Chemical Reactions - General Chemistry - Lecture Slides, Slides of Chemistry

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Chemical Reactions: An Introduction docsity.com Chemical Reactions • Reactions involve chemical changes in matter resulting in new substances • Reactions involve rearrangement and exchange of atoms to produce new molecules – Elements are not transmuted during a reaction Reactants  Products docsity.com Figure 6.2: Hot and cold pack reactions docsity.com Figure 6.3 (a): Chemical reactions docsity.com Figure 6.3 (b): Chemical reactions docsity.com Chemical Equations • Shorthand way of describing a reaction • Provides information about the reaction – Formulas of reactants and products – States of reactants and products – Relative numbers of reactant and product molecules that are required – Can be used to determine weights of reactants used and of products that can be made docsity.com Figure 6.4: The reaction between methane and oxygen to give water and carbon dioxide docsity.com Conservation of Mass • Matter cannot be created or destroyed • In a chemical reaction, all the atoms present at the beginning are still present at the end • Therefore the total mass cannot change • Therefore the total mass of the reactants will be the same as the total mass of the products docsity.com Writing Equations • Use proper formulas for each reactant and product • proper equation should be balanced – obey Law of Conservation of Mass – all elements on reactants side also on product side – equal numbers of atoms of each element on reactant side as on product side • balanced equation shows the relationship between the relative numbers of molecules of reactants and products – can be used to determine mass relationships docsity.com Symbols Used in Equations • symbols used after chemical formula to indicate state – (g) = gas; (l) = liquid; (s) = solid – (aq) = aqueous, dissolved in water docsity.com Balancing by Inspection 1. Count atoms of each element • polyatomic ions may be counted as one “element” if it does not change in the reaction Al + FeSO4 Al2(SO4)3 + Fe 1 SO4 3 • if an element appears in more than one compound on the same side, count each separately and add CO + O2  CO2 1 + 2 O 2 docsity.com Examples • when magnesium metal burns in air it produces a white, powdery compound magnesium oxide – burning in air means reacting with O2 3. count the number of atoms on each side – count polyatomic groups as one “element” if on both sides – split count of element if in more than one compound on one side Mg(s) + O2(g)  MgO(s) 1  Mg 1 2  O  1 docsity.com Examples • when magnesium metal burns in air it produces a white, powdery compound magnesium oxide – burning in air means reacting with O2 4. pick an element to balance – avoid element in multiple compounds 5. find least common multiple of both sides & multiply each side by factor so it equals LCM Mg(s) + O2(g)  MgO(s) 1  Mg 1 1 x 2  O  1 x 2 docsity.com Examples • when magnesium metal burns in air it produces a white, powdery compound magnesium oxide – burning in air means reacting with O2 6. use factors as coefficients in front of compound containing the element  if coefficient already there, multiply them together Mg(s) + O2(g)  2 MgO(s) 1  Mg 1 1 x 2  O  1 x 2 docsity.com Examples • Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 3. count the number of atoms of on each side – count polyatomic groups as one “element” if on both sides – split count of element if in more than one compound on one side NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 3  H  2 2  O  1 + 1 docsity.com Examples • Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 4. pick an element to balance – avoid element in multiple compounds 5. find least common multiple of both sides & multiply each side by factor so it equals LCM NH3(g) + O2(g)  NO(g) + H2O(g) 1  N 1 2 x 3  H  2 x 3 2  O  1 + 1 docsity.com Examples • Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 6. use factors as coefficients in front of compound containing the element 2 NH3(g) + O2(g)  NO(g) + 3 H2O(g) 1  N 1 2 x 3  H  2 x 3 2  O  1 + 1 docsity.com Examples • Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 10. Repeat – A trick of the trade, when you are forced to attack an element that is in 3 or more compounds – find where it is uncombined. You can find a factor to make it any amount you want, even if that factor is a fraction! – We want to make the O on the left equal 5, therefore we will multiply it by 2.5 2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g) 2  N 2 6  H  6 2.5 x 2  O  2 + 3 docsity.com Examples • Under appropriate conditions at 1000°C ammonia gas reacts with oxygen gas to produce gaseous nitrogen monoxide and gaseous water 11. Multiply all the coefficients by a number to eliminate fractions – 0.5 x 2, 0.33 x 3, 0.25 x 4, 0.67 x 3 2 x [2 NH3(g) + 2.5 O2(g) 2 NO(g) + 3 H2O(g)] 4 NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g) 4  N 4 12  H  12 10  O  10 docsity.com