Do chemical properties differ from chemical reactions
Characteristics of the chemical reaction
Change of material properties
If iodine or ice is heated, only the state of aggregation changes. If you cool down, the water solidifies again to ice. From the violet iodine vapors, crystals are formed again through resublimation. In both cases, the material properties have been retained, so there are no chemical reactions.
The situation is different if ammonium dichromate is heated as described in experiment 2. Under an appearance of fire, the water-soluble, orange-colored crystals form a green, water-insoluble powder. The fact that a new substance has now been created can be recognized by the newly created properties and therefore this process is a chemical reaction.
Analysis and synthesis are known as the basic types of chemical reactions. Chemical reactions are constantly taking place in our environment.
Chemical reaction and energy participation
With chemical reactions, the substances change and there is a turnover of energy. Energy is according to the definition of physicsthe ability to do work. It does not matter whether the work is carried out in a biological cell or by a machine. In experiments 1 and 2, thermal energy was released, and the synthesis of iron sulfide and zinc sulfide (Chapter 7) also took place with the release of thermal energy. The unit of thermal energy is the joule. To heat 1 kg of water from 14.5 to 15.5 ° C, 4.19 kJ (1 kilojoule = 1000 joules) are required.
Chemical reactions that take place with the release of thermal energy are called exothermic reactions. But there are also reactions that continuously require thermal energy; they are referred to as endothermic reactions. For example, the analysis of mercury oxide (Experiment 2, Chapter 7) stops if one does not heat continuously. In technology, reactions that provide energy are very important, just think of an oven or the engine of a car. Every movement of a muscle in the body is characterized by chemical reactions that involve energy and is vital for all living beings.
The activation of a chemical reaction
The energy involved in a chemical reaction is not always visible, for example when iron rusts. Nevertheless, it is there and can be detected with fine measuring instruments. In experiments 2 and 3 it was observed that chemical reactions can also involve energy in the form of light. Electrical energy can also appear in chemical processes.
Tests 1 and 2 proceeded with considerable heat emission (i.e. exothermic), but heating had to be carried out before the start in order to get the reaction going. Only then did the reactions take a strongly exothermic course. The energy required to "ignite" a chemical reaction is called activation energy. They put substances in a ready-to-react state.
Examples of exothermic reactions
- the neutralization of an acid by an alkali.
- the combustion of fuels and fuels.
- Breathing in the cells of the body. It provides energy to keep the heart and lungs going, to keep the body warm and the energy we need to move.
Examples of endothermic reactions
- Reactions that take place while cooking food.
- the polymerization of ethene to polyethylene.
- the reduction of silver ions to silver in photography.
- Electrolysis. The energy is provided in the form of electricity.
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