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Warm-up, 11/2/2011

Warm-up, 11/2/2011. Take out your homework so I can check it. Answer the following: a. Give an example of a physical property. b. Give an example of a chemical property. c. What is a physical change? d. What is a chemical change?

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Warm-up, 11/2/2011

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  1. Warm-up, 11/2/2011 • Take out your homework so I can check it. • Answer the following: a. Give an example of a physical property. b. Give an example of a chemical property. c. What is a physical change? d. What is a chemical change? e. What are two ways you know a chemical change has occurred?

  2. Law of Conservation of Matter Physical and Chemical Changes & Reactions

  3. Physical Change • Substance may seem different, but the way the atoms link up is the same.

  4. It’s a physical change if • It changes shape or size • It dissolves.

  5. It’s a physical change if... • It changes phase (freezes, boils, evaporates, condenses)

  6. Chemical Change • Changes the way the molecules link up • Makes new substances

  7. It’s a chemical change if…. • It burns • Temperature changes without heating/cooling

  8. It’s a chemical change if... • It bubbles (makes a gas)

  9. It’s a chemical change if... • It changes color • It forms a precipitate

  10. How Do I Know If A Reaction Has Occurred? There are 5 indicators of a chemical reaction Evolution of a gas Evolution of light Evolution of heat Color change Evolution of a precipitate Precipitate: an insoluble solid that is produced as result of a chemical reaction between two liquids

  11. How Is Matter Changed Into New Substances? Chemical reaction (also known as a chemical change) is a change in a substance or substances that results in a totally new substance Ex: 2H2(g) + O2(g)  2H2O(g) Notice that the reactants (the substances you start with) combine to form a new substance (the product)

  12. Why Do Chemical Reactions Occur? Chemical reactions occur to produce a more stable product than the existing reactants Ex: 2Na(s) + Cl2(g)  2NaCl(s) *The sodium is highly unstable and the chlorine gas is somewhat unstable. The resulting Sodium Chloride is VERY stable. **It is important to understand that the products have totally different properties than the reactants

  13. The Law of conservation of massstates that matter cannot be created or destroyed in any chemical reaction *The atoms in the reactants are rearranged to form new compounds, but none of the atoms disappear, and no new atoms areformed. O H H H H O O H O H H H Products Reactants

  14. Remember that atoms don’t change in a chemical reaction. • The number and kinds of atoms present in the reactants of a chemical reaction are the same as those present in the products. When stated this way, it becomes the law of conservation of atoms.

  15. Chemical Equations:An expression in which symbols and formulas are used to represent a chemical reaction. sodium metal + chlorine gas  table salt (sodium chloride)

  16. A chemical equation identifies the starting and ending chemicals as reactants and products: reactantsproducts Formation of water 2H2 + O2 2H20 A chemical equation is balanced when it reflects the law of conservation of mass. Graphic: http://www.chemistry.ohio-state.edu/betha/nealChemBal/

  17. Types of Reactions • There are 5 general types of reactions. - Synthesis - Decomposition - Combustion - Single Replacement - Double Replacement

  18. SYNTHESIS • A reaction in which two or more reactants yield a single product. • Also called composition or combination • General Equation A + B  AB • EX: 2 Li + Se ---> Li2Se

  19. Decomposition Reaction One Reactant Breaking Down into two or more products General Equation AB ---> A + B Example: 2 HgO  2 Hg + O2

  20. What IsCombustion? • One or more reactants combine with oxygen releasing heat or light • Any combustion reaction must include the reactant oxygen, O2 • General Equation: A + O2 AO Example: 2Mg(s) + O2(g) 2MgO(s)

  21. Single Replacement • Sometimes called Displacement • One element replaces a similar element in a compound A + BX  AX + B 2 Na + 2 HOH ----> 2 NaOH + H2

  22. Double Replacement • Reaction that has the interchanging of two ions from two different compounds. • general form: AB + CD----> AD + CB • Example: Pb(NO3)2 + 2 KI ----> PbI2 + 2 KNO3

  23. Reaction Checklist • 1) Is O2 a reactant? (combustion) • 2) One product? (synthesis) • 3) One reactant? (decomposition) • 4) Is an element being replaced? (single) • 5) 2 switches? (double) • Reaction typesCombustion Reaction

  24. Exothermic Reaction • Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may even be explosive. • An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced • Examples of exothermic reactions: •Digestion of food releases energy •All combustion reactions

  25. Endothermic Reaction • The chemical reactions that absorb energy in order to proceed are endothermic reactions. • Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. • Photosynthesis is an example of an endothermic chemical reaction. sunlight + 6CO2(g) + H2O(l) C6H12O6(aq) + 6O2(g)

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