PERTEMUAN12_EVIDENCE

EVIDENCES OF CHEMICAL REACTIONS

    Chemical reactions occur all the time in everyday life. Every time you eat, your body uses a chemical reaction to break your food into energy. Other examples include rusted metal, burning wood, electrically generating batteries, and photosynthesis in plants. Understanding chemical reactions.

      What are reactants, reactants, and products?

Reactants and reagents are substances used to perform chemical reaction processes. A reactant is a substance that is consumed or used during a reaction. Substances produced by chemical reactions are called products.

      All chemical bonds posses potential energy.  In a chemical reaction this energy is changed when old bonds are broken and new bonds are formed. Chemical changes are different from physical changes.  When a physical change occurs there is no breaking and forming of bonds.  There are certain things that will help us identify if a chemical reaction has taken place.  We call these evidences of chemical reactions.  

1. Light is given off: Potential energy in valance electrons is released in the form of light.
2. Gas is given off: Indicates an increase in kinetic energy of the atom (compound) as it is changed to a gas.
3. Temperature change: Exothermic is an increase in the speed of the molecules (heat). Exothermic is the taking in of energy.
4. Color change: the increase or decrease of potential energy stored in new bonds.
5. Precipitate formed: increase or decrease of potential energy in new bonds formed.
6. Electrical potential: The release of chemical bond energies to cause the movement of  valence electrons. 

A chemical change is characterized by the formation of new compounds that are different from the compound pereaksinya. Chemical reactions are said to take place if one thing is observed among them:

- The reaction changes color

- There is formed precipitate

- There is a temperature change or gas formation

EXAMPLE

  If Na2CO3 is reacted with HCl or with BaCl2 it will form the precipitate and the color of the solution becomes turbid.

  When NaOH is reacted with Pb (NO 3) 2 or with HCl there is no precipitate

      All chemical reactions, whether simple or complex, involve changes in substances. One or more starting substances, the reactants, are changed into one or more new substances, the products.

Reactants à Products

In a chemical reaction the ways in which atoms are joined together are changed. Bonds are broken and new bonds are formed as reactants are converted into products. The atoms are not created or destroyed. They are just rearranged.

Chemical reactions can be described in different ways. For example, we could say: "Iron reacts with oxygen to produce iron (III) oxide (rust)." Alternatively, we could identify the reactants and product in this reaction by writing a word equation.

 Chemical reactions can happen anywhere around us, not just in the laboratory. Matter interacts to form new products through a process called chemical reactions or chemical changes. Every time we cook or are cleaning, it is also a chemical in action. Our bodies live and grow thanks to chemical reactions.

1.      Fotosintesis

Photosynthesis is a process used by plants and other organisms to convert light energy, usually from the Sun, into chemical energy that can then be liberated to fuel organism activity. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. In most cases, oxygen is also produced as a waste product. Most plants, mostly algae, and cyanobacteria do photosynthesis, and these organisms are called photoautotrophs.

6 CO2 + 6 H2O + light → C6H12O6 + 6 O2

2.      Respiration

Aerobic cellular respiration is the reverse process of photosynthesis in molecular energy coupled with the oxygen we breathe to release the energy required by our cells plus carbon dioxide and water. The energy used by cells is the chemical energy in the form of ATP (adenosine triphosphate).

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (36 ATPs)

3.      Corrosion

Rust is iron oxide, usually a red oxide formed by redox reactions of iron and oxygen in the presence of water or air humidity. Some forms of rust are distinguished both visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron (III) oxide Fe2O3 · nH2O and iron (III) oxides-hydroxides (FeO (OH), Fe (OH) 3).

Here is the chemical equation for iron rust:

Fe + O2 + H2O → Fe2O3. XH2O

4.      Mixing chemicals

If we just combine vinegar and baking soda to make chemical volcanoes or milk with baking powder in a recipe, we experience the displacement or double reaction metathesis (plus some other things). The material recombines to produce carbon dioxide and water gases. The form of carbon dioxide bubbles in the volcano and can help increase roasting.

These reactions seem simple in practice, but often consist of several steps. Here is the overall chemical equation for the reaction between baking soda and vinegar:

HC2H3O2 (aq) + NaHCO3 (aq) → NaC2H3O2 (aq) + H2O () + CO2 (g)

5.      Chemical reactions by  Soaps and detergents

         Soaps and detergents can be cleaned by using chemical reactions. Soap emulsifies dirt, which means soap binds stain oil stains so they can be cleaned with water. The detergent acts as a surfactant, lowering the surface tension of the water so it can interact with oil, isolate them, and and clean it.