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Chapter 7:

Chapter 7:. Chemical Reactions. 7.1: Chemical reactions alter arrangements of atoms. Atoms interact in chemical reactions: Chemical reaction: produces new substances by changing the way in which atoms are arranged. Bonds between atoms are broken New bonds are formed.

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Chapter 7:

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  1. Chapter 7: Chemical Reactions

  2. 7.1: Chemical reactions alter arrangements of atoms • Atoms interact in chemical reactions: • Chemical reaction: produces new substances by changing the way in which atoms are arranged. • Bonds between atoms are broken • New bonds are formed

  3. Physical and Chemical changes • Physical changes: • Change in the state of a substance • Physical changes only • **substance is the same** • Chemical changes: • Changes in the chemical make up of the substance • Creates a new substance

  4. Reactants and Products • Reactants: • The substances present at the beginning of a chemical reaction • Bonds between the reactants are broken during a chemical reaction • Products: • The substances formed by a chemical reaction • New bonds are formed • CH4+ O2 CO2 + H2O productsreactants

  5. Evidence of Chemical Reactions • Color change • Formation of Precipitate • Precipitate: a solid product formed when chemicals in two liquids react • Formation of a Gas • Temperature Change • Flame from burning • May need thermometer to notice

  6. Chemical reactions can be classified • Synthesis reaction: • Forms a new compound by the combination of simpler reactants • N2 + 202 2NO2 • Decomposition reaction: • A reactant breaks down into simpler products such as elements or compounds • Like the reverse of a synthesis reaction • 2H20 2H2 + O2

  7. Combustion reaction: • One reactant is always oxygen and another reactant usually contains carbon and hydrogen • CH4 + 2O2 CO2 + 2H20 • Methane + Oxygen Carbon dioxide + water • The burning of methane is a combustion reaction

  8. The rates of chemical reactions can vary • Concentration: • Measurement of the number of particles present in a certain volume • The higher the concentration, the larger amount of particles available to react • Surface Area: • The area available for reactants to produce a chemical reaction. • Breaking apart a large area into smaller areas increases the overall surface area of the object • Increase in surface area = increase in the rate of the reaction

  9. Temperature: • By increasing temperature, you are adding energy to the reactants. • The increase in energy increases the rate of the reaction • Catalyst: • A substance that increases the rate of a chemical reaction but is not itself consumed in the reaction • After the reaction, the catalyst remains unchanged • Many reactions in your body would not take place without enzymes • Example: enzymes

  10. 7.2: The masses of reactants and products are equal • Law of conservation of mass: • In a chemical reaction atoms are neither created nor destroyed. • All atoms present in the reactants are also present in the products

  11. Chemical reactions can be described by chemical equations • To write a chemical equation you need to know: • The reactants and the products in the reaction • The atomic symbols and chemical formulas of the reactants and products in the reaction • The direction of the reaction • The indicates the direction of the reaction

  12. Chemical equations must be balanced Chemical equations must follow the law of conservation of mass so the same number of atoms must be present on both sides of the equation. You can balance a chemical equation by changing the amounts of reactants or products represented.

  13. Using coefficients to balance equations • Coefficients: • The number in front of the chemical formula • Indicate how many molecules take part in the reaction • If there is no coefficient then only one molecule of that type takes place • Only the coefficients can be changed in order to balance a chemical equation. • Look at page 211 for step by step instructions

  14. Using the conservation of mass A balanced chemical equation shows that no matter how atoms are rearranged during a chemical reaction, the same number of atoms must be present before and after the reaction. 2NaN3 2Na + 3N2 2 molecules of sodium azide yields 2 molecules of sodium and 3 molecules of nitrogen when it goes through a decomposition reaction.

  15. 7.3: Chemical reactions involve energy changes • Chemical reactions release or absorb energy • bond energy: the energy associated with bonds (breaking bonds requires energy, forming bonds releases energy) • Reactant molecules require energy to break the bonds • Energy is released when bonds are formed in product molecules

  16. Exothermic reactions: • A reaction in which energy is released • More energy is released during the formation of the products than is needed to break the bonds in the reactants releasing energy during the reaction. • Endothermic reactions: • A reaction in which energy is absorbed • More energy is required to break the bonds in the reactants than is released when the products form so energy must be added to the reaction.

  17. Exothermic reactions release energy Often produce an increase in temperature The bond energies of the reactants are less than the bond energies of the products which leads to a release in heat 2Al + Fe2O3 Al2O3+2Fe Enough heat is generated by this reaction to melt the iron that is produced.

  18. Exothermic reactions continued.. • All common combustion reactions are exothermic • Some chemical reactions release energy as light (glow sticks) • Exothermic chemical reactions occur in living things as heat or light • Fireflies

  19. Endothermic reactions All endothermic reactions absorb energy Often produce a decrease in temperature The bond energies of the reactants are greater than the bond energies of the products Energy is absorbed from the surroundings as heat = decrease in temperature

  20. Endothermic reactions continued • Photosynthesis: • Process by which plants absorb energy from sunlight to turn carbon dioxide and water into oxygen and glucose. • The energy is stored in the glucose molecules • 6CO2 + 6H20 C6H12O6 + 6O2

  21. Exothermic and endothermic reactions work together to supply energy. • In an exothermic reaction (releases energy), energy is found on the product side of the reaction • Reactants Products + Energy • In an endothermic reaction (absorbs energy), energy is found on the reactant side of the reaction • Reactant + Energy Products

  22. 7.4: Life and industry depend on chemical reactions • Living things require chemical reactions: • Photosynthesis • Respiration: the “combustion” of glucose to obtain energy • Energy released by respiration is used for new cell growth, movement, or any other life function • Catalysts (enzymes) are used to speed up the process

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