ionic or molecular? Ionic and Molecular Compounds

Transcription

1 ionic or molecular? Ionic and Molecular Compounds There are two major classes of compounds: Ionic compounds are formed by the attractions between oppositely charged ions. (metal + nonmetal or complex ion) Molecular compounds are formed when nonmetallic atoms share attractions for each other's electrons. Due to their different ways of bonding, each class of compounds has its own unique properties. 1

2 Physical Properties Physical properties of matter: A physical property refers to any characteristic such as colour, solubility, electrical conductivity, melting point, etc. For example: The physical properties of table salt include; white solid at room temperature, cubic crystal structure, high melting point, soluble in water, and conducts electricity when in solution (dissolved in water). Physical Change A physical change refers to a change in the characteristics of matter when there is a change in its physical environment. For example, ice (solid water) will melt to form liquid water when heated. This change in the physical state of matter is an example of a physical change. During a physical change the atoms that make up the compound are not rearranged in any manner. Ice and liquid water still are the same in terms of the arrangement of the atoms in the compound. Ice and liquid water are both H2O. 2

3 Chemists use symbols to represent states of matter. For example: water has three physical states of matter - solid, liquid and gas. Can you predict the meaning of the following: H2O (s), H2O (l), H2O (g) What is the meaning of the subscripts in parentheses, ()? Chemical Property and Change A chemical property refers to the behaviour of matter during a chemical reaction. During a chemical reaction there is a chemical change, atoms are rearranged to form new substances, new types of matter with new chemical properties are formed as products. For example: Water can be decomposed into hydrogen and oxygen gas when electricity is passed through it. Since hydrogen and oxygen have totally different physical and chemical properties than water, this is an example of a chemical change or chemical reaction. 3

4 Properties State of Matter: The state of matter refers to matter as either solid, liquid, or gas. Ionic compounds are solids at room temperature whereas molecular substances as a group are variable in their states of matter - some are solids, but many are liquids or gases. Since all ionic compounds are solids, you can classify any pure liquid or gas substance as molecular. oalcohol (a liquid at room temperature) has the formula CH3CH2OH. Is alcohol ionic or molecular? How do you know? oalcohol is a molecular compound because alcohol is a liquid. Ionic compounds are always solids (never liquids or gases) at room temperature. 4

5 Properties Melting Point: Generally, molecular substances like sugar melt at temperatures below 300 C, whereas ionic substances tend to have melting points above 300 C. However, melting point data alone is usually insufficient evidence to classify a substance as ionic or molecular. In general, molecular compounds tend to have low melting points compared to the very high melting points of ionic compounds. Properties Solubility in Water : When a substance dissolves in water it forms what is known as an aqueous solution. aqueous solution - a solution in which water is the solvent. take some table sugar, C12H22O11,(molecular compound) dissolve it in pure water and there you have it, an aqueous solution 5

6 Another example: Place some table salt (NaCl) (ionic compound) in water and you have another example of an aqueous solution - salt water! In chemical equations, chemists use the symbol (aq) to the lower right of the symbol or formula to represent an aqueous solution. NaCl(aq) - sodium chloride dissolved in water. Both ionic and molecular compounds may or may not dissolve in water, so this evidence alone cannot be used to classify a solid compound. Properties Electrical Conductivity - (Electrical Conductivity Test of Aqueous Solutions) When solubility in water is combined with an electrical conductivity test we have an excellent way of classifying solids as either ionic or molecular. In our examples, both sugar and salt are soluble in water. How can we determine which is ionic and which is molecular? 6

7 In order for electricity to flow it needs a path to follow. The idea behind the electrical conductivity test is that free ions in the solution offer a path for the movement of electrons. Thus any solutions containing ions will conduct electricity. Ionic compounds are composed of ions. When dissolved in water the ionic compounds break up into its ions to form electrically conductive solutions electrolytic solutions Molecular compounds are composed of neutral molecules. When dissolved in water they stay as neutral molecules. Molecules do not allow the movement of electrons, thus do not conduct electricity. nonelectrolytic solutions (except for molecular acids) 7

8 Conductivity Tester One beaker contains an aqueous solution of table salt. The second beaker contains an aqueous solution of table sugar. Examine the image and identify which beaker contains the salt (an ionic compound) and which beaker contains the sugar (a molecular compound). Is it the beaker on the left, or the beaker on the right? How do you know? All known ionic compounds which dissolve in water to form aqueous solutions will conduct electricity! Molecular compounds that dissolve in water to form aqueous solutions do not conduct electricity. The beaker on the left (as indicated by the light on) contains salt. The beaker on the right (light off) contains sugar since it is a molecular compound and does not conduct electricity 8

9 Properties But what if the substance you want to test is not soluble in water? In such a case you have to melt it. If the molten (melted) substance conducts electricity, then the compound is ionic (ie: there must be free ions present). If the molten substance does NOT conduct electricity, then the compound is molecular (no ions present). Properties of Ionic and Molecular Compounds Create a table to compare Molecular and Ionic Compounds Composition (type of elements) State of matter (at room temperature) Melting point Solubility in water Electrical conductivity Bonding (type of bonding, valence electrons) 9

10 Properties of Ionic and Molecular Compounds 10