we have +2 + (-1) = +1 left over. We need another (-1) charge of Br to make it to zero. Therefore, MgBr 2 Naming ionic bonds with roman numerals (Stock number system) Some transition metals can have more than one charge. These are called multi-valent ions. Iron (Fe) for example can exist as Fe 2+ or Fe 3+. To name compounds such as Iron Oxide, you must show if the Iron is the Fe 2+ or Fe 3+ since it will affect the number of oxygen ions present. If Fe 2+ is bonded with O 2-, the bond would be FeO If Fe 3+ is bonded with O 2-, the bond would be Fe 2 O 3 To designate the charge used, use Roman Numerals after the element in question. For example: FeO would be Iron (II) oxide Fe 2 O 3 would be Iron(III) oxide. Naming ionic bonds with Polyatomic ions Polyatomic ions are ions that contain more than 1 element to make up the charge. In other words, instead of having just 1 element with a charge such as Cl 1- or Na 1+, polyatomic ions, hence poly will have more than 1 atom to form the ion. Examples such as phosphate - PO 4 3- or CO 3 2- all have more than 1 element in the ion with a charge. Polyatomic ions are treated as 1 large ion (cation or anion) instead of them separately. Look at the polyatomic table and familiarize yourself with the different ones. The faster you can familiarize with the different polyatomic ions, the easier the naming. Naming compounds with polyatomic ions follow the same rules as basic ionic bonds except that instead of replacing the suffix with ide, you keep the same name as the polyatomic ion. For example: CaCO 3 has Ca 2+ and CO 3 2- The compound is called Calcium Carbonate ß Same polyatomic ion name

5 K 3 PO 4 has K 1+ and PO 4 3- The compound is called Potassium Phosphate ß Same polyatomic ion name Naming Oxyanions Now that you have familiarize yourself with naming covalent and ionic compounds, we will look at naming oxyanions. As the name suggests, oxyanions are anions with oxygen in them. They are all polyatomic ions and have a negative charge (anions remember?). An example of an oxyanion is Chlorate ClO 3. In chlorate, there is an oxygen with chlorine atom. However, chlorates can come in different forms depending on the number of oxygen atoms present. Chlorate ClO 3 Chlorite ClO 2 Hypochlorite ClO Perchlorate ClO 4 Naming oxyanions is actually not too complicated. The key thing you need to memorize are the oxyanions that end in ate ending such as chlorate, bromate, phosphate, sulfate, and carbonate (See they all have ate endings!) Steps in naming oxyanions. 1. Start with your ate oxyanion. This is your reference oxyanion as in all the other ones will have 1 more oxygen atom or 1 or 2 less oxygen atom. Example: Sulfate SO 4 2. Once you have established your ate oxyanion If the same ate oxyanion gains 1 more oxygen atom, it will become have a prefix of PER- in front of the name. Example: Sulfate SO 4 à Persulfate SO 5 If the same ate oxyanion has 1 less oxygen atom, it will have an ending of ite instead of ate. Example: Sulfate SO 4 à Sulfite SO 3 If the same ate oxyanion gains 1 less oxygen atom from the ite, it will have a prefix of hypo- with the ite ending. Example: Sulfate SO 4 à HYPOSulfite SO 2