Class notes for Chapter 7: Chemical Reactions and Quantities in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include chemical and physical change, chemical and physical properties, stoichiometry, and the thr...
Chemical and Physical Change
●Chemical Change: change that alters the molecular composition of a substance
○Atoms rearrange and original substance transforms into a different substance
○Ex: rust, sugar burning, photosynthesis
●Physical Change: change that alters the state or appearance of a substance but not its chemical composition
○Atoms do not change identity
○Ex: water boiling, cutting paper
●Chemical Property: property that a substance displays only by changing its composition via chemical change
○Ex: flammability, corrosiveness, acidity
●Physical Property: property that a substance displays without changing its chemical composition
○Ex: odor, color, melting/boiling point
●Only chemical examination can confirm whether a particular change is chemical or physical
○However both chemical and physical changes have characteristic changes
■Chemical changes usually have temperature/color change
■Physical changes often have change in physical condition or state of matter
Writing and Balancing Chemical Equations
●Chemical Reaction: process by which one or more substances are converted to one or more different substances
●Chemical Equation: symbolic representation of a chemical reaction
○Balanced equation contains equal numbers of the atoms of each element on both
sides of the equation
○Ex: Combustion of natural gas is represented by the equation:
●Reactants: starting substance in a chemical reaction
○Appear on the left side of the equation
●Products: substance produced in a chemical reaction
○Appear on the right side of the equation
●We often specify the state of each reactant/product in parentheses next to the formula ●(g) indicates gases, (l) indicates liquid, (s) indicates solid, (aq) indicates aqueous (water solution)
●This equation for natural gas combustion is not complete – unequal number of O and H atoms on each side!
○Rewrite equation showing unequal sides
○Need to balance the equation
●Balancing equations:
○Change the coefficients (number in front of chemical formula)
○Keep the subscripts (the numbers within the chemical formula)
●Guidelines for balancing equations to make it easier
○Balance atoms in most complex substances first and simplest substances/elements last
■When balancing free elements, only adjust the coefficient of the free element
●Ex: O2 → H2O = ½O2 → H2O
●May come out as a fraction
○If balanced equation contains coefficient fractions, clear these by multiplying the entire equation by the denominator of the fraction
●EX: C4H10(g) + O2(g) → CO2(g) + H2O (g)
○Balance complex substances first
■Balancing C
●C4H10(g) + O2(g) → 4CO2(g) + H2O(g)
■Balancing H
●C4H10(g) + O2(g) → 4CO2(g) + 5H2O(g)
○Balance free elements last
■Balancing O
●C4H10(g) + 13/2O2(g) → 4CO2(g) + 5H2O(g)
○If there are coefficient fractions, multiply entire equation by denominator
■13/2 coefficient before O
●2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)
○Check to make sure it’s balanced!
■8 C on both sides
■20 H on both sides
■26 O on both sides
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