Chemical reactions
chemical equations tell us what is happening in reaction
CH4 +O2 --> CO2 +H2O
reactants products
How do we know when a chemical reaction has taken place?
Signs a reaction has occurred
Temperature change
Gas released
Possible color change
Possible shape change (phase change)
Solid forms
Change in magnitism
When a reaction occurs--> breaking or making bonds
What part of the atom does this involve?
What are two types of chemical bonds?
Reactions rearranges atoms--> does not create or destroy atoms
Three “basic groups” of chemical reactions
Precipitation/dissolution reaction--> solid forms or dissolves
Acid-Base reaction--> joining acid with base creates water + salt
Oxidation-reduction--> transfer of electrons
Dissolution/Precipitation reactions
ionic compounds (salts) dissolve or precipitate…Why?
Dissolution
Dissolve ionic compound in Water
Barium Nitrate [Ba(NO3)2]
Water is a polar molecule
polar nature of water “pulls” the ionic compound apart
Solution no longer has Ba(NO3)2
Solution contains Ba2+ and NO3- ions
Precipitation
solid forms when bonds are created
ex// mix together 2 ionic solutions
solution above with Ba2+ and NO3- ions with a solution of K+ and CrO42- ions
Will a solid form (I.e. will a reaction take place)?
Solubility of compound determines if a reaction occurs
very soluble--> stays in solution or dissolves readily
slightly soluble--> will bond and create compound or dissolves slightly
insoluble--> won’t dissolve in solution
“Guessing” if reaction goes is based on solubility rules
Ex// most nitrate salts are soluble (NO3-)
most sulfide, carbonate and phosphates are only slightly soluble etc…
You don’t need to know these…know what is meant by solubility of compounds
How does this type of chemical reaction relate to weathering of rocks?
Polar water molecules cause dissolution of minerals
destroys ionic bonds
Supersaturation of solutions cause precipitation of new minerals
(more on this type of sedimentary rock later)
Acid/Base reactions
Arrhenius Concept of Acids and Bases
Acid: a substance that produces H+ ions when dissolved in water
HNO3 à H+ + NO3-
Base: a substance that produces OH- ions when dissolved in water
NaOH à Na+ + OH-
Combining an acid with a base creates water + salt
HNO3 + NaOH à H2O + NaNO3
Solids (metal and non-metal oxides)
non-metal oxides--> SO3, CO3 ect…
react with water and create acid (H2SO4, H2CO3)
metal oxides--> CaO, Li2O
react with water to form bases (CaO(OH)2, 2LiOH)
Acid/base Strengths
how much of the acid or base reacts
complete reaction (none of the original molecules left)
partial reaction (some of the original molecules left)
pH scale measures the strength of an acid or base
based on the H+ concentration
low numbers acid, 7 is neutral, high numbers are basic
How do acid/base reactions affect weathering of rocks?
Water can behave as a weak acid
Combines with natural materials to form stronger acids on earths surface.
Increasing the acidic nature of H2O
Silicic acid created by weathering silicate minerals
Carbonic acid (H2CO3):
Organic matter
Limestone
Atmosphere
Sulfuric acid:
Car exhaust
Volcanic eruptions
Acid will break down minerals/rocks in natural environments by replacing cations in minerals structures with H+ cations. Known as Hydrolysis…changes the minerals to something new
Oxidation/reduction reactions
reactions of metals with non-metals
transfer of electrons from one element to another
creates ions
ex// 2 Mg (s) + O2 (g) --> 2MgO
Produces white flash in flashbulbs
often known as combustion reactions
Oxidation--> losing electrons
Reduction--> gaining electrons
can’t have one without the other--> something is oxidized and something is reduced
How does oxidation/reduction weather rocks?
Often occurs between Iron and Oxygen
minerals rich in iron (olivine, pyroxenes etc) dissolve or breakdown into other components
Based on these reactions...
Where would you expect chemical weathering to dominate?
Where would you expect mechanical weathering to dominate?