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P. Sci.

Learn about the different types of chemical reactions, including synthesis, decomposition, single-displacement, double-displacement, and combustion. Also, understand how to balance chemical equations in order to satisfy the law of conservation of matter.

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P. Sci.

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  1. P. Sci. Unit 13 Chemical Reactions Chapter 7

  2. Essential Questions • What is the difference between an endothermic reaction and an exothermic reaction. • Distinguish among the five general types of chemical reactions.

  3. What are the steps to balance a chemical equation? Explain chemical equilibrium in terms of equal forward and reverse reaction rates. Explain how the law of conservation of matter is related to balancing equations.

  4. Review • Chemical change –occurs when one substance changes into another substance. • You will have to know how to write a chemical formula from a name. (last chapter) • ex. Dihydrogen monoxide = H2O

  5. Review cont. • Law of Conservation of … • Matter – Matter cannot be created nor destroyed, it can only be changed. • Energy – Energy cannot be created nor destroyed, it can only be changed.

  6. Chemical Reactions • Occur when substances undergo chemical changes to form new substances. • Atoms are re-arranged as bonds are broken and formed.

  7. Produce Yields forms Reactants → Product The chemicals you start with What you end up with after the reaction occurs

  8. Signs of a Chemical Reaction • Production of a gas • Production of a precipitant • Change in color • Change in odor • Production of light or heat

  9. Energy and Reactions • All chemical reactions release or absorb energy – (heat, light, sound or electricity) • To break bonds – takes energy • To form bonds – releases energy

  10. Energy and Reactions cont. • endothermic – when more energy is required to break bonds than is released when new ones are formed. (ex: photosynthesis) • exothermic – less energy is required to break the original bonds than is release when new bonds form. (ex: bioluminescence – fireflies.

  11. Energy is conserved. • Chemical energy is the energy stored in a chemical bond. • Example: isooctane (gas) and oxygen plus a tiny spark will set off an explosion – the energy of the explosion comes from the chemical energy stored in the isooctane bonds.

  12. Reaction Types • There are 5 main types of chemical reactions. • Synthesis • Decomposition • Single-displacement (replacement) • Double-displacement (replacement) • Combustion

  13. Synthesis Reactions • two or more substances combine to form another . • A + B → AB Ex.: 2H2 + O2 → 2H2O • Synthesis reactions always join substances so the product is a more complex compound than the reactants.

  14. Decomposition Reaction • opposite of synthesis – one substance breaks down into it’s parts. • AB → A + B ex: 2H2O → 2H2 + O2 • Decomposition reactions break apart so the reactants are more complex than the products.

  15. Single-displacement Reactions • When one element replaces another element in a compound. • A + BC → AC + B Ex: Cu + 2AgNO3 → Cu(NO3)2 + 2Ag • Generally, a more reactive element will take the place of a less reactive one.

  16. Double-displacement Reactions • The positive ion of one compound replaces the positive ion of the other to form two new compounds. • AB + CD → AD + CB Ex: Ba(NO3)2 + K2SO4 → BaSO4 + 2KNO3

  17. Combustion Reactions • Use oxygen as a reactant • AB + O2→ A + BO2 ex: 2CH4 + 3O2 →2CO + 4H2O • Usually the products include water and a gas (CO, CO2)

  18. Balancing Chemical Equations • Chemical Equations summarize reactions by using formulas for each substance. • Because of conservation of matter (mass) the left side (reactants) must equal the right side (products).

  19. Steps for Balancing Equations • Write the equation (write the reactants and products as chemical compound formulas and elements) Ex: H2 + O2→ H2O

  20. Draw a table and fill in all elements in the equation as well as how many atoms of each element are on each side of the equation. Ex: H2 + O2→ H2O

  21. If the reactant numbers are not the same as the product numbers for each element, you have to balance the equation. 2 1

  22. Add co-efficients in front of the formulas until they balance. (remember – you can’t change the formula by changing subscripts – you can only change the amount of the substance you have. )

  23. Example 2 2 H2 O2 H2 O 2 2 2 1 + → Un-balanced H / / 4 4 balanced O / 2 balanced

  24. Practice • Balance the following equations: • HgO → Hg + O2 • N2 + H2 → NH3 • KClO3 → KCl + O2 • KBr + Cl2 → KCl + Br2 • CO + O2 → CO2

  25. Answers • 2HgO → 2Hg + O2 • N2 + 3H2 → 2NH3 • 2KClO3 → 2KCl + 3O2 • 2KBr + Cl2 → 2KCl + Br2 • 2CO + O2 → 2CO2

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