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Answers
Answer from Librarian50

A trial-and-error approach to balancing chemical equations involves playing with the equation


Goals for Balancing Chemical Equations

1. The number of atoms of each element on both sides of the equation is the same and therefore mass is conserved.

2. The sum of the positive and negative charges is the same on both sides of the equation and therefore charge is conserved. (Charge is conserved because electrons are neither created nor destroyed in a chemical reaction.)

There are two situations in which relying on trial and error can get you into trouble. Sometimes the equation is too complex to be solved by trial and error within a reasonable amount of time. Consider the following reaction, for example.

3 Cu(s) + 8 HNO3(aq) 3 Cu2+(aq) + 2 NO(g) + 6 NO3-(aq) + 4 H2O(l)

Other times, more than one equation can be written that seems to be balanced. The following are just a few of the balanced equations that can be written for the reaction between the permanganate ion and hydrogen peroxide, for example.
Sources: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/oxred_2.php

 

Answer from onsakthi

The Half-Reaction Method of Balancing Redox Equations.


Balancing Oxidation-Reduction Equations

A trial-and-error approach to balancing chemical equations involves playing with the equation--adjusting the ratio of the reactants and products--until the following goals have been achieved.

Goals for Balancing Chemical Equations

1. The number of atoms of each element on both sides of the equation is the same and therefore mass is conserved.

2. The sum of the positive and negative charges is the same on both sides of the equation and therefore charge is conserved. (Charge is conserved because electrons are neither created nor destroyed in a chemical reaction.)

There are two situations in which relying on trial and error can get you into trouble. Sometimes the equation is too complex to be solved by trial and error within a reasonable amount of time. Consider the following reaction, for example.

3 Cu(s) + 8 HNO3(aq) -----> 3 Cu2+(aq) + 2 NO(g) + 6 NO3-(aq) + 4 H2O(l)

Other times, more than one equation can be written that seems to be balanced. The following are just a few of the balanced equations that can be written for the reaction between the permanganate ion and hydrogen peroxide, for example.
2 MnO4-(aq) + H2O2(aq) + 6 H+(aq) -----> 2 Mn2+(aq) + 3 O2(g) + 4 H2O(l)
2 MnO4-(aq) + 3 H2O2(aq) + 6 H+(aq) -----> 2 Mn2+(aq) + 4 O2(g) + 6 H2O(l)
2 MnO4-(aq) + 5 H2O2(aq) + 6 H+(aq) -----> 2 Mn2+(aq) + 5 O2(g) + 8 H2O(l)
2 MnO4-(aq) + 7 H2O2(aq) + 6 H+(aq) -----> 2 Mn2+(aq) + 6 O2(g) + 10 H2O(l)

Equations such as these have to be balanced by a more systematic approach than trial and error.
Sources: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch19/oxred_2.php

 

 

Comments on this question:

Did the question really write Zn + HCl --> Zn2+ + H2? That's odd, because there should be a Cl on the right side (Cl2, actually if I remember correctly). So the second half-reaction should be:

HCl -> H2 + Cl2

which you then have to balance. I feel that something is missing from the equation.


 

 
that is exactly what is says in my book. and it does some kind of weird like you mentioned.