Chemical reactions
Chemical reactions
Assumed perquisite knowledge:
Students understand the difference between a physical change and a chemical change
Introduction
- A chemical reaction occurs when substances combine or breakdown to produce something NEW.
- A chemical reaction is accompanied by a chemical change. eg. permanment change in color or the production of a gas.
Chemical changes indicate that something new has been produced. eg. the permanent color change produced as bread is toasted in a toaster or the smoke from a burning match.
- Chemical reactions are generally difficult to reverse. eg. turning a slice of toast into a slice of fresh bread.
The chemicals that react are called the reactants. The new materials that are produced are called the products.
A chemical reaction can be shown by the use of a word equation. The ==> means 'give or produce'
| Reactants | ==> | Products |
| chemicals that react | ==>
|
chemicals that are produced |
| hydrogen + oxygen | ==> | water |
| hydrogen peroxide | ==> | water + oxygen |
| magnesium + oxygen | ==> | magnesium oxide |
| sodium chloride | ==> | sodium + chlorine |
Types of chemical reactions.
- There are many different types of reactions which include
- Corrosion of metals. Eg. The rusting of iron
- Combustion: Eg. The burning of fossil fuels
- Acid and base : Eg. Hydrochloric acid and sodium hydroxide.
- Acid and carbonate: Eg. Vinegar and bicarb of soda
- Acid and active metal: This type of reaction is also called a single displacement reaction. Eg. The dissolving of magnesium ribbon in hydrochloric acid.
- Precipitation: This type of reaction is also called a double displacement reaction. Eg. Mixing colorless lead nitrate and potassium iodide to yellow produce lead iodide.
- Decomposition: Eg. Hydrogen peroxide being broken down into water and oxygen
- Synthesis: Eg. The burning of magnesium in air to produce magnesium oxide.
- Displacement:. Eg. The production of copper wool by from steel wool being placed into a solution of copper sulphate.
- Redox (Reduction/ Oxidation): Eg. Making a battery.
- Condensation: Eg. Glucose and Fructose producing sucrose
- Dehydration: Eg. Adding concentrated sulphuric acid to sugar
- Substituion: Eg. Bromine reacting with octane in the presence of UV light.
- Addition: Eg. Adding bromine to cyclohexene
- Polymerisation: Eg. making Nylon, polyethylene
- Fermentation: Eg. The making of wine
- Exothermic: Eg. The thermite reaction is used to weld rail tracks together. The reaction is so hot molten iron is produced!
- Endothermic: Eg. An ice pack. Mix the contents inside the packet and the mixture becomes cold
Chemical change
Chemical change
A chemical change is a sign that indicates that a chemical reaction has occurred.
Chemical changes include
- The production of a gas. Eg. Fizz from sherbet
- A permanent color change. Eg. A red solution changing to a colorless solution
- The production of light. Eg. Glow in the dark light sticks which is an example of chemiluminesce
- The production of a flame. Eg. A Bunsen burner
- The dissolving of an insoluble solid. Eg. Magnesium dissolving in acid.
- The production of an insoluble solid. Eg. The production of a precipitate
These changes indicate that something new has been produced.
Physical changes on the other hand are easy to reverse and don't produce anything new. They only involve a change in the physical states (solid. liquid and gas). eg. the melting of an ice cube and steam being produced from a kettle.
Corrosion
Corrosion reactions
Corrosion is the loss of metallic properties of a metal as the metal reacts with the atmosphere or water. e.g. strength, lustre or shine and electrical conductivity.
Rust is brownish red in color and is formed from the corrosion of iron. Other metals like copper and aluminium also corrode or weaken.
For corrosion to occur both water and oxygen need to be present.
Examining the corrosion of iron using nails:
| Oxygenated water (shaken) |
Humid air |
Dry air (anhydrous CaCl2) |
Deoxygenated water (Boiled water/ oil layer) |
Salt water |
|
|
|
|
|
|
|
Rust |
Rust |
No rust |
No rust |
Extensive rust |
If a nail is placed in deoxygenated water it will not rust. If a nail is placed in dry air it will not rust. For rust to occur both oxygen and water need to be present. The presence of salt accelerates or speeds up the rate of corrosion.
The chemical formula of rust is Fe2O3.nH2O
The process of rust formation is due to a series of chemical reactions. Points of stress acts in the nail allow the iron to be easily oxidised. Carbon impurities in the nail allow water to be reduced. An internal battery or galvanic cell is set up.
Oxidation of iron at points of stress in the crystal lattice: 2Fe(s) ==> 2Fe2+(aq) + 4e-
Reduction of water at the site of carbon impurities: O2(g) + H2O(l) + 4e- ==>4OH-(aq)
Overall equation: 2Fe(s) + O2(g) + H2O(l) ==>Fe(OH)2
The iron(II) hydroxide is converted to rust through a serious of reactions.
The ion(II) hydroixde firstly oxides to iron(III) oxide.
1. Fe(OH)2(s) =oxidation-=> Fe(OH)3
The iron(III) oxide then changes to rust through a dehydration reaction.
2. Fe(OH)3(s) =dehydration=> Fe2O3.nH2O(s) or rust
Rust does adheres loosely to the surface of the metal. This exposes the metal to more and more water and oxygen allowing rust to continue to proceed.