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Chapter 20

Chapter 20. Chemical Reactions. 20.1 Chemical Changes. When you bite into your food, you chew it into smaller pieces. Chewing does not alter the chemical composition of your food. Chewing causes a physical change in your food.

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Chapter 20

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  1. Chapter 20 Chemical Reactions

  2. 20.1 Chemical Changes • When you bite into your food, you chew it into smaller pieces. • Chewing does not alter the chemical composition of your food. • Chewing causes a physical change in your food. • A physical change is a change that affects only the physical properties of a substance (size, shape, and state).

  3. Physical Changes • The physical properties of ice can be changed by crushing it or by letting it melt. • The only difference between solid water (ice), liquid water, & water vapor is the amount of energy involved in each state. • Water vapor is the most energetic form of water.

  4. Chemical Changes • A chemical change is a change in a substance that involves breaking and reforming of chemical bonds to make one or more different substances. • So… when you chew your food, glands in your mouth produce saliva. This immediately breaks down complex molecules by breaking bonds.

  5. Chemical Changes cont… • Chemical changes are the result of chemical reactions. • They are also the breaking of bonds in one or more substances, & reforming of new bonds to create new substances.

  6. Chemical Changes cont… • How do you know when a chemical change has occurred? • When one or more starting substances are mixed & you get products that appear to be different from those staring substances. • This involves ENERGY!

  7. A Chemical Change • Making a cake: • You need flour, water, sugar,eggs….. • Mix the ingredients together and bake it in a oven. • When the cake is done, a chemical change has occurred. But how do you know? • Because none of the ingredients can go back to their original substance!

  8. Chemical Reactions • A chemical reaction involves breaking the chemical bonds in one or more reactants and reforming chemical bonds into one or more products. • Reactants Products • Reactants are substances that change. • Products are substances that are formed.

  9. 20.2 Chemical Equations • Evidence that chemical changes are occurring: • Bubbling • Turns cloudy • Temperature change • Color change

  10. Examples of Chemical Changes

  11. How Are Chemical Reactions Written? • Use chemical formulas that correspond to the elements & compounds in the reaction. • When chemical formulas and symbols are used to represent a reaction instead of using words, it is called a Chemical Equation.

  12. How Chemical Reactions Are Written • Words: Methane gas reacts with oxygen gas to produce carbon dioxide and water. • Chemical Equation: CH4 + O2CO2 + H20

  13. How Chemical Reactions Are Written cont… • A few things to keep in mind: • Most elemental gases do not exist as single atoms (with the exception of most gases).

  14. How Are Chemical Reactions Written cont… • Observe this chemical equation: • CH4 + O2CO2 + H2O • This is technically not correct! It does not agree with the Conservation of Atoms. • This principle says that the number of each type of atom on the reactants side must equal the number of each type of atom on the products side of the equation.

  15. Chemical Equation & Conservation of Atoms • It is necessary to Balance the number and type of atoms on the reactants and products sides of the equation. • Observe: CH4 + O2 CO2 + H2O

  16. Chemical Equations & Conservation of Atoms cont… • The chemical equation is not balanced because the number of hydrogen atoms and oxygen atoms are different on both sides of the equation. • To balance the equation, you must figure out what number to multiply each compound by in order to make the numbers add up!

  17. Chemical Equations & Conservation of Atoms cont… • NOTE: You CANNOT change the number of individual atoms in a compound. You can only change the number of molecules of that compound.

  18. Chemical Equations cont… • To change the number of molecules of a compound, you can write a whole number “Coefficient” in front of the chemical formula. • When this happens, all of the types of atoms in that formula are multiplied by that number. • 2CH4 gives you…. • 2 x 1 C = 2 Carbon • 2 x 4 H = 8 Hydrogen

  19. Chemical Equations cont… • Look at this balanced equation: • CH4 + 2O2 CO2 + 2H2O

  20. Chemical Equations cont… • The balanced equation reads as follows • CH4 + 2O2 CO2 + 2H2O • One molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

  21. Let’s Practice

  22. 20.3 Conservation of Mass • Chemical reactions were a big mystery to people in the eighteenth century. • A scientist named Antoine Laurent Lavoisier (1743-94) stated that the total mass of the products of a reaction is equal to the total mass of the reactants. • This is known as the Law of Conservation of Mass.

  23. Conservation of Mass • Lavoisier showed that a “closed” system must be used when studying chemical reactions. • When chemicals are reacted in a closed container, you can show that the mass before & after the reaction is the same.

  24. Antoine Lavoisier • He placed 10.0 grams of mercury (II) oxide into a sealed container. He heated the container so that the mercury (II) oxide reacted to produce oxygen and mercury. • He observed the reaction, and noticed that the mercury (II) oxide bubbled and turned into a smaller amount of a silvery liquid. • In the reaction, 10.0 grams of mercury (II) oxide produced 0.7g of oxygen gas & 9.3g of mercury.

  25. THE END!!

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