Minerals and gases, in this case, carbon dioxide and oxygen, are the two main factors that speed up chemical reaction rates and weathering in rocks and soils.
Do you want to learn how the two factors affect chemical reactions?
Continue reading to learn that and much more!
What Two Factors Speed Up Rates of Chemical Reaction and Weathering in Rocks and Soils? (EXPLAINED)
Definition of Terms
It is a process where molecules in one or more substances known as reactants bond or leave their former structures to create new products, either compounds or elements.
A chemical reaction recombines the atoms that make up the reactants into different products. An example is:
Silicates + water= clay + water
Chemical reactions break down and weaken the rock along areas of weakness or faults into softer substances which are easily carried away by agents of transport, thus increasing the rate of weathering of rocks.
The term weathering refers to the whole process of rock degradation where rocks break down through the influence of other factors into other smaller and softer substances such as soils, clays, sediment, and other water-soluble substances.
The process of weathering usually kicks off when the earth’s crust is impacted by tectonic forces causing fractures, fault lines, and areas of weakness.
The cracks allow infiltration of other substances into the rocks, which begins the process of weathering.
After the chemical decay of uncovered rocks and physical breakup, the loose rock particles and other products leave the rock through erosion.
Erosion uses transporting agents like snow, ice, wind, rivers, and avalanches to carry away the weathered products from the mother rock, which exposes the fresh rocks beneath to further weathering.
The natural cycle will repeat until a whole mountain flattens over long periods.
Weathering occurs in two ways, namely;
(i) Chemical weathering occurs when the elements that form the rocks, such as minerals, chemically react internally or with external agents like water and air to form new substances.
Oxygen will oxidize the substances to altered products while water catalyzes this process and dissolves minerals into new substances like clay.
(ii) Physical weathering occurs when the rock breaks down through physical forces or mechanical processes like freeze-thaw cycles, wind erosion, rock fracturing, and erosion during transport by glaciers and rivers.
Factors That Speed Up the Rate of Chemical Reactions and Weathering
Chemical reactions and weathering naturally occur slowly over many years.
That’s why mountains and hills are still around.
However, certain elements have been observed to aid these processes to happen much faster.
They speed up the energy of the atoms, which increases collision.
Here are some of them.
Oxygen and Carbon Dioxide
Oxygen is essential for many life processes and increases chemical reactions in almost everything from digestion, respiration, combustion to weathering of rocks.
Oxygen is abundant in the atmosphere and affects chemical weathering rates.
Exposure of rocks and soils to the surface permits the free atmospheric oxygen to oxidize the elements increasing the rates of chemical reactions and weathering.
Oxidation occurs when certain elements such as iron (Fe) exchange electrons with oxygen to form metal oxides.
During oxidation, oxygen steals one or more electrons from Fe.
metal + oxygen = metal oxide
The positive ions in iron, for example, dissolve in water and react with the oxygen to form oxides with Fe2+ – Fe3+ (highest) as shown below;
4Fe + 3O2 = 2Fe2O3
Another example is when iron-bearing silicates like pyroxene react with oxygen dissolved in water to form Fe2+ and Fe3+, which react with more oxygen to form an iron oxide hematite precipitate.
Don’t be fooled by the chemical equations.
Simply put, the rocks contain iron and silicates above rust, which is evidenced by red and brown colors observed in desert sediments and humid regions with red soils.
Rust breaks down soils and rocks over time to form new substances.
The only areas spared from rapid oxidation are swamps and deep lakes since oxygen cannot reach certain depths, and the available oxygen is consumed by animals and plants for biological processes rapidly.
Carbon dioxide is another gas freely found in the atmosphere that increases the rate of chemical weathering in rocks and soils through acidity.
Acid is bad news because it corrodes or eats away substances.
Atmospheric carbon is not solely to blame because living organisms in soils and rocks produce or perform activities that release high carbon dioxide concentrations.
The carbon dioxide dissolves in rainwater, forming weak carbonic acid as shown by the following equation;
CO2 + H2O = H2CO3
The acidic rainwater, in turn, reacts with exposed rock surfaces to form new elements.
Slightly acidic water enhances the transformation of silicates to clay.
A chemical reaction between silicates and weak carbonic acid to form clay also releases Si and other positive ions, which dissolve in water, as shown below.
+ carbonic acid + silicate + water = dissolved SiO2 + clay + dissolved bicarbonate (Na+,K.) +dissolved cations
Rainwater and rivers transport dissolved cations and dump them into the oceans and other water bodies.
The elements or minerals that make up certain rocks will determine how fast those rocks react chemically with other substances such as water and weak carbonic acid.
For example, when feldspar in granite and calcite in limestone is exposed to similar conditions, hydrolysis will occur faster in calcite than in feldspar.
Quartz is very tough and will resist chemical weathering for longer than calcite.
If some rock were composed of calcite cemented grains, it would disintegrate faster than a rock with quartz cemented grains.
As shown below, acid rainwater can also speed up chemical reactions in non-silicate rocks or soils.
CaCO3 + H2CO3 = Ca2+ 2HCO3-
Halite is also water soluble
NaCl = Na+ + Cl-
Minerals that dissolve in acids or water are generally the easiest to weather, for example, calcite, halite, and gypsum.
Silicates such as those made of single chains or isolated silica tetrahedra are easier to weather than those composed of multiple silica tetrahedra sheets because they have lower silica to oxygen rations.
Surprisingly, minerals made up of chemical weathering by-products successfully resist further chemical reactions except that they are now more susceptible to physical weathering.
One other factor that can speed up the rate of chemical reactions and weather in rocks and soils apart from gases and minerals include;
Climate is a key determinant of both physical and chemical weathering because it controls water and temperature.
Remember that water is key in the hydrolysis of minerals.
Climate decides whether the water is in physical or liquid form.
For example, liquid water increases the rate of chemical weathering while solid water such as ice increases the rate of physical weathering in rocks and soils.
Climate also dictates the existing temperatures and higher temperatures speed up chemical weathering.
Climate also determines the kind of plant and animal life in certain areas.
Plants wedge their roots in cracks speeding up physical weathering.
Microorganisms release organic acids in soils, thus aiding in chemical weathering.
This means that chemical weathering in warmer tropical rainforests is higher than in cold Arctic deserts.
However, physical weathering will occur more predominantly in the Arctic.
Several factors increase the rate of physical and chemical breakdown in rocks and soils.
They include oxygen, carbon dioxide, minerals, and climate.
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