AS & A Levels: Chromosomes part 1

A living organisms grow and reproduce. since organism

The nucleus contains chromosomes

Before a eukaryotic cell divides, a number of thread like structures called chromosomes gradually 

become visible in the nucleus. They are easily seen, because stain intensely with particular stains. they were originally termed chromosomes, because 'chromo' means colored and 'some' means body.

The number of chromosomes is characteristic of the species. such as, in human cells there are 46 chromosomes, and in fruit fly there are only 8 chromosomes.

• There are matching pairs of chromosomes called homologous pairs. Each pair is given a number.
• Human chromosome is 46 which means 23 from the father and 23 from the mother
• Sex chromosomes, which determine the sex of the individual. All other chromosomes are called autosomes.
• The pairs of chromosomes can be distinguished not only by size and shape, but because each pair has a distinctive banding pattern when stained with certain, as











Set of chromosomes in human  (Karyogram)





A Level: Enthalpy changes part 3

A Level: Enthalpy changes part 2

AS & A Levels: The Avogadro constant

IGCSE: Moles Concept part 1

IGCSE: Moles Extra Task

A Level: Enthalpy Changes part 1

 

A Level  chemistry 2/5/2014

Enthalpy Changes  Part 1

 students should:

1.     know that reactions can be endothermic or exothermic

2.     understand that enthalpy change (ΔH) is the heat energy change  measured under conditions of constant pressure

3.      know that standard enthalpy changes refer to standard conditions,   i.e. 100 kPa and a stated temperature (e.g. ΔH298)

4.      be able to recall the definition of standard enthalpies of combustion  (ΔHc ) and formation (ΔHf )

                   Exothermic and endothermic reactions

     If a reaction produces heat (increases the temperature of the surroundings)  it is exothermic

     If the temperature of the reaction mixture decreases (i.e. heat is absorbed) then the reaction is endothermic.

Exothermic a reaction which produces heat (ΔH has a negative value by convention, -ve)

Endothermic a reaction which absorbs heat (ΔH has a positive value by convention, +ve)

Enthalpy of reaction: The change in internal (chemical) energy (H) in a reaction   = ΔH. 

    The most stable state is where all energy has been released.

    When going to a more stable state, energy  will be released, and when going to a less stable state, energy will be gained (from the surroundings).

    On an enthalpy level diagram, higher positions will be less stable (with more internal energy)  therefore, if the product is lower, heat is released (more stable, ΔH is -ve) but if is higher, heat is gained (less stable, ΔH is + ve).

    

If

A Level: Chemistry Class Test

1.          Calculate the volume occupied by 2,50 mol of carbon dioxie at a pressure of 100 KPa and a temperature of 19 oC.

(R= 8,31 JK -1 mol -1

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2.             A flask volume 5,00 dm3 contained 4.50 gram of oxygen. Calculate the pressure exerted by the gas at a temperature of 250  oC.

(Rr = 8,31 JK -1 mol -1 , Mr Oxygen = 32.0)

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3.             Calculate the volume occupied by 272 g of methane at a pressure of 250 KPa and  a temperature of 55 0C

(R = 8,31 JK-1 mol -1); Mr methane = 16 .0)

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4.       Describe 4 the behaviors of liquid state?

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5.          Explain the following:

a.       Why most metals are the strong, but ionic solids are brittle?

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b.      Why is an alloy of copper and tin stronger than either copper or tin alone?

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6.          Crystals of sodium chloride have a lattice structure.

a.       Describe a sodium chloride.

b.      Explain the following properties of sodium chloride.

(i)                  Sodium chloride has a high melting point

(ii)                Sodium chloride conducts electricity when molten but not when solid

(iii)               Sodium chloride is hard but brittle

  

7.             Why purifying and remolding aluminium is much cheaper than extracting aluminium from bauxite ore?

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8.          What is ceramics?

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9.          Describe the terms alloy !

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10.         Why is brass stronger than pure copper?

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