Chemistry_Grade_10-12 (1).pdf

Na cl nacl answer step 1 determine which atom will

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Na + Cl NaCl Answer Step 1 : Determine which atom will have a positive or negative oxidation number Sodium will have a positive oxidation number and chlorine will have a negative oxi- dation number. Step 2 : Determine the oxidation number for each atom Sodium (group 1) will have an oxidation number of +1. Chlorine (group 7) will have an oxidation number of -1. Step 3 : Check whether the oxidation numbers add up to the charge on the molecule In the equation Na + Cl NaCl , the overall charge on the NaCl molecule is +1-1=0. This is correct since NaCl is neutral. This means that, in a molecule of NaCl, sodium has an oxidation number of +1 and chlorine has an oxidation number of -1. 86
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CHAPTER 4. ATOMIC COMBINATIONS - GRADE 11 4.11 Worked Example 21: Oxidation numbers Question: Give the oxidation numbers for all the atoms in the reaction between hydrogen and oxygen to produce water. The unbalanced equation is shown below: H 2 + O 2 H 2 O Answer Step 1 : Determine which atom will have a positive or negative oxidation number Hydrogen will have a positive oxidation number and oxygen will have a negative oxidation number. Step 2 : Determine the oxidation number for each atom Hydrogen (group 1) will have an oxidation number of +1. Oxygen (group 6) will have an oxidation number of -2. Step 3 : Check whether the oxidation numbers add up to the charge on the molecule In the reaction H 2 + O 2 H 2 O , the oxidation numbers for hydrogen and oxygen (on the left hand side of the equation) are zero since these are elements. In the water molecule, the sum of the oxidation numbers is 2(+1)-2=0. This is correct since the oxidation number of water is zero. Therefore, in water, hydrogen has an oxidation number of +1 and oxygen has an oxidation number of -2. Worked Example 22: Oxidation numbers Question: Give the oxidation number of sulfur in a sulphate ( SO 2 4 ) ion Answer Step 1 : Determine which atom will have a positive or negative oxidation number Sulfur has a positive oxidation number and oxygen will have a negative oxidation number. Step 2 : Determine the oxidation number for each atom Oxygen (group 6) will have an oxidation number of -2. The oxidation number of sulfur at this stage is uncertain. Step 3 : Determine the oxidation number of sulfur by using the fact that the oxidation numbers of the atoms must add up to the charge on the molecule In the polyatomic SO 2 4 ion, the sum of the oxidation numbers must be -2. Since there are four oxygen atoms in the ion, the total charge of the oxygen is -8. If the overall charge of the ion is -2, then the oxidation number of sulfur must be +6. 87
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4.12 CHAPTER 4. ATOMIC COMBINATIONS - GRADE 11 Exercise: Oxidation numbers 1. Give the oxidation numbers for each element in the following chemical com- pounds: (a) NO 2 (b) BaCl 2 (c) H 2 SO 4 2. Give the oxidation numbers for the reactants and products in each of the fol- lowing reactions: (a) C + O 2 CO 2 (b) N 2 + 3H 2 2NH 3 (c) Magnesium metal burns in oxygen 4.12 Summary A chemical bond is the physical process that causes atoms and molecules to be attracted together and to be bound in new compounds.
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