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Chemical Reactions: Energy Transformations and Reaction Types

Explore the energy changes in chemical reactions, from exothermic to endothermic processes, and delve into different reaction types such as synthesis and decomposition reactions. Understand the conservation of energy and mass, as well as the significance of balanced equations in determining mole ratios and quantities.

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Chemical Reactions: Energy Transformations and Reaction Types

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  1. Chapter 7Chemical Reactions

  2. The Nature of Chemical Reactions • Chemical reactions occur when substances go through chemical changes to form new substances • Reactant is a substance that participates in a chemical reaction • Product is a substance that forms in a chemical reaction

  3. The Nature of Chemical Reactions • Chemical reactions always involve changes in energy • Energy must be added to break bonds • Forming bonds releases energy • Energy is conserved in chemical reactions • Energy in a chemical reaction can change form, energy is never created or destroyed

  4. The Nature of Chemical Reactions • Reactions that release energy are exothermic • Products have less energy than the reactants • Reactions that absorb energy are endothermic • Products have more energy

  5. Cold Pack • Cold Packs A cold pack comes in a plastic bag made of tough white plastic. This bag is filled with a smaller bag and ammonium nitrate crystals. The smaller bag contains water, and is made of a thin weak plastic, so it is easy to break. When a cold pack is used, it must be "broken" by rupturing the inner bag. Breaking the bag releases the water, which dissolves the ammonium nitrate. The water and ammonium nitrate react completely, and within fifteen to twenty the cold pack will no longer feel cold. 

  6. Cold Pack • Cold packs make use of the heat transfer that occurs during chemical reactions, but in contrast to heat packs, utilize endothermic reactions. In the endothermic reaction between ammonium nitrate crystals and water, the heat required for the reaction to proceed from reactants to products is absorbed from the surrounding environment, resulting in a decrease in temperature of the pack noticeable to the touch. This reaction, equation (2) NH4NO3(s) + H2O(l) + heat → NH4+(aq) + NO3-(aq) (2)

  7. Hand Warmers • Air activated hand warmers contain cellulose, iron, water, activated carbon (evenly distributes heat), vermiculite (water reservoir) and salt (catalyst) and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours. • Hand warmers work because of a rusting process. The rusting is a redox reaction and the equation is as follows: 4Fe(s) + 3O2(g) -> 2Fe2O3(s) .

  8. Chemical Equations • Uses symbols to represent a chemical reaction and shows the relationship between the reactants and products of a reaction • Reactants yield Products • CH4 + O2 CO2 + H2O • Not Balanced?????

  9. Chemical Equations • Balanced chemical equations account for the conservation of mass • CH4 + 2O2 CO2 + 2H2O

  10. Chemical Equations • A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction • Equation: 2Mg + O2 2MgO • Moles: 2 + 1 2 • Molar mass(g/mol): 24.3 + 32.0(16x2) 40.3 • Mass calculations: • 24.3g/molx2mol + 32.0g/molx1mol  • 40.3g/molx2mol • Mass(g): 48.6g + 32.0g  80.6g

  11. Chemical Equations • Mole ratios tell you the relative amounts of reactants and products • Mole ratios can be converted to masses

  12. Reaction Types • Synthesis reactions combine substances • A + B  AB • 2Na + Cl2  2NaCl • Metal sodium reacts with chlorine gas to form sodium chloride (table salt)

  13. Reaction Types • Decomposition reactions break substances apart • AB  A + B • 2H2O  2H2 + O2 • Electrolysis of water breaks down water into hydrogen gas and oxygen gas

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