Chemical Reaction Equations Section 7.1 pg. 278-285
Reaction Assumptions Reactions are spontaneous – reactions will occur when all the reactants are mixed together Reactions are fast – the reaction must occur within a reasonable time (see pg. 280) Reactions are quantitative – one that is more than 99% complete; in other words, at least one reactant is completely used up Reactions are stoichiometric – means that there is a simple whole-number ratio of chemical amounts of reactants and products (the coefficients for a balanced equation do not change)
Net Ionic Equations A chemical reaction equation that includes only reacting entities (molecules, atoms and/or ions) and omits any that do not change Writing Net Ionic Equations: 1) Write a complete balanced chemical equation 2) Dissociate all high-solubility ionic compounds, and ionize all strong acids to show the complete ionic equation strong acids to show the complete ionic equation 3) Cancel identical entities that appear on both the reactant and product sides product sides 4) Write the net ionic equation, reducing coefficients if neccessary neccessary
Practice Write the net ionic equation for the reaction of aqueous barium chloride and aqueous sodium sulfate. (Refer to the solubility table) 1) BaCl 2(aq) + Na 2 SO 4(aq) BaSO 4(s) + 2NaCl (aq) 2) Ba 2+ (aq) + 2Cl - (aq) +2Na + (aq) + SO 4 2- (aq) BaSO 4(s) + 2Na + (aq) + 2Cl - (aq) (Complete ionic equation) 3) Ba 2+ (aq) + 2Cl - (aq) +2Na + (aq) + SO 4 2- (aq) BaSO 4(s) + 2Na + (aq) + 2Cl - (aq) 4) Ba 2+ (aq)) + SO 4 2- (aq) BaSO 4(s) (Net ionic equation) Ions that are present but do not take part in (change during) a reaction are called spectator ions (like spectators at a sports game: they are present but do not take part in the game) When cancelling spectator ions, they must be identical in every way: chemical amount, form (atom, ion, molecule) and state of matter
Practice Write the net ionic equation for the reaction of zinc metal and aqueous copper (II) sulfate (Refer to the solubility table) 1) Zn (s) + CuSO 4(aq) Cu (s) + ZnSO 4(aq) 2) Zn (s) + Cu 2+ (aq) + SO 4 2- (aq) Cu (s) + Zn 2+ (aq) + SO 4 2- (aq) (Complete ionic equation) 3) Zn (s) + Cu 2+ (aq) + SO 4 2- (aq) Cu (s) + Zn 2+ (aq) + SO 4 2- (aq) 4) Zn (s) + Cu 2+ (aq) Cu (s) + Zn 2+ (aq) (Net ionic equation)
Practice Write the net ionic equation for the reaction of hydrochloric acid and barium hydroxide 1) 2HCl (aq) + Ba(OH) 2(aq) BaCl 2(aq) + 2HOH (l) aka: 2H2O (l) aka: 2H2O (l) 2) 2H + (aq) + 2Cl - (aq) + Ba 2+ (aq) + 2OH - (aq) Ba 2+ (aq) + 2Cl - (aq) + 2H 2 O (l) (Complete ionic equation) 3) 2H + (aq) + 2Cl - (aq) + Ba 2+ (aq) + 2OH - (aq) Ba 2+ (aq) + 2Cl - (aq) + 2H 2 O (l) 4) H + (aq)) + OH - (aq) H 2 O (l) (Net ionic equation) – coefficients reduced to 1 For entities involving strong acids, H + (aq) is used as a matter of convenience over H 3 O + (aq)
Limiting and Excess Reagents Cu (s) + AgNO 3(aq) What is in the container when the reaction is finished? Cu(NO 3 ) 2(aq) + Ag (s) (Cu 2+ (aq) = blue) Cu(NO 3 ) 2(aq) + Ag (s) (Cu 2+ (aq) = blue) But how do you know the reaction is done?? There is still copper left! (a) Copper wire and a beaker with aqueous silver nitrate solution (b) A few moments after the wire is immersed (c) The beaker contents after 24 h
Limiting and Excess Reagents When no further changes appear to be occurring, we assume that all of the AgNO 3(aq) that was initially present has now been completely reacted. A limiting reagent is the reactant whose entities are completely consumed in a reaction, meaning the reaction stops. In order to make sure this happens, more of the other reactant must be present than is required An excess reagent is the reactant whose entities are present in surplus amounts, so that some remain after the reaction ends.. In our reaction: much more copper was used than needed (evidenced by the unreacted copper) so we assume the reaction ended when no more silver ions were left, so silver nitrate was the limiting reagent.
Homework: Net Ionic Worksheet Pg.284 #10-14 Pg. 285 #3-7
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