Chemistry CH 4 - 4 Chemical Reactions Contents 4-1 Chemical...

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4 Chemical Reactions Contents 4-1 Chemical Reactions and Chemical Equations 4-2 Chemical Equations and Stoichiometry 4-3 Chemical Reactions in solution 4-4 Determining the Limiting Reactant 4-5 Other Practical Matters in Reaction Stoichiometry
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We are all aware that iron rusts and natural gas burns.These processes are chemical reactions.Chemical reactions are the central concern,not just of this chapter but of the entire science of chemistry. 4-1 Chemical Reactions and Chemical Equations A chemical reaction is a process in which one set of substances called reactants is converted to a new set of substances called products.In other words,a chemical reaction is the process by which a chemical change occurs.
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We need evidence before we can say that a reaction has occurred.Some of the types of physical evidence to look for are shown here. a color change • formation of a solid (precipitate) with a clear solution • evolution of a gas • evolution or absorption of heat When none of these signs of a chemical reaction appears,we need chemical evidence.This requires a detailed chemical analysis of the reaction mixture to discover whether any new substances are present. Just as we use symbolic for elements and formulas for compounds, we have a symbolic or shorthand,way of representing a chemical reaction- the chemical equation .In a chemical equation,formulas for the reactants are written on the left side and formulas for the products are written on the right.The two sides of the equation are joined by an arrow ( )or equal sign (=).We say that the reactants yield the products.
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Consider the reaction of colorless nitrogen monoxide and oxygen gases to form red-brown nitrogen dioxide gas,a reaction that occurs in the manufacture of nitric acid nitrogen monoxide + oxygen nitrogen dioxide 1.Substitute chemical formulas for names,to obtain the following expression. NO + O 2 NO 2 On the left there are three O atoms On the right there are two O atoms Atoms are neither created or destroyed in a chemical reaction,this expression needs to be balanced!
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2.Balance numbers of atoms to obtain a chemical equation In a balanced equation,for each element present,the total number of atoms of the element is the same on both sides.We see this below,both in the symbolic equation and in the molecular representation of the reaction. 2NO + O 2 2NO 2 +
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The coefficients required to balance a chemical equation are called stoichiometric coefficients.These coefficients are essential in relating The amounts of tractants used and products formed in a chemical reaction,through a variety of calculations. In balancing a chemical equation,keep in mind that the equation can be balanced only by adjusting the coefficients of formulas,as necessary.In particular, Never introduce extraneous atoms or molecules into an equation.
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