Physical states, of reactants and products

Note that the physical states of reactants and products must be specified as solid (s), liquid (0, gaseous (g), or aqueous (aq) when enthalpy changes are reported. The enthalpy change for the reaction of propane with oxygen is A H = -2043 kj if water is produced as a gas but AEf = -2219 kj if water is produced as a liquid. 

In addition to specifying the physical state of reactants and products when reporting an enthalpy change, it s also necessary to specify the pressure and temperature. To ensure that all measurements are reported in the same way so that different reactions can be compared, a set of conditions called the thermodynamic standard state has been defined. 

The physical states of reactants and products are included where necessary. The symbols used are (s) for solid, (/) for liquid, (g) for gas, and (aq) for aqueous (water) solutions. In the case of sodium chloride formation, the equation is modified accordingly. 

To give a more complete description of reactions, we indicate the physical states of reactants and products (g) for gases, (f) for liquids, and (s) for solids. The notation (aq) following ions indicates that they are hydrated in aqueous solution that is, they interact with water molecules in solution. The complete ionization of a strong electrolyte is indicated by a single arrow (—>). 

If the physical states of reactants and products are not given, an uninformed person might try to bring about the reaction by mixing solid KBr with solid AgN03. These solids would react very slowly or not at all. Imagining the process on the microscopic level, we can understand that for a product like silver bromide to form, the Ag+ and Br ions would have to come in contact with each other. However, these ions are locked in place in their solid compounds and have little mobility. (Here is an example of how we explain a phenomenon by thinking about what happens at the molecular level, as discussed in Section 1.2.)... 

For simplicity, we omit the physical states of reactants and products.) All three elements (K, Cl, and O) appear only once on each side of the equation, but only for K and Cl do we have equal numbers of atoms on both sides. Thus KCIO3 aud KCl must have the same coefficient. The next step is to make the number of O atoms the same on both sides of the equation. Since there are three O atoms on the left and two O atoms on the right of the equation, we can balance the O atoms by placing a 2 in front of KCIO3 and a 3 in front of O2. 

The physical states of reactants and products may be shown in parentheses. 

What does it mean to "balance" an equation Why is it so important that equations be balanced What does it mean to say that atoms must be conserved in a balanced chemical equation How are the physical states of reactants and products indicated when writing chemical equations ... 

The physical states of reactants and products must always be included in thermochemical equations because they influence the overall amount of energy as heat gained or lost. For example, the energy needed to decompose water would be greater than 483.6 kJ if we started with ice, because extra energy would be needed to go from ice to liquid and then to vapor. 

Chemists usually indicate the physical states of reactants and products withi itahciKd letters in parentheses following each species in the equation. Gases, liquids, and solids are labeled with (g), (/), and (s), respectively. Chemical species that ate dissolved in water are said to be aqueous and are labeled aq). The equation examples given previously can be written as follows ... 

Chemical Reactions and Chemical Equations—Chemical reactions are processes in which one or more starting substances, the reactants, form one or more new substances, the products. A chemical reaction can be represented symbolically in a chemical equation, with formulas of reactants on the left and formulas of products on the right reactants and products are separated by an arrow. The equation must be balanced. A balanced equation reflects the true quantitative relationships between reactants and products. An equation is balanced by placing stoichiometric coefficients before formulas to signify that the total number of each kind of atom is the same on each side of the equation. The physical states of reactants and products can be indicated by symbols, such as (s), (1), (g), and (aq), signifying solid, liquid, gas, and aqueous solution, respectively. 

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