Chapter 5: Structure and properties of materials. Flashcards
What is an element?
- An element is a pure substance that cannot be broken down by (its) chemical means.
- There are 118 known elements in the periodic table and each element is represented by a symbol.
- E,g. of elements: calcium (Ca), Chlorine (cl), lead (Pb), and nitrogen (N).
What is a compound?
- A compound is a pure substance that contains 2 or more elementa bonded chemically in a fixed ratio
- A compound usually has an appearance and properties that are different from those of the elements it is made up of.
- E.g. of compounds: Lead (II) chloride is a yellow solid and is made up of the elements lead (a dull and grey solid), and chlorine (a yellow and green gas)
- A compound can be broken down into its constituent elements by its chemical means such as thermal decomposition and electrolysis (the passing of electricity through a compound).
What is a mixture?
- A mixture is formed when 2 or more substances are mixed together without being chemically bonded.
- The constituents of a mixture are not in a fixed ratio.
- E.g. of mixtures: Air, sea water and cement, because they are made out of different elements and compounds.
What are the differences between an element, a compound and a mixture?
- Element:
composition :
- Made up of only 1 type ofatom
method of separation: -
Properties : -
energy change: -
melting and boiling points: Fixed
- E.g. melting point: 0ยฐc
boring point: 100ยฐc - Compound:
composition:
- Made upof 2 or more eiements in a fixed ratio:
Method of separation:
- (Based on the substancesโ/substanceโs) chemical means.
properties:
- Different from those of its constituent elements.
Energy change:
- Arises due to a chemica reaction
melting and boiling points:
- Fixed - Mixture:
composition:
- 2 or more elements and or compounds in a variable ratio.
method of separation:
- physical means
properties:
- similar to those of the elements and or compounds present.
Energy change:
- Does not arise due to no chemicai reaction melting and bolihg points:
- not fixed
- Vary according to its composition and what it is made of.
What are ionic compounds?
- Ionic compounds have melting and boiling points.
- Ionic compounds have giant ionic crystal lattice structures.
- The positive and negative ions in the lattice are held together by strong electrostatic forces of attraction.
- A lot of energy is required to overcome the strong electrostatic forces.
What is the relationship of ionic compounds with the charge of ions, and the electrostatic forces of attraction between the ions and the melting/boiling points of compounds.
- In ionic compounds, the greater the charge of the ions, the stronger the electrostatic forces of attraction between the ions and the higher the melting and boiling points of the compounds.
- E.g. Potassium chloride consists of K+ and cl- ions while calcium oxide consists of Ca2+ and O2- ions. Since the ions in calcium oxide have a greater charge, the electrostatic forces of attraction between the ions are STRONGER and require more energy to overcome.
- Thus, calcium oxide has a higher melting and boiling point than potassium chloride.
Are all ionic compounds soluble in water?
- No, only most ionic compounds that are soluble in water, but insoluble in organic solvents.
What is the condition that ionic compounds are in the aqueous state?
- Ionic compounds that are dissolved in water are in the aqueous state.
What are the states that ionic compounds can conduct electricity in?
- Ionic compounds can conduct electricity in the molten and aqueous states.
- Mobile charge carriers must be present to conduct electricity.
- When an ionic compound is in the molten and aqueous state, the ions move freely and act as a mobile charger carrier. Thus, the compound can conduct electricity.
- When an ionic compound is in a solid state, the ions are held together in a giant lattice structure and cannot move around. thus, the compound cannot conduct electricity.
What is a TEXTURE characteristic of an ionic compound?
- An ionic compound is hard but brittle.
What are some characteristics and properties of simple covalent substances?
- Simple covalent substances have low melting and boiling points. Thus, they are said to be volatile.
- Simple covalent substances have simple molecular structures.
- Weak intermolecular forces of attraction exist between the molecules of the substances.
- A small amount of energy is needed to overcome these weak intermolecular forces.
What is the relationship of a simple covalent substance with the intermolecular forces and the melting/boiling points of the substances?
- The larger the molecule of a simple covalent substance, the stronger the intermolecular forces of attraction and the higher the melting and boiling points of the substance.
- E.g. Chlorine is a larger molecule than nitrogen, and it has stronger intermolecular forces of attraction which require more energy to overcome. Thus, chlorine has higher melting and boiling points than nitrogen.
Are all simple covalent substances insoluble in water and soluble in organic solvents?
- YES, simple covalent substances are insoluble in water, but soluble in organic solvents.
Why are the simple covalent substances unable to conduct electricity?
- Simple covalent substances are unable to conduct electricity as they do not have delocalised electrons/ions, which act as charge carriers.
Are covalent substances able to conduct electricity?
- YES, covalent substances that dissociate in water can conduct electricity.
- E.g. Hydrogen chloride (HCl) is soluble in water.
- E.g. Aqueous solution can conduct electricity, due to the presence of mobile H+ and Cl- ions.
What are giant covalent substances?
- Giant covalent substances are usually in the solid state as they consist of many atoms connected to one another by strong covalent bonds.
- E.g. of giant covalent substances: Diamond, graphite, and silicon dioxide.
What are allotropes?
- Allotropes are different forms of the same element with just different structural arrangements of atoms.
- E.g. of allotropes: Diamonds and graphite.
What is a diamond, and does it have any melting and boiling points?
- Diamond (material) has high melting and high boiling points.
- Diamond has a giant covalent structure and each carbon atom is covalently bonded to 4 other carbon atoms in a TETRAHEDRAL ARRANGEMENT.
- A large amount of energy is needed to break the strong covalent bonds.