Fertilizer Chemistry Chemistry 521

Transcription

1 Agriculture Learning Outcome #3 Agriculture Learning Outcome #6 Chemistry Learning Outcome #114-7 Chemistry Learning Outcome # Chemistry Learning Outcome # Critical thinker Problem solver and decision maker Nature of science and technology Perform stoichiometric calculations related to chemical equations Identify various stoichiometric applications Assignment: Recognize the practical applications of chemistry in a routine agricultural situation. Preparation: Prior knowledge and skills: Materials/Resources: Chemistry concepts of moles, Avogadro s number and molar mass. Work with SI and significant figures in measurements and calculations. Wilbraham et als. Chemistry. Addison-Wesley Publishing Company: Don Mills, Ontario, Activity: Answer questions on Fertilizer Chemistry student handout following classroom instruction on stoichiometric applications. Adapted with permission from the California Foundation for Agriculture in the Classroom's educational unit, The Chemistry of Fertilizers;

2 1. Ammonium nitrate is used as a fertilizer; ammonia is also used as a fertilizer. Compute the percentage of nitrogen in each. If both cost the same amount per tonne, which is the better buy in nitrogen content? 2. A farmer has two different fertilizers in her barn. One is K 2 SO 4 and the other is KCl. The two fertilizers cost the same per tonne, but she wants to use the one with the highest percentage of potassium. Which fertilizer should she use? Substantiate your explanation with numbers. 3. Calcium nitrate and ammonium nitrate are used as fertilizers. Compute the percent nitrogen in each. If calcium nitrate costs $225 per tonne and ammonium nitrate costs $275 per tonne, which fertilizer is more cost effective? 4. Animal manures are often used to add nutrients to soils. Assume that the average price for chicken manure is $15 per tonne and contains 16 kg of nitrogen per tonne. a. What percentage nitrogen does the manure contain? b. What is the cost per kilogram of nitrogen? c. Which is more cost effective to apply chicken manure, calcium nitrate or ammonia nitrate? Explain. d. What are some issues, other than price, that might affect which of the fertilizers a farmer or gardener might choose to use? 5. Super phosphate fertilizer is made by treating phosphate rock, Ca3(PO4)2 with sulfuric acid according to this equation: Ca 3 (PO 4 ) 2 + 2H 2 SO 4 Y Ca(H 2 PO 4 ) 2 + 2CaSO 4 If this reaction has a 52.5% yield, how much Ca(H 2 PO 4 ) 2 could be obtained from 5.2 metric tonnes of phosphate rock? 6. Review the questions and answers in the problems above. How could this type of information help you as a home gardener? Why is it crucial that farmers understand how to do basic chemistry such as this?

3 Answer Key 1. Ammonium nitrate is used as a fertilizer; ammonia is also used as a fertilizer. Compute the percentage of nitrogen in each. If both cost the same amount per tonne, which is the better buy in nitrogen content? Ammonium nitrate NH 4 NO 3 vs. Ammonia NH 3 N 1 x = N 1 x = H 4 4 x = H 3 3 x = N 1 x = O 3 3 x = %N = / x 100 %N = / x 100 %N = = 35%N %N = 82.2%N Ammonia is the better buy. It is applied as anhydrous ammonia, a colourless gas. The gas is injected into the soil, usually about 10 centimetres deep. While ammonia is the cheapest source of N, many safety precautions must be used in its applications. Students should understand that factors other than price are considered in selecting nitrogen material. 2. A farmer has two different fertilizers in her barn. One is K 2 SO 4 and the other is KCl. The two fertilizers cost the same per tonne, but she wants to use the one with the highest percentage of potassium. Which fertilizer should she use? Substantiate your explanation with numbers. K 2 SO 4 KCl K 2 2 x = K S 1 x = Cl O 4 4 x = %K = / x 100 %K = / x 100 %K = 49.4% %K = 52.4% The farmer would get a better buy if KCl was used.

4 3. Calcium nitrate and ammonium nitrate are used as fertilizers. Compute the percent nitrogen in each. If calcium nitrate costs $225 per tonne and ammonium nitrate costs $275 per tonne, which fertilizer is more cost effective? Ca(NO 3 ) 2 NH 4 NO 3 Ca 1 x = N 2 x = N 2 x = H 4 x = x = O 3 x = %N = / x 100 = 17.1% %N = / x 100 = 35.0% For every tonne of calcium nitrate, only 17.1% is the desired, needed nitrogen. Therefore, for every thousand kilograms of calcium nitrate, only.171 x 1000 or 171 kilograms are actually nitrogen. For ammonium nitrate, 35% of the compound is nitrogen, so a tonne of ammonium nitrate has 350 kilograms of actual nitrogen. To calculate which is more cost effective, you take the cost of each material per kilogram of actual N in each material. cost of N in calcium nitrate $225 x 1000 kg Ca(NO 3 ) 2 = $225 = $1.32/kg 1000 kg Ca(NO 3 ) kg N 171 kg N cost of N in ammonium nitrate $275 x 1 tonne NH 4 NO 3 = $275 = $0.79/kg 1 tonne NH 4 NO kg N 350 kg N The basic premise is that cost per tonne of material and % N must be used to determine which N source is most cost effective. In this case, even though ammonium nitrate costs more for a tonne of material, it has a lot more nitrogen. This high concentration of nitrogen spreads the cost out and makes ammonium nitrate a cheaper source of desired nitrogen. This same theory applies to the consumer at the grocery store. Should you buy a whole chicken for $1.50/kg or bu boneless, skinless all white meat chicken breasts for $2.50/kg? Which one is the best buy of desirable, usable, edible chicken?

5 4. Animal manures are often used to add nutrients to soils. Assume that the average price for chicken manure is $15 per tonne and contains 16 kg of nitrogen per tonne. a. What percentage nitrogen does the manure contain? 16 kg N x kg 1 = = 1.6% nitrogen b. What is the cost per kilogram of nitrogen? $15 x 1 tonne x 1000 kg $0.94/kg 1 tonne 1000 kg 16 kg c. Which is more cost effective to apply chicken manure, calcium nitrate or ammonia nitrate? Explain. chicken manure = $0.94 calcium nitrate = $1.32 ammonium nitrate = $0.79 d. What are some issues, other than price, that might affect which of the fertilizers a farmer or gardener might choose to use? safety ease of application reduced nitrogen loss to the environment effect on soil microbes effect on soil organic matter plant s need for other nutrients availability of nitrogen for immediate and/or delayed uptake by plants transportation costs

6 5. Super phosphate fertilizer is made by treating phosphate rock, Ca3(PO4)2 with sulfuric acid according to this equation: Ca 3 (PO 4 ) 2 + 2H 2 SO 4 Y Ca(H 2 PO 4 ) 2 + 2CaSO 4 If this reaction has a 52.5% yield, how much Ca(H 2 PO 4 ) 2 could be obtained from 5.2 metric tonnes of phosphate rock? 5.2 tonnes x x molecular mass of Ca(H 2 PO 4 ) 2 ( ) = 2.06 tonne molecular mass of Ca 3 (PO 4 ) 2 ( ) Ca(H 2 PO 4 ) 2 Ca 3 (PO 4 ) 2 Ca 1 x = Ca 3 x = H 4 x = P 2 x = P 2 x = O 8 x = O 8 x = Review the questions and answers in the problems above. How could this type of information help you as a home gardener? Why is it crucial that farmers understand how to do basic chemistry such as this? Answers will vary.