Atomic balance

Of course, these reactions may be very much more complicated. Since the pH is specified, H + is not included as a reactant, and a reactant may be a sum of species if the reactant has pKs in the pH region of interest. These biochemical reactions do balance atoms of elements other than hydrogen, but they do not balance electric charges. When the half-reactions occur in half-cells connected by a KC1 salt bridge, the difference in electric potential between the metallic electrodes... 

Matter is conserved in chemical reactions The total mass of the products equals the total mass of the reactants. Chemical equations reflect this conservation. It is why chemical equations must be balanced. Atoms have mass, and the numbers of each kind of atom on each side of the equation must be the same. Coefficients, the numbers to the left of the formulas, are used to balance equations. Many equations can be balanced directly by simply adjusting the coefficients, as illustrated in the equations given above. Other equations are more difficult to balance, such as that for the decomposition of nitroglycerine (an explosive)... 

This represents the reaction at a specified pH, and so it does not balance hydrogen atoms or electric charge, but it does balance atoms of all other elements. The expression for the apparent equilibrium constant K is given by... 

This shows that for the system of two reactions, the components can be taken to be atoms of the elements C, H, and O or molecules of CH4, O2, and CH2O. The last two columns indicate that CO2 = - CH4 + 2CH2 O and H2 O = CH4 + O2 -CH2 O. The second reaction is the same as reaction 7.1-15, but the first reaction is not the same as reaction 7.1-14. However, it does balance atoms. The use of NuIlSpace and RowReduce above provides a more organized way to compare conservation matrices with stoichiometric matrices for larger systems. 

Given that all three methods of carrying out material balances on reactive systems— molecular species balances, atomic species balances, and extents of reaction—necessarily yield the same results, the question is which one to use for a given process. There are no hard and fast rules but we suggest the following guidelines ... 

The feed to the reactor contains 7.80 mole% CHi, 19.4% O2, and 72.8% N2. The percentage conversion of methane is 90.0%, and the gas leaving the reactor contains 8 mol C02/mol CO. Carry out a degree-of-freedom analysis on the process. Then calculate the molar composition of the product stream using molecular species balances, atomic species balances, and extents of reaction. 

If only one reaction is involved, all three balance methods (molecular species balances, atomic species balances, extent of reaction) are equally convenient. If several reactions occur simultaneously, however—such as combustion of a fuel to form both CO and CO2— atomic species balances are usually most convenient. 

Balance atoms one element at a time by inserting coefficients. 

The stoichiometry of the reaction defines the reaction elemental balance (atoms of H and Br, for instance) and therefore relates the number of molecules of reactants and products participating in the reaction. The stoichiometric coefficients are not unique for a given reaction, but their ratios are unique. For instance, for the FIBr synthesis above we could have written the stoichiometric equation Vs.H.2 + VaBrj <=> HBr as well. 

How can you tell whether to balance atoms or charges first Look at the equation after you have balanced the redox part. Decide which is simpler, and do that. In the preceding equation, it is easier to balance charges than to balance atoms. 

Step 2. Balance atoms and charges in each half-reaction. We use H2O to balance O atoms, to balance H atoms, and e to balance positive charges. 

The G atoms are balanced here—one C on each side of the arrow. But the O and H atoms are not balanced. Often, it is easiest to first balance atoms that... 

The small integers appearing in (F) to balance atomic species are known as stoichiometric coefficients. Notice that the net charge on both sides of the reaction is balanced as well. 

In each case, the atomic number of the daughter is obtained by balancing atomic numbers in the equation. The symbol for the daughter is obtained by matching atomic numbers to symbols in the periodic table. 

Balance atoms first, then charges, in the oxidation half-reaction ... 

Balance the atoms. To balance the oxygen in this half-reaction, 4 water, H2O, molecules must be added to the left side. This gives 10 extra hyi ogen atoms on that side of the equation. Therefore, 10 hydrogen ions are added to the right side. In basic solution, OH ions and water may be used to balance atoms. 

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