Classifying Chemical Reactions PDF

Classifying Chemical Reactions Lavoisier and the Conservation of Mass Chemical reactions are taking place all around you and even within you. A chemical reaction is a change in which one or more substances are converted into new substances. The substanc...

Classifying Chemical Reactions Lavoisier and the Conservation of Mass Chemical reactions are taking place all around you and even within you. A chemical reaction is a change in which one or more substances are converted into new substances. The substances The new substances that react are produced are called called reactants. products. Lavoisier and the Conservation of Mass The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants. For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products. One of the questions that motivated Lavoisier was the mystery of exactly what happened when substances changed form. He began to answer this question by experimenting with mercury. Lavoisier and the Conservation of Mass Lavoisier placed a carefully measured mass of solid mercury (II) oxide, which he knew as mercury calx, into a sealed container. When he heated this container, he noted a dramatic change. The red powder had been transformed into a silvery liquid that he recognized as mercury metal, and gas was produced. When he determined the mass of the liquid mercury and gas, their combined masses were exactly the same as the mass of the red powder he had started with. Lavoisier also established that the gas produced by heating mercury(II) oxide, which we call oxygen, was a component of air. Writing Equations A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. Scientists have developed a shorthand method to describe chemical reactions. Types of Reactions Chemists have defined five main categories of chemical reactions: Synthesis Decomposition Combustion Two or more The reverse of a synthesis. (burning) substances combine One substance breaks down, or decomposes, into two or Occurs when a substance to form another more substances. reacts with oxygen to substance. General Formula: produce energy in the General Formula: AB → A + B form of heat and light. A + B → AB Most decomposition reactions produce one or more require the use of heat, light, products that contain the or electricity. elements in the reactants. Types of Reactions Single displacement Double displacement When one element replaces another One compound's positive ion replaces the element in a compound, it is called a other's positive ion to form two new single-displacement reaction. compounds. General formula: A double displacement reaction takes place A + BC → AC + B if a precipitate, water, or gas forms when two ionic compounds in a solution are We can predict which metal will replace combined. another using the diagram shown which lists metals according to how reactive they are. A metal will replace any less active metal. Types of Reactions Single displacement Double displacement A precipitate is an insoluble compound that comes out of the solution during this type of reaction. General formula: AB + CD → AD + CB Oxidation-Reduction Reactions One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons. Chemists use the term oxidation to describe the loss of electrons and the term reduction to describe the gain of electrons. Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements. Corrosion of metal is a visible result. Oxidation-Reduction Reactions The substance that gains an electron or electrons obviously becomes more negative, so we say it is reduced. On the other hand, the substance that loses an electron or electrons then becomes more positive, and we say it is oxidized. The electrons that were Reduction is the partner pulled from one atom to oxidation; the two were gained by another always work as a pair, atom in a chemical which is commonly reaction called reduction. referred to as redox. Oxidation-Reduction Reactions Types of Reactions