Writing Chemical Reaction Equations

A chemical reaction involves any number of chemicals, say, chemical species A, B, C, D. Let us suppose that A and B react chemically and, as a result, C and D would form. In other words, a chemical reaction involves change in chemical species. There is one more thing that needs to be added in order to be able to write this chemical reaction equation. That is the so-called stoichiometric relationship. It is about the relationship among the quantities of these chemical species when they react. For example, two molecules of A react with three molecules of B and form two molecules of C and one molecule of D. This chemical reaction can then be represented by the following type of equation, though the equal sign = is now usually replaced by an arrow sign  

In this particular case, the reaction is supposed to proceed from the left-hand side to the right-hand side, and the species on the left-hand side (A and B in this case) are called reactants and those on the right-hand side are products. The stoichiometric relationship implies only the overall (relative) quantitative relationship among the chemical species involved and would not imply how the reaction takes place. The latter issue is called reaction mechanism. That is, the chemical reaction equation such as shown above does not necessarily imply the reaction mechanism. The reaction mechanism is a very difficult issue, and its delineation involves a lot of detailed studies on the reaction, and currently is one of the most interestingly studied in the chemical/biological science. 

The stoichiometric balance is based on the principle that in a chemical reaction no change (both quantity and identity) should take place regarding all the individual atoms involved. In other words, the total number of atoms on the left-hand side should be equal to that on the right-hand side. And this applies to all the elements involved. 

Sodium metal (represented by the symbol Na) bums (reacts with) in chlorine 

Chemical species and their arrangements are correct, but the stoichiometric relationship is not correctly represented. In other words, the total number of atoms in chemical compounds should be equal on both sides. You have the same number of Na on both sides, but there are two Cls on the left-hand side and only one Cl on the right. To adjust the number you can do the following  

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Physical chemists like to write a single equation that can apply to every possible case. In order to do this, we write chemical reaction equations with the symbols for all substances on the right-hand side and replace the —symbol by an equals sign. The three reactions of Eqs. (2.7-1) through (2.7-3) become... 

By standard thermodynamics, a dimensionless equilibrium constant can be derived from this condition and, for a general reversible chemical reaction, Equation 3.7, we may write Equation 3.8 which defines the equilibrium condition ... 

In writing chemical equations and biochemical equations it is important to be careful with names of reactants. Chemical reactions are written in terms of species. In chemical reaction equations, atoms of all elements and electric charges must balance. Biochemical reaction equations are written in terms of reactants, that is in terms of sums of species, H+ is not included as a reactant and electric charges are not shown or balanced. In biochemical reaction equations, atoms of all elements other than hydrogen must balance. The names of the reactants that must be used in making calculations with this data base are given later. 

In writing chemical reaction rate equations we will generally u.se [A] to denote the concentration of species A, rather than c.,. 

By now, you are familiar with enough chemical compounds to begin to write your own chemical reaction equations. Write two examples of what we mean by a combustion reaction. 

Write the chemical reaction equation for the hydrolysis of the dipeptide glycylalanine into the constituent amino acids. 

Write the two balanced chemical reaction equations (one for HCl and one for SO2), including the excess feed. 

The differential material balances contain a large number of physical parameters describing the structure of the porous medium, the physical properties of the gaseous mixture diffusing through it, the kinetics of the chemical reaction and the composition and pressure of the reactant mixture outside the pellet. In such circumstances it Is always valuable to assemble the physical parameters into a smaller number of Independent dimensionless groups, and this Is best done by writing the balance equations themselves in dimensionless form. The relevant equations are (11.20), (11.21), (11.22), (11.23), (11.16) and the expression (11.27) for the effectiveness factor. 

Write chemical equations for the reaction that takes place... 

As a reactant molecule from the fluid phase surrounding the particle enters the pore stmcture, it can either react on the surface or continue diffusing toward the center of the particle. A quantitative model of the process is developed by writing a differential equation for the conservation of mass of the reactant diffusing into the particle. At steady state, the rate of diffusion of the reactant into a shell of infinitesimal thickness minus the rate of diffusion out of the shell is equal to the rate of consumption of the reactant in the shell by chemical reaction. Solving the equation leads to a result that shows how the rate of the catalytic reaction is influenced by the interplay of the transport, which is characterized by the effective diffusion coefficient of the reactant in the pores, and the reaction, which is characterized by the first-order reaction rate constant. 

Crystals of sodium hydroxide (lye) react with carbon dioxide from air to form a colorless liquid, water, and a white powder, sodium carbonate, which is commonly added to detergents as a softening agent. Write a balanced equation for this chemical reaction. 

Strategy The first step is to write a balanced equation for the chemical reaction. Then calculate AH° using Table 8.3 (page 209) and AS° using Table 17.1 (page 456). Finally, use the Gibbs-Helmholtz equation to determine AG° at 230°C (3 sig. fig.)... 

In this generalized equation, (75), we see that again the numerator is the product of the equilibrium concentrations of the substances formed, each raised to the power equal to the number of moles of that substance in the chemical equation. The denominator is again the product of the equilibrium concentrations of the reacting substances, each raised to a power equal to the number of moles of the substance in the chemical equation. The quotient of these two remains constant. The constant K is called the equilibrium constant. This generalization is one of the most useful in all of chemistry. From the equation for any chemical reaction one can immediately write an expression, in terms of the concentrations of reactants and products, that will be constant at any given temperature. If this constant is measured (by measuring all of the concentrations in a particular equilibrium solution), then it can be used in calculations for any other equilibrium solution at that same temperature. 

Expression (2) applies to a solubility equilibrium, provided we write the chemical reaction to show the important molecular species present. In Section 10-1 we considered the solubility of iodine in alcohol. Since iodine dissolves to give a solution containing molecules of iodine, the concentration of iodine itself fixed the solubility. The situation is quite different for substances that dissolve to form ions. When silver chloride dissolves in water, no molecules of silver chloride, AgCl, seem to be present. Instead, silver ions, Ag+, and chloride ions, Cl-, are found in the solution. The concentrations of these species, Ag+ and Cl-, are the ones which fix the equilibrium solubility. The counterpart of equation (7) will be... 

Sometimes we need to construct a balanced chemical equation from the description of a reaction. For example, methane, CH4, is the principal ingredient of natural gas (Fig. H.3). It burns in oxygen to form carbon dioxide and water, both formed initially as gases. To write the balanced equation for the reaction, we first write the skeletal equation ... 

H.3 The first box below represents the reactants for a chemical reaction and the second box the products that form if all the reactant molecules shown react. Using the key below write a balanced equation for the reaction. Assume that if two atoms... 

The implication of this equation is that, because chemical reactions typically take place at constant pressure in vessels open to the atmosphere, the heat that they release or require can be equated to the change in enthalpy of the system. It follows that if we study a reaction in a calorimeter that is open to the atmosphere (such as that depicted in Fig. 6.11), then the measurement of its temperature rise gives us the enthalpy change that accompanies the reaction. For instance, if a reaction releases 1.25 kj of heat in this kind of calorimeter, then we can write AH = q — —1.25 kj. 

Some tasks in the Test of Gained Knowledge required students to connect observations about the macro course of chemical reactions with their notations in the submicro and/or symbolic types of representation. The results indicate that most students were able to rearticulate the information about reactants and products of a chemical reaction from the textual description of chemical reaction into the form of word chemical equation (textual description of macros word equation of macro Task 8.2, f(o/ )=89.82% Task 9.1, f(o/ )=87.61%). This action corresponds to the first step in learning to write down chemical equation in the LON approach. It can easily be explained, because teachers described the learning process to be very efficient to this point, as is illustrated below ... 

C04-0002. Although gasoline is a complex mixture of molecules, the chemical reaction that takes place in an automobile engine can be represented by combustion of one of its components, octane (Cg Hig). Such burning of fossil fuels releases millions of tons of carbon dioxide into the Earth s atmosphere each year. Write a balanced equation for the combustion of octane. 

We can use the ideal gas equation to calculate the molar mass. Then we can use the molar mass to identify the correct molecular formula among a group of possible candidates, knowing that the products must contain the same elements as the reactants. The problem involves a chemical reaction, so we must make a connection between the gas measurements and the chemistry that takes place. Because the reactants and one product are known, we can write a partial equation that describes the chemical reaction CaC2(. ) +H2 0(/) Gas -I- OH" ((2 q) In any chemical reaction, atoms must be conserved, so the gas molecules can contain only H, O, C, and/or Ca atoms. To determine the chemical formula of the gas, we must find the combination of these elements that gives the observed molar mass. 

Consider the case when the equilibrium concentration of substance Red, and hence its limiting CD due to diffusion from the bulk solution, is low. In this case the reactant species Red can be supplied to the reaction zone only as a result of the chemical step. When the electrochemical step is sufficiently fast and activation polarization is low, the overall behavior of the reaction will be determined precisely by the special features of the chemical step concentration polarization will be observed for the reaction at the electrode, not because of slow diffusion of the substance but because of a slow chemical step. We shall assume that the concentrations of substance A and of the reaction components are high enough so that they will remain practically unchanged when the chemical reaction proceeds. We shall assume, moreover, that reaction (13.37) follows first-order kinetics with respect to Red and A. We shall write Cg for the equilibrium (bulk) concentration of substance Red, and we shall write Cg and c for the surface concentration and the instantaneous concentration (to simplify the equations, we shall not use the subscript red ). 

Interpreting Data Write balanced chemical equations for each of the reactions performed. If no reaction was observed write No Reaction. Be sure to show the state for each reactant and product. 

Write the mathematical equation used to determine the average rate of a chemical reaction. What factor is held constant What are the variables ... 

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