Predicting the Rate Law for an Elementary Reaction
If the following reaction occurs in a single elementary reaction, predict its rate law:
H2(g) + Br2(g)→2 HBr(g)
Consider the following reaction: 2 NO(g) + Br2(g) →2 NOBr(g). (a) Write the rate law for the reaction, assuming it involves a single elementary reaction. (b) Is a single-step mechanism likely for this reaction?
Lecture 16 – Hydrology Hydrology Why do we care We can’t survive very long without access to water There is a finite amount of water on Earth Hydrologic Cycle Ocean is the largest water storage Although people believe that rivers and lakes would be the second largest, glacial ice and groundwater are actually larger storage Part 1 – Groundwater Runoff: water that s running off the surface of the land Infiltration: water soaks into the ground; water is infiltrating the surface This water becomes our groundwater supply Infiltration Properties Has to be room for water to go into. If water fell onto a completely solid surface, it wouldn’t infiltrate Porosity: the total amount of open space in a given area Usually expressed as a percentage Types of Porosity 1) Intergranular pores: little spaces in between grains Each pore space is usually very, very small But when you add each of the little spaces, it becomes one of the largest pore spaces in an area Huge reservoir when combined 2) Fracture porosity: lots of crack within the rock that can be filled in with water Since fractures are bigger than individual pores, you can get a lot of storage of water Amount of water you can store depends on how many fractures you have 3) Vuggy porosity: Vugs: an opening in a rock that represent an area that has been dissolved away in a rock Can be very small, but can also be big enough for a person to easily fit through Can hold a tremendous amount of water because of their size, but they aren’t very common everywhere Controls on Porosity Determined by sediment/rock properties Sorting- when the sediments or rocks are well sorted, there is high porosity Rocks don’t usually fit very well together like puzzle pieces do, so there is a lot of room in between the rocks Poorly Sorted: when the sediments/rocks are porrly sorted, there is low porosity Since the rocks are such different sizes, instead of water filling the spaces in between rocks, smaller sediments will fill the space, so water won’t be able to fit as well Cementation: if the rocks are becoming lithified and have particles holding the sediments together, there isn’t going to be much room for water to fit through and vice versa Infiltration Properties Permeability: measure of how much water can flow through an area Have to have permeability and porosity to have a good water system Can have high permeability and low porosity or high porosity and low permeability, and that won’t lead to a great water system Need both conditions at the same time for a good water system Groundwater Zone of Aeration (aka Unsaturated Zone or Vadose Zone): area close to surface when water first goes into the ground Zone of Saturation (aka Phreatic Zone): where the pores in the ground become full of water Water table (WT): boundary between the unsaturated zone and saturated zone Above water table, lots of empty pore space, and below water table, pores are filled with water Groundwater Supply Aquifer: any layer of sediment or rock that will produce a supply of water Recover for human use via wells Some areas may have multiple aquifers Types of Aquifers 1) Unconfined aquifer: above the aquifer, there is no area of material that disturbs the flow of water System that can easily sustain itself 2) Aquitard: a layer of material below aquifer that will prohibit water flow 3) Confined Aquifer: an aquifer in between two aquitards Difficult to put water back into it Will have a lot of runoff, but hardly any infiltration Have to find some spot in the area that rainfall will be able to infiltrate the area Artesian well: in confined aquifers, you build up a lot of water pressure since in between two areas If you know where water pressure is high, you can put a hole there and water can make its way to the surface with its own high pressure Pumping equipment is expensive, so if water is able to get to the surface using its own water pressure, its much less expensive More common towards the bottom of a hill or slope 4) Perched Aquifer (aka Perched WT): an aquifer that is perched halfway up a slope because the water infiltrates the area above an impervious layer The impervious layer doesn’t go all the way down the hill Don’t have to drill as deep, so less expensive Water Supply Recharge: how much water you’re putting back into the aquifer Discharge: how much waterflowing out of the aquifer If recharge is greater than the discharge, the water table will rise Bigger aquifer, and bigger water supply Can cause problems With construction, if the WT is too high, it will be difficult to Overdrafting: if discharge is greater than recharge Leads to many problems Effects of Overdrafting 1) Cone of Despression: water table around a well gets sucked downward Happens because water is flowing out faster than it’s being replaced Appears like a cone and the water table at that area is being depressed Eventually, won’t be able to get water out of well because WT is too low Area will be useless until water table can rise up again To fix the problem, slow down the rate of pumping 2) Subsidence: the ground willsink downward When you pump water out very quickly, you remove some of the support for the ground Water pressure in pores can help keep the land in place When you pump out the water too fast, the ground level will sink It will be very difficult to refill the aquifer because when the water gets pumped out so quickly, the porosity changes Can try to pump water back into aquifer, but usually nowhere for water to go Ex) California 3) Salinity Contamination: if in a coastal area, salinity can contaminate the aquifer Ocean water can seep into the aquifer and replace the fresh water that’s already there Desalinization: taking salt out of water We have the technology to do this, but very expensive Groundwater Movement Groundwater moves very slowly, especially compared to surface water because it has to move through such small pore spaces Good: most of your water stays in the same place for a long time Bad: if the water becomes contaminated, the contaminated water just sits there for a long time Could filter out the contaminants, but would be expensive Erodes, even at slow speeds Groundwater carries dissolved substances CO2 and SO2 Dissolve carbonate rocks Can cause sink holes—dissolve materials under the surface slowly over time until the ground collapses Groundwater Contamination—Case Study: Love Canal Love Canal, Niagra Falls NY Early 1900s tried to build a canal along the Niagra River, but never got finished, so empty canals just sat there lined with cement 1940s: bought to hold chemical waste because was already lined with cement 1960s: became a place for people to move to and the town grew because became a nice place to live The chemical company didn’t want to sell the area because filled with chemical waste, but was forced to sell Contract 1970s: a lot of impermeable sediments were removed with construction of basements and in this time, recharge was very high so water table rose Rising water table brought up chemical wastes with it People started noticing that health was decreasing Over 50% of infants were born with defects 1978: local homeowners learned that there was 21000 tons of chemical waste underground Children would get chemical burns just by falling and rolling around in grass, which demonstrates how much the chemical waste has risen Who’s responsible August 7, 1978: Jimmy Carter declared a federal state of emergency, ordering families to evacuate 9/11 and Hurricane Katrina were also federal states of emergency, so this was a huge deal, and it was just caused by groundwater 1980: “Superfund Act” allowed federal money to be used to help clean up the area Became the very first superfund site 1994: the legal cases were resolved; the chemical company was found responsible and had to pay millions of dollars Love Canal Aftermath: 2008: survey of 4 states found that 500,000 kids in schools were less than half a mile from waste dumps, including one on top of a PCB dump (can cause cancer)