2.3 continued

Chemical reactions occur when atoms bond together to form molecules or when bonds between atoms break apart.


Molecules-

  •    bonded atoms (02,H2, C6H1206)

Compounds

  •     molecule with more than one element. (H20)


Types of bonds

  •     Covalent-   

    • Polar covalent=

    • Nonpolar covalent=

  • Hydrogen.

  • Ionic.



  • Ionic=strong bonds (stealing of electrons) creates two opposite charges/

  • Sodium loses electron to chlorine

  • Sodium becomes positive (+)

  • Chlorine becomes negative (-)

  • They are attracted (like magnets)



Types of Chemical bonds: Covalent


  • Covalent bonds are weak, 2 atoms share electrons to complete outer shell.

  • Non-polar Covalent 2 atoms ahre electrons equally to fill their outer shell.

  • Polar covalent bond: 2 atoms share electrons unequally.

    • E.g Oxygen holds electrons more ofen= slightly negative

    • E.g Hydrogen holds electrons less often=slightly positive.


Double Covalent Bonds

  • 2 atoms share 4 electrons

  • Double bonds are stronger than single bonds




Hydrogen bonds

  • Weak “bonds” because slight charges (from polar covalent)

    • The hydrogen bonds give water its properties

      • Covered in a few slides.


How to write formulas


  • Structural formula-

    • Uses straight lines H-H

    • 1 line indicates 1 pair of shared electrons (bond).

  • Molecular formula

    • simply shows a number of atoms involved.


Chemical Formulas and Reactions

Equation is balanced if the same number of each type of atom occurs on both sides of the arrow.

  • An overall equation for photosynthesis

    • reactnats 6CO2 + 6H20 ——> C6H12O6 +6O2. Products


  • 6 carbon diozide +6 water → 1 glucose molecule +6 oxygen molecules

  • 6 carbon+ 18 oxygen +12 hydrogen→ 6 carbon + 18 oxygen +12 hydrogen.

  • Practice: What does this mean 6NH4 = 6 Nitrogen and 24 Hydrogen.



Waters importance to life

  • Life began in water

  • All organisms 70-90% water

  • Water has unique properties that make it life-supporting

    • The properties come from chemical structure (polar covalent bond)

    • Hydrogen bonds.


Properties of Water

  • Solvency

    • Water is a solvent.

    • Polarity and H-bonds of water dissocate (break up into individual elements) substanaces that are ionic (NaCl-salt) or

    • Dissolve substances that are polar (sugar) (compounds stay intake, but they are no longer lumped together)

  • Cohesion and Adhesion

    • Cohesion: water clingts to water

    • Ahdesion: water clingts to polar surfaces

      • This helps water move through animals and plants

  • Surface tension

    • Water cohesion keeps water bonded together

    • Surface resists breaking or evaporating.

  • Specific heat, Latent Heat Vaporization, latent heat of fusion.

    • High specific heat (1mcpas/gram/1 deg. C)

    • H-bonds allow water to absorb a lot of heat.

    • Takes a lot of energy to warm up or cool down water.

  • Heat of vaporizatiuon (540 cal/g/deg.) and heat of fusion (80 cal/g/deg.)

    • A lot of energy is needed to change from liquid to water vapor (steam), or to/from a sold.

    • Evaporation cools us down.


  • Varying Density

    • Ice is less dense (floats) than water

    • Water most dense at 4 degrees Celsius

    • Warm water floats on cold water

    • Ice insulates water and keeps it warmer.

  • Acids and Bases

    • Water. dissociated into H+ and OH-

    • Acids= lemon juice, vinegar, coffee.

      • Hydochloride acid HCl → H+ and Cl

    • Bases= baking soda (NaOH), chalk (tums)

    • pH= tells how many H+ in solution

      • 7 is netural (water)

      • Greater than 7 is basic= has more OH- than H+

      • Less than 7 is acidic = has more H+ than OH-