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chemistry news Written by Suzanne Boniface Classifying chemical reactions
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Chemical reactions may be classified according to different criteria, such as: the nature of the species transferred during a reaction, visible changes that occur, or the nature of the atomic arrangement that occurs during the reaction. A simple classification based on the number and nature of the reactants and products and the type of transformation that is needed to convert the reactants to products can be used to classify most chemical reactions. Reactions can be classified as decomposition, combination, displacement and exchange. 1. Combination: elements or simple compounds come together to form a single more complex compound. More than one reactant joins together to form only one product – 2 or more particles combine to form only 1 particle. e.g. S(s) + O2(g) SO2(g) A
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2. Decomposition: a compound breaks apart into either elements or less complex compounds. One reactant visibly becomes two products – 1 particle breaks into 2 (or more) particles. This is the opposite of combinations. e.g. CuCO3(s) CuO(s) + CO2(g) A
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3. Displacement: a single element replaces another element in a compound. This means that there will be an element and a compound in the reactants, and a different element and a compound in the products. Often the elements will be metals, but the action of a metal on an acid to release hydrogen gas is also an example of a displacement reaction.
National Secondary Schools Quiz A number NZ Institute of Chemistry Branches have been running regional quizzes for secondary schools for a number of years now. These have proved to be lots of fun and popular with students. In 2011, the winners of the regional competitions were invited to a National Competition to help celebrate International Year of Chemistry (IYC). This event was organised by the Wellington Branch of NZIC on 5 July and was hosted by Dr Rob Keyzers. The six regional teams (23 boys and one girl) were from: James Hargest College (Invercargill); St Paul’s Collegiate School (Hamilton); Burnside High School (Christchurch); Macleans College (Auckland), Palmerston North Boys’ High School; and Wellington College. There were six rounds of nine questions containing material from the senior syllabus and beyond. Some were lighthearted but all encompassed chemistry as the central science.
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Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)
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Level 1 Science AS90947 and Level 1 Chemistry AS90934
New Zealand Association of Science Educators
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4. Exchange: atoms or groups of atoms are exchanged between two compounds. For ionic compounds the exchange reaction will be recognised when the ions are swapped between the dissolved compounds, and one of the products is insoluble and precipitates out. e.g. BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq) A
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Many more complex reactions can be classified as a sequence of two or more of the above reactions e.g. an exchange reaction for which one of the products is unstable will be followed by a decomposition reaction. When hydrochloric acid and limestone (calcium carbonate) react, carbon dioxide is produced. This could be considered in two steps, exchange followed by decomposition. Exchange: CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2CO3(aq) Decomposition: H2CO3(aq) H2O + CO2(g) Overall reaction: CaCO3(aq) + 2HCl(aq) CaCl2(aq) + H2O + CO2(g) This simple classification allows students to explain their observations of chemical reactions using their understanding of what is happening at the ‘molecular’ or submicroscopic level of particles and link these to the symbolic level of formulae and equations. As their understanding of the particles involved in reactions becomes more sophisticated, so too will their explanations and they will find new ways of classifying chemical reactions such as proton or electron transfer, nucleophilic or electrophilic substitution.
Competition was tight with the top two places only being separated by less than 2 marks. The results were: First: Macleans College; Second: St. Paul’s Collegiate School, and Third: Burnside High School. The complete prize package was generously provided by Agilent Technologies with the all members of the winning team and school receiving $300; second prize was $200 for each member of the team and school; and third prize was an MP3 player for each competitor.
Global Water Experiment It is not too late to get involved in the Global Water Experiment. Go to: http://water.chemistry2011.org/web/ iyc and sign up so that your students can collect and contribute NZ data to the global database. For more information about IYC events visit: http:/yearofchemistry.org.nz/ and http://www.chemistry2011.org/