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Targets for this Unit

Learn about chemical reactions, properties of products & reactants, balancing equations, energy changes, and more in matter. Explore examples and explanations on chemical changes and energy involvement in reactions.

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Targets for this Unit

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  1. Targets for this Unit • the properties of the products of a chemical reaction are different from the properties of the reactants. • that in a chemical reaction no atoms are lost or created, they are simply rearranged. • how to balance a simple chemical reaction equation. • chemical reactions that absorb heat are called endothermic reactions. • chemical reactions that release heat are called exothermic reactions.

  2. Observing Chemical Change Chapter 6.1 Page 214

  3. Suppose you fill a sealable bag with ice cubes, and you allow the ice to melt. How would the mass of the bag and ice before melting compare to the mass of the water and the bag after melting? Explain your answer. • What happens during a chemical reaction?

  4. Section 1:Observing Chemical Change • How can changes in matter be described? • How can you tell when a chemical reaction occurs?

  5. Review • Matter --- anything that has mass and takes up space • Chemistry --- the study of matter and how it changes • Changes in matter come in two forms: physical changes and chemical changes

  6. Properties and Changes in Matter • Matter can undergo both physical change and chemical change.

  7. Chemical changeinvolves both reactants and products C16H22O11 + 2 O2 7 C2 + 11 H2O + 2 CO2 reactantsproducts Marshmallow + oxygen carbon (ash) + water (vapor) + carbon dioxide Reactants are the compounds you begin with in a chemical reaction. Products are the compounds produced in a chemical reaction.

  8. Properties and Changes in Matter • Chemical changes occur when bonds break and new bonds form. Reactants Products

  9. Evidence for Chemical Reactions Chemical reactions involve changes in properties and changes in energy that you can observe. 1. Changes in properties 2. Changes in energy

  10. 1. Changes in Properties • Physical change • Color - Gas • Precipitate - Texture • Chemical change • When two or more reactants chemically combine to form new products, the chemical properties of the products are different from the properties of the reactants.

  11. 2. Changes in Energy Chemical reactions usually absorb heat or release heat. When a reaction absorbs heat it is called an endothermic reaction. In an endothermic reaction, the products are cooler than the reactants, because the reaction absorbs heat from the reactants.

  12. 2. Changes in Energy Chemical reactions usually absorb heat or release heat. When a reaction releases heat it is called an exothermic reaction. In an exothermic reaction, the products are warmer than the reactants, because the reaction releases heat to the reactants.

  13. Energy in Chemical Changes • A student places two substances in a flask and measures the temperature once per minute while the substances react. The student plots the time and temperature data and creates the graph at left.

  14. At 3 minutes the temperature in the flask was about 30ºC. The first time the temperature was 6ºC was at about 7 minutes. Reading Graphs: What was the temperature in the flask at 3 minutes? When was the first time the temperature was at 6ºC? Energy in Chemical Changes

  15. About 20ºC Calculating: How many degrees did the temperature drop between2 minutes and 5 minutes? Energy in Chemical Changes

  16. The reaction was endothermic, because it absorbed thermal energy from the reaction mixture, causing the temperature to drop. Interpreting Data: Is the reaction endothermic or exothermic? Explain. Energy in Chemical Changes

  17. The reaction stopped at about 2ºC. You can tell because that is the lowest temperature reached. Inferring: At what temperature did the reaction stop? How can you tell? Energy in Chemical Changes

  18. If the temperature increased instead, the reaction would be exothermic, because an exothermic reaction is one in which energy is released. Drawing Conclusions: Suppose the temperature in the flask increased instead of decreased as the reaction occurred. In terms of energy, what kind of reaction would it be? Energy in Chemical Changes

  19. Links on Chemical Changes • Click the SciLinks button for links on chemical changes.

  20. End of Section:Observing Chemical ChangeDo Cornell Notes for Chapter 6, Section 1

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