Showing posts with label Chemistry Form 4. Show all posts
Showing posts with label Chemistry Form 4. Show all posts

[Chemistry Form 4] Forming Theories

Man's first step in modern chemistry probably dated back to the discovery of fire. Thanks to it, man could cook food, make glass and also extract metals from ores.

Man was so content with the results of what they had done that they did not try to explain what had happened or why something had happened.

Only two thousand and five hundred years ago, philosophers began to question what things are made of, and what happens when things change.

The following is a brief history of chemists who had pondered about matter;
  1. It was the 5th century BC in Greece. Empedocles suggested that all things on Earth are made from four fundamental substances he called elements. What were these four elements?
    • Fire, water, air and earth.

  2. Another Greek philosopher, born at the time Empedocles died, proposed that nature is made up of tiny particles he called atoms, which, in Greek, means indivisible. Who was this Greek philosopher?
    • Democritus.

  3. The atomic hypothesis was contested by the greatest philosopher at the time who remained faithful to the element theory. Because of his great reputation, the false element theory dominated scientific thought for two thousand years. Who was this famous Greek philosopher?
    • Aristotle.

  4. In later years, history has it that the Alexandrians were the first to devote themselves to alchemy, and they were soon followed by the Arabs, who introduced it into Western Europe. When was this?
    • 4th century AD.

  5. Chemistry then made little headway for years. In 1525, a Swiss doctor and scientist challenged his students to tear up their books, which recorded old theories that had been developed through reasoning, and told them to find out for themselves through experiments whether a scientific theory was right or wrong. This scientist had an imposing name. Who was this scientist?
    • Theophrastus Bombastus Paracelsus Von Hohenheim.

  6. Later on, Robert Boyle, an Englishman, succeeded in killing off the old idea of the four elements by establishing that there are other elements - substances that cannot be formed by or broken into other substances. When was this?
    • 1661
A hundred years after Boyle made his discovery, a Swede, Karl Scheele, and an Englishman, Joseph Priestley, succeeded in isolating oxygen. Modern chemistry was thus born.

[Chemistry Form 4] Two-way Conversion

Electrochemistry is the science that studies the relation between electricity and chemical changes. So how are they related?

An electric current can bring about an otherwise non-spontaneous chemical reaction, such as the recovery of metals from their ores and electroplating of surfaces.

In the process called electrolysis, electrical energy is converted into chemical energy, which is stored in the products of the reaction.

Aluminium and copper are some examples of the products of electrolysis.

Conversely, chemical energy can be converted into electrical energy.

This process can be seen in spontaneous chemical reactions and forms the basis for batteries and fuel cells in supplying electrical power.

An electrolyte is an electrically conductive substance that contains positively- and negatively- charged particles called ions.

The following are electrolytes and non-electrolytes;
  • Electrolytes
    1. Copper (II) chloride
    2. Molten aluminium oxide
    3. Sodium hydroxide solution
    4. Silver nitrate solution

  • Non-electrolytes
    1. Aqueous ammonia
    2. Copper (II) sulphate crystal
    3. Glucose solution
    4. Ethanol
    5. Solid sodium chloride
    6. Molten naphthalene
    7. Acetamide
    8. Tetrachloromethane

Did You Know?
  • A battery is a device that converts chemical energy into electrical energy. In 1800, Italian physicist Alessandro Volta developed the first electric battery that generated a steady stream of electricity. His invention was called the Voltaic pile.

    Since then, the principles applied to the Voltaic pile have been used in batteries. In 1881, the volt (V), an electrical unit, was named after him.

[Chemistry Form 4] Ions In Solution

Swedish chemist Arrhenius defined acids as substances that, when dissolved in water, produce hydrogen ions (H+). Eg: When gaseous hydrogen chloride reacts with water, the amount of hydrogen ions in the water increases and hydrochloric acid is thus produced.



HCl (g) ---> H+ (aq) + Cl- (aq)

Acid

(H+ producer)



Some of the food and medicines that we take contain acids. Ethanoic (acetic) acid in vinegar and acetylsalicyclic acid in aspirin, for instance.



Contrary to acids, bases are defined as substances that, when dissolved in water, yield hydroxide ions (OH-). Eg: When sodium hydroxide dissolves in water, the amount of hydroxide ions in the water increases and the alkaline solution is thus produced.



NaOH (s) ---> Na+ (aq) + OH- (aq)

Base

(OH- producer)



Did You Know?

The compounds responsible for colours in plants are often sensitive to acids and alkalis. For example, blue hydrangeas grow only in acidic soils; in neutral or alkaline soils, they turn to pink.

[Chemistry Form 4] Alkaline Aqueous

Hydroxides of sodium, potassium, magnesium and calcium are bases. Bases have many common features: they have a bitter taste, are slippery to the touch, change the colour of indicators, etc. Aqueous solutions of soluble bases are called alkalis.



Solution vs Ionisation

Some strong bases like calcium hydroxide aren't very soluble in water. That doesn't matter as what does dissolve is 100% ionised into calcium ions and hydroxide ions. Thus, calcium hydroxide still counts as a strong base because of that 100% ionisation.



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Hydroxide that are alkalis:

  • NaOH
  • Mg(OH)2
  • KOH
  • LiOH
  • NH4OH


The following the dissociation of acids and base.



Hydrochloric acid

Formula: HCl

Equation to show dissociation: HCL -> H+ + Cl-



Nitric acid

Formula: HNO3

Equation to show dissociation: HNO3 -> H+ + NO3-



Ethanoic acid

Formula: CH3COOH

Equation to show dissociation: CH3COOH -> CH3COO- + H+



Potassium hydroxide

Formula: KOH

Equation to show dissociation: KOH -> K+ + OH-



Sodium hydroxide

Formula: NaOH

Equation to show dissociation: NaOH -> Na+ + OH-



Calcium hydroxide

Formula: Ca(OH)2

Equation to show dissociation: Ca(OH)2 -> Ca2+ + 2OH-



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[Chemistry Form 4] Concentration Of Solution

The concentration of a solution refers to the quantity of solute in a given volume of solution. The units can be expressed in two forms:



Concentration of solution in g dm-3

= Mass of solute (g) / Volume of solution (dm3)



Concentration of solution in mole dm-3 (molarity)

= Number of moles of solute (mole) / Volume of solution (dm3)



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1. 20g of sodium hydroxide is dissolved in 500cm3 of solution. Calculate the

a) concentration of the solution in g dm-3

40g dm-3



b) molarity of the solution in mole dm-3

( Relative atomic mass of Na = 23; O = 16; H = 1)

1 mole dm-3



2. Calculate the number of moles of calcium hydroxide in 250cm3 of 0.5 mole dm-3 calcium hydroxide solution.

Hint: The number of moles = Molarity X Volume in dm3

0.5 X (250/1,000)



3. Calculate the mass of calcium hydroxide in 200cm3 of 0.4 mole dm-3 calcium hydroxide solution.

(Relative atomic mass of Ca = 40; O = 16; H = 1)

Hint: Find the number of moles of calcium hydroxide first.

0.4 X (200/1,000) X RMM of Ca(OH)2



4. You are required to prepare 250cm3 of 5 mole dm-3 sodium hydroxide solution. Calculate the mass of sodium hydroxide required.

(Relative atomic mass of Na = 23; O = 16; H = 1)

Hint: Find the number of moles of sodium hydroxide first.

5 X (250/1,000) X RMM NaOH

[Chemistry Form 4] Acid Meets Base

One easy way to distinguish acid and base from one another is through indicators. An example is the litmus indicator.

Also, many natural dyes found in fruits vegetables, and flowers can act as indicators. For example, red cabbage extract is red in an acidic solution and blue in a base solution.

When an acid reacts with a base, the reaction yields water and an ionic compound known as salt. This acid-base reaction is called a neutralisation reaction.

***
Indicator: Litmus
Acid solution: RED
Neutral solution: PURPLE
Alkali solution: BLUE

***
Indicator: Phenolphthalein
Acid solution: Colourless
Neutral solution: Colourless
Alkali solution: PINK

***
Indicator: Methyl Orange
Acid solution: PINK
Neutral solution: ORANGE
Alkali solution: YELLOW

***
Indicator: Bromothymol Blue
Acid solution: YELLOW
Neutral solution: GREEN
Alkali solution: BLUE

***
Indicator: Cresol Puple
Acid solution: YELLOW
Neutral solution: GREEN
Alkali solution: PURPLE

***

[Chemistry Form 4] Conductive

  • Conductors are substances that can conduct electricity in a solid or molten state, and are not chemically changed.

  • Electrolytes are substances that can conduct electricity either in a molten state or an aqueous solution, as well as undergo chemical changes at the same time.

  • Non-electrolytes are substances that cannot conduct electricity either in a molten state or an aqueous solution.

  • Electrolysis is the decomposition of a chemical compound into its constituent elements when an electric current passes through an electrolyte.

  • An electrolytic cell ia a device that uses electrical energy to generate chemical reactions.

  • Electrodes are conductors that transmit electricity through the electrolyte during electrolysis.

  • Cathode is the electrode that is connected to the negative terminal of a battery.

  • Anode is the electrode through which the electrons leave the electrolyte and move to the external circuit.

  • Molten compounds or aqueous solutions can conduct electricity due to the presence of free moving ions.

  • During electrolysis, cations move towards the cathode, where they will be discharged by accepting electrons, whereas anions move towards the anode, where they will be discharged by donating electrons.

  • A voltaic or galvanic cell is a device that converts chemical energy into electrical energy.
 

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