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Chemical Reactions

Chemical Reactions. Chapter 7.2 Notes. Chemical Change. A chemical change occurs when a substance reacts and forms one or more new substances. Signs of a chemical change include color change, temperature change, light produced, gas formed, precipitate formed. Reactions.

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Chemical Reactions

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  1. ChemicalReactions Chapter 7.2 Notes

  2. ChemicalChange • A chemical change occurs when a substance reacts and forms one or more new substances. • Signs of a chemical change include color change, temperature change, light produced, gas formed, precipitate formed.

  3. Reactions • Reactions are often classified by the type of reactant or the number of reactants and products.

  4. Types of Reactions • We will be looking at six different types of reactions. • Synthesis • Decomposition • Single replacement • Double replacement • Combustion • Acid-Base

  5. Synthesis • Synthesis • A reaction in which two or more substances react to form a single substance. 2H2+ O2 2H2O • 2 hydrogen gas molecules react with one oxygen gas molecule to form 2 liquid water molecules.

  6. Decompostion • Decomposition • A reaction in which a compound breaks down into two or more simpler substances. 2KClO3 2KCl + 3O2

  7. Single Replacement • Single Replacement • A reaction in which one element takes the place of another element in a compound. 2Al + 3CuSO4 Al2(SO4)3 + 3Cu

  8. Double Replacement • Double Replacement • A reaction in which two different compounds exchange positive ions and form two new compounds. Pb(NO3)2+ 2KI  PbI2 + 2KNO3

  9. Combustion • Combustion • A reaction in which a substance reacts rapidly with oxygen, often producing heat and light. CH4+ 2O2 CO2 + 2(H2)O

  10. Acid-Base • Acid-Base • This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water.

  11. Acid-Base HBr+ NaOH ---> NaBr + H2O • Hydrobromic acid reacts with sodium hydroxide to produce sodium bromide and water.

  12. Electron Transfer Reactions • Reactions as Electrons Transfer • The discovery of subatomic particles enables scientists to classify certain chemical reactions as transfers of electrons between atoms.

  13. Oxidation • Oxidation is classified as any process in which an element loses electrons during a chemical reaction.

  14. Oxidation • For example, when iron reacts with oxygen to form rust or when calcium reacts with oxygen to form calcium oxide. • 2Ca + 02 2CaO • Ca Ca2+ + 2e- • Even though the reactants and products are balanced, the elements themselves have a charge. In this case, calcium lost two electrons and has a charge of +2.

  15. Reduction • Reduction is the process in which an element gains electrons during a chemical reaction. A reactant is said to be reduced if it gains an electron. • For example, in the previous calcium oxide reaction, calcium gave away two electrons and became positive, which is oxidation, however, oxygen gained two electrons and became negative, which is reduction. • O + 2e- O2-

  16. Oxidation-Reduction • Oxidation and reduction always occur together. When one element loses electrons another element must gain electrons. • Oxidation-Reduction Reaction • An oxidation-reduction reactions is one in which electrons are transferred from one reactant to another.

  17. Energy Changes in Reactions Chapter 7.3

  18. Chemical Energy • Chemical Energy is the energy stored in the chemical bonds of a substance. • Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

  19. Types of Reactions • During a chemical reaction, energy is either released or absorbed. • Exothermic reaction – A reaction in which energy is released into its surroundings • Endothermic reaction – A reaction in which energy is absorbed from its surroundings.

  20. Catalyst • A catalyst is a substance that affects the reaction rate without being used up in the reaction. • Chemists use catalysts to speed up a reaction or enable a reaction to occur at a lower temperature.

  21. Catalysts • Since catalysts are neither reactants or products they are written over the arrow in a chemical equation. V2O5 2SO2+ O2 2SO3

  22. Conservation of Energy • Conservation of Energy • In an exothermic reaction, the chemical energy of the reactants is converted into heat plus the chemical energy of the products. While it may appear energy is lost due to heat, it is not. The total amount of energy before and after the reactions is the same.

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