9: Relating the Properties of Substances to Structures and Bonding Flashcards
What is the name of the structure of ionic solids?
Giant ionic structure
Describe the structure of sodium chloride solid.
Sodium chloride consists of sodium ions and chloride ions that are packed together in a regular arrangement called a lattice. Billions of sodium ions and chloride ions are arranged in this way to make up a giant ionic structure. Ionic bonding extends throughout the whole structure. Each sodium ion is surrounded by 6 chloride ions and each chloride ion is surrounded by 6 sodium ions. They are arranged in a cubic pattern, so sodium chloride crystals are cubic in shape.
What are the general properties of ionic compounds?
- They are hard but brittle
- They have high melting and boiling points
- They are usually soluble in water
- They conduct electricity in molten or aqueous states (they are electrolytes)
Why are ionic compounds hard and brittle?
Ionic compounds are hard because the relative motion of ions is restricted by the strong ionic bonds between the ions.
Since an ionic compound forms a lattice of alternating positive and negative ions, when an external force is applied, the layers of ions may move relative to each other. This can bring ions of the same charge close to each other an result in replusion between the ions. As a result, the lattice splits.
Why do ionic compounds have high melting and boiling points?
There are strong ionic bonds between the ions with opposite charges. In order to melt and boil ionic compounds, lots of strong ionic bonds between the ions have to be overcome. This takes a lot of heat, so ionic compounds have high melting points and boiling points. All ionic compounds are crystalline solids at rtp.
Why are ionic compounds usually soluble in water?
In order for a substance to dissolve in a solvent:
1. the attractive forces between its particles and solvent particles > (can overcome) attractive forces between particles in the pure substance
2. particles in the substance must separarte from each other and become surrounded by the solvent particles.
Since water has partial positive and partial negative charges, the partial positive ends attract the negative ions while the partial negative ends attract the positive ions. The attraction is sufficient to remove the ions from the lattice.
Why are ionic compounds usually insoluble in non-aqueous solvents?
The strong ionic bonding within the lattice > the weak van-der Waals’ forces between the ions and the solvent molecules. The solvent cannot pull the ions out of the lattice.
What are the conditions that must be met for a substance to conduct electricity?
It must contain mobile charged particles.
Why cannot ionic compounds conduct electricity in solid state?
The ions are in fixed positions and not free to move around (not mobile).
Why can ionic compounds conduct electricity in molten or aqueous forms?
When an ionic compound is melted, the ions are mobile throughout the liquid. When electricity is passed through the molten compound, cations move towards a negative electrode while anions move towards a positive electrode, allowing the conduction of electricity.
When soluble ionic compounds are dissolved in water, these aqueous solutions can conduct electricity because the ions are free to move around in aqueous solutions.
List 3 forms of elements that have giant covalent structures.
Carbon (in the form of diamond and graphite), silicon
Give an example of a compound having giant covalent structure.
Silicon dioxide (found in quartz)
What are allotropes?
Allotropes are two or more forms of the same element in which the atoms or molecules are arranged in different ways.
Describe the structure of diamond.
A diamond has a giant covalent structure. In a diamond, each carbon atom is covalently bonded to 4 neighbouring carbon atoms. All the caron atoms are arranges tetrahedrally. The strong covalent bonds extend in all directions throughout the structure.
Describe the structure of graphite.
In graphite, the carbon atoms are arrnaged in flat layers. Each layer contains billions of carbon atoms arranged in hexagons. Each carbon atom is covalently bonded to 3 neighbouring carbon atoms so every layer has a giant covalent structure. Only weak van der Waals’ forces exist between the layers of carbon atoms.
Why can graphite conduct electricity?
With three covalent bonds formed between carbon atoms within the layers, an unbonded outermost shell electron is present on each atom. These unbonded electrons become delocalised along the layers of carbon atoms. They can only move within the layer.
Describe the structure of silicon dioxide.
Each silicon atom is bonded to 4 oxygen atoms while each oxygen atom is bonded to only 2 silicon atoms. Silicon atoms and oxygen atoms in the whole structure are held together by strong covalent bonds.
What is the chemical formula of silicon dioxide and why?
SiO2: The ratio of atoms in silicon dioxide is two oxygen atoms for every one silicon atom. (Si:O = 1:2)
What are the general properties of giant covalent substances and exceptions?
- They are very hard (exception: graphite)
- They have high melting points
- They are insoluble in water or non-aqueous solvents
- They do not conduct electricity (exception: graphite)
Explain why giant covalent substances are hard, thus suggest a usage of diamond and quartz.
In giant covalent structures, strong covalent bonding extends throughout the structure. Relative motion of the atoms is restricted. This makes the substances very hard and useful as abrasives.
Sand paper is coated with grains of quartz.
Diamond is the hardest substance known. It is used for cutting and grinding hard materials such as glass and stone.
Why do giant covalent structures have a high melting point?
In order to melt a giant covalent substance, lots of strong covalent bonds between atoms have to be broken. These bonds need a lot of heat to break. Thus, these substances have high melting points.
Why are giant covalent substances insoluble in water or non-aqueous solvents?
Weak van der Waals’ forces between their atoms and solvent molecules < (is not strong enough to overcome) the strong covalent bonds that hold the atoms together.
Why are giant covalent substances electrical insulators (except for graphite)?
In most giant covalent substances, all the outermost shell electrons are either held by individual atoms or used to form covalent bonds. There are no mobile electrons or ions. Thus, these substances do not consuct electricity.
List two properties of graphite that are different from other giant covalent substances.
- Only weak van der Waals’ forces exist between the layers of carbon atoms, so the layers can slide past each other easily. Therefore, graphite is soft and feels slippery to touch.
- Graphite is a conductor of electricity.