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Chemical Reactions

Chemical Reactions. Chapter 21. Describing Chemical Reactions. 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. A chemical reaction occurs when you bake a cake.

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Chemical Reactions

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  1. Chemical Reactions Chapter 21

  2. Describing Chemical Reactions • Chemical reactions are taking place all around you and even within you. • A chemical reactionis a change in which one or more substances are converted into new substances. A chemical reaction occurs when you bake a cake.

  3. Physical Change • Change that alters the form of a substance but not the composition • Tearing of paper • Chopping wood • Melting ice • Change of state • Dissolving

  4. Chemical Changes • Result of a chemical reaction • Breaking of bonds and formation of new bonds • Atoms are rearranged • Energy is released or absorbed

  5. Examples of Chemical Reactions • Combustion – substance and oxygen are combined and release heat or light • Metabolism – chemical change that releases energy through digestion • Fermentation – break down of carbohydrates by living things in the absence of oxygen • Corrosion- Rusting • Electrolysis

  6. Signs of a Chemical Reaction • Rapid release of energy– heat, light, sound • Production of a gas– bubbles • Forming a precipitate– a solid that doesn’t dissolve in liquid • Substance getting colder • Examples: Fire, antacid tablet in water, rusting, silver tarnish

  7. Exothermic/Endothermic • More energy is produced than it takes to break the bonds of the reactants • Energy is released from the reaction • Gives off heat and/or light • Fire • MRE’s – wrapped in special sleeve with Mg – addition of water starts reaction that heats food • Takes more energy to break bonds in the reactants than is released by forming products • Energy has to be put in for the reaction to occur • Gets colder • Instant ice pack – water and ammonium nitrate • Rusting iron

  8. 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.

  9. Lavoisier's Contribution • 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.

  10. Lavoisier's Contribution • 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 a gas was produced.

  11. Lavoisier's Contribution • 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.

  12. Conservation of mass • the mass of the reactants will always equal the mass of the products in a closed system(you have to “catch all of the products” • 2H2 + O2 = 2H2O • H – 1.01 amu • O – 16 amu • ReactantsProducts • 4H – 4.04 2H2O – 36.04 • 2O – 32 amu • Total – 36.04 amu

  13. Conservation of atoms • the number of atoms in the reactants will always equal the number of atoms in products in a closed system • 2H2 + O2 = 2H2O • ReactantsProducts • 4H 4H • 2O 2O

  14. Writing Equations • Scientists have developed a shorthand method to describe chemical reactions. • A chemical equationis a way to describe a chemical reaction using chemical formulas and other symbols. • “Recipe” for chemical reactions • Bonds are breaking and reforming • CH4 + O2 → CO2 + H2O + heat + light • Methane and oxygen yield carbon dioxide and water • One or more substance (reactants) change into one or more new substances (products)

  15. Writing Equations • Reactants to the left of the arrow • Products to the right of the arrow • Arrow is yields • Reactants yield the products • Some of the symbols used in chemical equations are listed in the table. • Example: (aq) stands for aqueous, which means dissolved in water

  16. Balanced Equations • Lavoisier's mercury(II) oxide reaction can be written as: • Notice that the number of mercury atoms is the same on both sides of the equation but that the number of oxygen atoms is not the same.

  17. Balanced Equations • Start with a skeleton equation like Lavoisier’s • Each side of the equation must have the same number of atoms of each element • We use coefficients to balance equations – (whole number in front of the chemical formula) • H2 + O2 → H2O • 2H2 + O2 → 2H2O

  18. Balancing • Two things you CANNOT do when balancing an equation • You cannot change a subscript • You can only write the coefficient in front of a chemical formula

  19. Limiting and Excess Reactants • Limiting Reactant – the reactant that is used up first limits the amount of product • Excess Reactant – the reactant that is not completely used up because there is extra • If I have 1 cup flour and 2 eggs but only need 1 egg for my recipe – the flour is the limiting reactant and the egg is the excess reactant

  20. Inhibitors and Catalysts • Catalysts speed up reactions. They do not change what goes in or comes out, but they make the reactions happen faster- example- enzymes • Catalysts are often affected by other factors such as pH and temperature • Inhibitors slow down reactions– such as food preservatives that help keep food from spoiling too quickly

  21. Types of reactions • Combustion • Synthesis (Addition) • Decomposition • Single Displacement (Single Replacement) • Double Displacement (Double Replacement)

  22. Combustion • A chemical reaction in which something is burned • Lavoisier was one of the first to observe this type of reactions • A combustion reaction occurs when a substance reacts with oxygen to produce heat and/or light • C6H12O6 + 6O2  6CO2 + 6H20 • Products are always the same

  23. Synthesis Reactions • Also called addition • Two or more substances combine to form another substance • General Formula: A + B  AB • Example: 2H2 + O2  2H20

  24. Decomposition • A substance breaks down, or decomposes into its components • General Formula: AB  A + B • Example: 2H20 + 2H2 + O2

  25. Single Displacement • Also called single replacement • One atom displaces, or replaces another to form a new substance • General Formula: A + BC  AC + B • Example: • Cu (s) + 2AgNO3 (aq)  Cu (NO3)2 + 2Ag (s)

  26. Double Displacement • Also called double replacement • The positive ion of two compounds switch with each other, making two entirely new compounds • General Formula: AB + CD  AD + CB • Example: • Ba (NO3)2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KNO3 (aq)

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