Ch 9.3: Energy Changes and Chemical Reactions


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Grade 8 Integrated Science Chapter 9 Lesson 3 on energy changes, chemical reactions, endothermic and exothermic reactions, and activation energy. Understanding a reaction potential energy diagram.

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Ch 9.3: Energy Changes and Chemical Reactions

  1. 1. Energy Changes and Chemical Reactions Chapter 9.3 p 317-323
  2. 2. Energy Changes • To propel a space shuttle, scientists use rocket fuel. • The shuttle’s main engines burn almost 2 million L of liquid hydrogen and liquid oxygen. • The reaction produces water vapor and a lot of energy. • The heat energy causes the water vapor to heat to high temperatures. • The rapidly expanding water vapor pushes the shuttle.
  3. 3. Chemical Energy in Bonds • Chemical bond contain a form of energy called chemical energy – Breaking a bond absorbs energy from the surroundings. – The formation of a bond releases energy to the surroundings. – Some chemical reactions release more energy then they absorb. – Others absorb more than they release – You can detect these energy changes by recording the temperature of the surroundings.
  4. 4. Endothermic Reactions – Energy Absorbed • Chemical reactions that absorb thermal energy are endothermic reactions • For an endothermic reaction to continue, energy must be constantly added. • In these reactions more energy is required to break bonds of the reactant than is released when the products form. • Therefore, the overall reaction absorbs energy.
  5. 5. Exothermic Reactions – Energy Released • An exothermic reaction is a chemical reaction that releases thermal energy • In an exothermic reaction, more energy is released when the products form than is required to break the bonds in the reactants. • Therefore, the overall reaction releases energy
  6. 6. Activation Energy
  7. 7. Activation Energy • Many reactions do not start by themselves. – Paper does not burn when it touches oxygen. Fire is required. • All reactions require energy to start the breaking of bonds. • This is called activation energy. • Activation energy is the minimum amount of energy needed to start a chemical reaction. – Different reactions require different activation energies
  8. 8. Activation Energy • The rusting of iron is a reaction that has a low activation energy. – The energy in the surroundings is enough to start the process. • Other reactions require more energy and have high activation energy – The burning of wood requires the energy of a flame. – Once the reaction starts, the reaction itself releases enough energy to keep the reaction going.
  9. 9. Reaction Rates • Some reactions happen quickly, while others happen very slowly – Fireworks explode in seconds – The rate of reaction is the speed at which it occurs. • For a chemical reaction to occur, particles must collide in the right orientation with enough energy to break the bonds – Chemical reactions occur faster of particles collide more often or move faster when they collide.
  10. 10. Several Factors Affect Reaction Rates • • • • • Surface Area Temperature Concentration and Pressure Catalysts Inhibitors
  11. 11. Surface Area • Surface area is the amount of exposed, outer area of a solid. • Increased surface area increases reaction rate because more particles on the surface if a solid come into contact with the particles of another substance. – Consider a piece of chalk versus the same amount of chalk powder.
  12. 12. Temperature • At higher temperatures, the average speed of particles is greater • This speeds reactions in two ways – First, particles collide more often – Second, collisions with more energy are more likely to break chemical bonds
  13. 13. Concentration and Pressure • Increasing the concentration of one or more reactants increases collisions between particles. • More collisions results in a faster reaction rate. • In gases an increase in pressure pushes gas particles closer together • When they are close, more collisions occur
  14. 14. Catalysts • A catalyst is a substance that increases reaction rate by lowering the activation energy of a reaction. – One way catalysts speed reactions is by helping reactant particles contact each other more often – A catalyst isn’t changed in a reaction, and it doesn’t change the reactants or products – A catalyst doesn’t increase the amount of reactant used or the amount of product that is made.
  15. 15. Catalyst • Catalyst are not changed, so they are not considered reactants • Your body is filled with catalysts called enzymes. • An enzyme is a catalyst that speeds up chemical reactions in living cells. – The enzyme protease breaks the protein molecules that can be absorbed by your intestine. – Without protease the reaction would occur too slowly and we could not survive.
  16. 16. Inhibitors • An inhibitor is a substance that slows, or even stops, a chemical reaction caused by an enzyme. • Inhibitor are often used on our foods. Preservatives are inhibitors that slow the processes that cause food to spoil.
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