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Chemistry 121 Chapter 3 Study material for this chapter : All the sections of chapter 3 in the textbook CALCULATIONS WITH CHEMICAL FORMULAS AND EQUATIONS 3.1. Chemical Equations Lavoisier observed that mass is conserved in a chemical reaction, this is known as the law of the conservation of mass The quantitative aspects of dealing with chemical formulas and reactions is called stoichiometry Chemical reactions are represented by chemical equations o In any chemical equation there are: o reactants (to the left of the arrow) o and products (to the right of the arrow) HCl + NaOH NaCl + H 2 O There are two types of number is a chemical equation o Numbers in front of the chemical formulas, called coefficients o Numbers that appear as subscripts in the formulas o Stoichiometric coefficients give the ratio in which the reactants and products exist The subscripts give the ratios of the atoms in the molecule o Example : H 2 O 2 shows that there are two hydrogen atoms and two oxygen atoms per molecule of hydrogen peroxide H 2 O 2 o 2 H 2 O shows that there are two molecules of water – in total there are 4 hydrogen atoms and 2 oxygen atoms present on H 2 O Balancing Equations Matter cannot be lost or created in a chemical reaction o The atoms on the right hand of the chemical equation must therefore be equal to the atoms on the left-hand side o C 6 H 14 + O 2 CO 2 + H 2 O o If the atoms are counted on the right there are o 1 C, 2 H and 3 O-atoms o on the left-hand side we have 6 C, 14 H and 2 O o it appears as if atoms have been created or destroyed, and this cannot be o to balance the chemical equation, we adjust the coefficients as follows: o 2 C 6 H 14 + 19 O 2 12 CO 2 + 14 H 2 O o we now have 12 C atoms on both sides, 28 H atoms and 38 O-atoms 1
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Indicating the States of the Reactants and Products The physical state of each reactant and product may be added to the equation CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(g) (g) denotes a gas; (l) denotes a liquid; (s) denotes a solid Homework Problems: 3.11 and 3.13 in Text 3.2. Patterns of Chemical Reactivity Combustion in Air Combustion reactions are rapid reaction that produce a flame Combustion is the burning of a substance in air An example is propane in a gas stove which combusts to give H 2 O and CO 2 C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) Combination and Decomposition Reactions In combination reactions two or more substances react to form one product Combination reactions therefore have more reactants than products o 4Al (s) + 3 O 2 (g) 2 Al 2 O 3 (s) In decomposition reactions on substance undergoes a reaction to produce two or more substances Decomposition reactions therefore have more products than reactants o Consider the reaction in an automobile air bag 2NaN 3 (s) 2Na(s) + 3N 2 (g) Homework Problems: 3.17 and 3.19 33 Formula Weights Formula and Molecular Weights Formula weight (FW) is the sum of atomic weights of all elements in a chemical
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