2.1-2 – chemistry and water · 2020-04-08 · regents biology! ph scale 10–1 h+ ion...
TRANSCRIPT
Regents Biology
2.1-2 – Chemistry and Water
Prepared by Kim Foglia. Adapted and modified by Nhan Pham.
Regents Biology
Objectives § Discuss why we study chemistry in biology § Review structure of an atom § Explain the role of valence electrons in chemical
bonding § Distinguish different types of bonds
Regents Biology
Why study chemistry in a biology class?
Regents Biology
Life requires ~25 chemical elements § About 25 elements are essential for life § 4 elements make up 96% of living matter
• carbon (C) • hydrogen (H) • oxygen (O) • nitrogen (N)
§ Four elements make up most of remaining 4% • phosphorus (P) • calcium (Ca) • sulfur (S) • potassium (K)
Regents Biology
The Atom § indivisible unit of matter § 100 million atoms side
by side ~ 1 cm
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The Octet Rule § Electrons
determine the chemical behavior of atom
§ Valence electrons (on the outer most shell)
§ 8 is the magic number!
Regents Biology
Bonding properties
How does this atom behave?
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–
–
–
Chemical reactivity Atoms tend to: § complete a partially filled valence shell OR § empty a partially filled valence shell
This tendency drives chemical reactions…
and creates bonds
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Sodium Metal in Water
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–
–
H2 (hydrogen gas)
Covalent bond
Bonds in Biology § Weak bonds - ionic - hydrogen bonds - hydrophobic and hydrophilic
interactions - van der Waals interaction
§ Strong bonds - covalent bonds
Hydrogen bond
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Bond Strength
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Ionic Bond § 1 or more electrons are
transferred from one atom to another atom
§ Ex: NaCl (table salt) § Why?
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Covalent bonds § Why are covalent bonds strong? - two atoms share a pair of electrons - both atoms “holding onto” the electrons - very stable
§ forms molecules
–
–
H2 (hydrogen gas)
H — H
H2O (water)
H
H
Oxygen H
H O
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Multiple covalent bonds
§ 2 atoms can share more than 1 pair of electrons § double bonds § triple bonds
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van der Waals § slight attraction
between oppositely charged regions of nearby molecules
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Objectives § Explain why water is polar § Examine different properties of water and explain
how they are good for living things
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Station 1 – Surface Tension and Adhesion § Water molecules
“stick” to each other
§ Cohesion § Such “stickiness”
creates high surface tension
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Water is a Polar Molecule
§ Oxygen wants electrons a lot more than hydrogen à pulls electrons toward itself à becomes a slightly negative “pole”
§ The hydrogen atoms become positive “poles”
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Hydrogen Bond § This is why water
molecules stick to each other like little magnets
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Station 2 – Climbing Property § Water molecules not
only stick to each other, they also… stick to other molecules (adhesion)
§ climb up the piece of paper… and inside tree trunks
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How does H2O get to top of trees? Transpiration is built on cohesion and adhesion
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Station 3 – Polarity of Water § Water is polar § Oil is non-polar à They don’t MIX
§ How is this good for living things?
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Station 4 – Universal Solvent § Solvent – substance that
dissolve, or break apart, another substance (solute)
§ Water dissolves many substances that are polar, ionic, and even non-polar
§ How is this good for living things?
Regents Biology
Water is the solvent of life § What makes H2O a good solvent? POLARITY § polar H2O molecules surround (+) and (–) ions
Regents Biology
Dissolved Oxygen in Blood
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Station 5 – High Specific heat § It takes a long time for water
to change temperature à high specific heat
§ How is this good for living things?
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Specific heat § H2O resists changes in temperature § H2O moderates temperatures on Earth
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The special case of ice § Most (all?) substances are more dense when they are
solid… but not water. § Why is this property important?
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Ionization of water & pH § Water ionizes
§ pH scale
H2O → H+ + OH–
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pH Scale 10–1
H+ Ion Concentration
Examples of Solutions
Stomach acid, Lemon juice 1
pH
100 Hydrochloric acid 0
10–2 2
10–3 Vinegar, cola, beer 3
10–4 Tomatoes 4
10–5 Black coffee, Rainwater 5
10–6 Urine, Saliva 6
10–7 Pure water, Blood 7
10–8 Seawater 8
10–9 Baking soda 9
10–10 Great Salt Lake 10
10–11 Household ammonia 11
10–12 Household bleach 12
10–13 Oven cleaner 13
10–14 Sodium hydroxide 14
tenfold change in H+ ions
pH1 → pH2 10-1 → 10-2
10 times less H+
pH8 → pH7 10-8 → 10-7
10 times more H+
pH10 → pH8 10-10 → 10-8
100 times more H+
Battery acid
Regents Biology 1 0 0
1
2
3
4
5
6
7
8
9
3
Amount of base added
Buffering range
4 5 2
pH
Buffers and Cellular Regulation § pH of cells must be kept ~7. Why?
§ pH affects shape of molecules
§ shape of molecules affect function
à pH affects cellular function
§ Control pH by buffers
reservoir of H+ - donate H+ when
[H+] falls - absorb H+ when
[H+] rises