Transcript Chapter 24
Chapter 24 Chemical Reactions Section 1-Chemical Changes • Chemical reactions are taking place all around you and even within you • Chemical reaction-a change in which one or more substances are converted into new substances • Reactants-substances that react • Products-new substances produced Conservation of Mass • Lavoisier-famous scientist who discovered that masses of reactants and products were the same in an experiment • SO, he concluded: • Law of Conservation of Mass-the starting mass of reactants equals the final mass of the products OR • Matter is neither created nor destroyed during chemical reactions Unit Managers • Atoms in a chemical reaction are rearranged, but never lost or destroyed. • Coefficients- represent the number of units of each substance taking part in a reaction – (can be thought of as unit managers) – Knowing the number of units of reactants enables chemists to add correct amounts of reactants to a reaction Section 2-Chemical Equations • Balancing an equation doesn’t change what happens in a reaction-it simply changes the way the reaction is represented. • The balancing process involves changing coefficients in a reaction • Balanced chemical equation-has the same number of atoms of each element on both sides of the equation Choosing Coefficients • Finding out which coefficients to use to balance an equation is often a trial-&error process, but with practice the process becomes easier. • Steps to Balancing Equations: 1. Write a chemical equation for the reaction using formulas and symbols 2. Check the Equation for atom balance 3. Choose coefficients that balance the equation 4. Recheck the numbers of ea/atom on Section 3-Classifying Chemical Reactions • Combustion Reactions-(burning) occurs when a substance reacts with oxygen to produce energy in the form of heat and light Synthesis Reaction- two or more substances combine to form another substance. A + B ---AB • Decomposition Reaction- occurs when one substance breaks down, or decomposes, into two or more substances AB --- A + B Single Displacement- when one element replaces another element in a compound A + BC --- AC + B OR D + BC --- BD + C The Activity Series • A metal will replace any less active metal • Notice: copper, silver and goldleast active metals • SO: that is why these elements often occur as deposits of the relatively pure element. More on types of reactions • Double Displacement- the positive ion of one compound replaces the positive ion of the other to form two new compounds • Precipitate- an insoluble compound that comes out of solution during a double displacement reaction • AB + CD --- AD + CB Section 4-Chemical Reactions and Energy • All chemical reactions release or absorb energy • This energy can take many forms, such as heat, light, sound and electricity • Chemical bonds are the source of this energy • When chemical reactions take place, some chemical bonds in the reactants must be broken • Breaking these bonds takes energy • Bond formation releases energy Exergonic Reactions- Chemical reactions that release energy – Less energy is required to break the original bonds than is released when new bonds form – Exothermic Reaction- when the energy given off in a reaction is primarily in the form of heat – *Exothermic reactions provide most of the power used in homes and industries Endergonic Reactions- a chemical reaction that requires more energy to break bonds than is released when new ones are formed • The energy absorbed can be in the form of light, heat, or electricity • Endothermic reaction- when the energy needed is in the form of heat • (Endothermic can also refer to phase/physical changes as well) Catalysts • Catalyst- a substance that speeds up a chemical reaction without being permanently changed itself • The mass of the product that is formed remains the same • The catalyst remains unchanged and can be reused Inhibitors • Inhibitors— substances that are used to combine with one of the reactants in a reaction. • The inhibitor ties up the reactant and prevents it from undergoing the original reaction. – Ex: food preservatives BHT and BHA are inhibitors that prevent spoilage of certain foods, s