Physical Science Unit 4

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  • 1. UNIT 4: CHEMICAL REACTIONS & SPECIAL TOPICS
  • 2. BILL NYE THE SCIENCE GUY  http://www.youtube.com/watch?v=PlwuxpMh8nk
  • 3. 7.1: DESCRIBING REACTIONS The most useful way of describing reactions (or a chemical change) is by stating what was present before and after the chemical change.
  • 4. CHEMICAL CHANGE  chemical reaction  process in which one or more substances are changed into new substances  reactants  substance that reacts  product the new substance that is formed
  • 5. CONSERVATION OF MASS The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants.  This is called the Law of Conservation of Mass (mass is not created nor destroyed in a chemical reaction).
  • 6. WRITING EQUATIONS chemical equation shorthand method to describe a chemical reaction using chemical formulas and other symbols Reactants HgO(s)  Products Hg(l) + O2(g)
  • 7. WRITING EQUATIONS (aq) – aqueous (substance dissolved in water) (s)– solid (l) – liquid (g) – gas coefficients – the numbers to the left of the formulas used to help balance the equation
  • 8. BALANCING EQUATIONS  If you notice that the number of atoms on the left side does not equal the number of atoms on the right side then we must balance the equation.  Since mass is conserved before and after a chemical reaction, an equation MUST BE balanced in order for it to be true.
  • 9. BALANCING EQUATION RULES 1. You should NEVER change the subscripts in a formula. 2. Start by, counting the number of atoms of each element on each side of the equation. 3. Change one or more coefficients until the equation is balanced
  • 10. EXAMPLE  Balance the following equation NiCl2(aq) + NaOH(aq)  Ni(OH)2(s) + NaCl(aq)
  • 11. EXAMPLE HgO(s)  Hg(l) + O2(g)
  • 12. 7.2: TYPES OF REACTIONS  Reactions are classified by the type of reactant or the number of reactants and products. There are 4 different types of reactions that we will discuss  Synthesis  Decomposition  Single-Replacement  Double-Replacement
  • 13. SYNTHESIS A synthesis reaction is a reaction in which two or more substances react to form a single substance.  The product synthesized is always a compound  Examples A + B  AB 2Na + Cl2 2NaCl
  • 14. DECOMPOSITION  The opposite of synthesis A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances.  The reactant MUST BE a compound.  Examples AB A + B 2H2O 2H2 + O2
  • 15. SINGLE REPLACEMENT A single replacement reaction is a reaction in which one element takes the place of another element in a compound. Example Form: A + BC B + AC Cu + 2AgNO3  2Ag + Cu(NO3)2
  • 16. DOUBLE REPLACEMENT A double replacement reaction is one in which two different compounds exchange positive ions and form two new compounds. Example Forms: AB + CD AD + CB Pb(NO3)2 + 2KI PbI2 + 2KNO3
  • 17. 7.3: ENERGY CHANGES IN REACTIONS Chemical Energy – the energy stored in the chemical bonds of a substance. Chemical Reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.
  • 18. BREAKING BONDS Breaking Bonds REQUIRES energy.  This means we need to ADD energy in order to break the bonds of reacting molecules in order to get the reaction started.
  • 19. FORMING BONDS The formation of chemical bonds RELEASES energy.  When new chemical bonds are formed, a bit of energy is released usually in the form of heat or light.
  • 20. ENERGY IN REACTIONS During a chemical reaction, energy is either ABSORBED or RELEASED.  We describe these reactions in two different ways either Exothermic or Endothermic.
  • 21. EXOTHERMIC REACTIONS A chemical reaction that RELEASES energy is called an exothermic reaction.  The energy released as the products form is greater than the energy required to break the bonds in the reactants.  Think of it as energy is EXITING the reaction  EXiting _ Exothermic
  • 22. ENDOTHERMIC REACTIONS A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction.  This means that there is more energy require to break the bonds of the reactants than is released by the formation of the products.
  • 23. CONSERVATION OF ENERGY The total amount of energy BEFORE a reaction is EQUAL to the total amount of energy AFTER a reaction.  This is called the Conservation of Energy.
  • 24. 8.1: FORMATION OF SOLUTIONS A solution is a mixture that forms when substances dissolve and form a homogeneous mixture  In order for a solution to form, one substance must dissolve in another.
  • 25. DISSOLVING Every solution has two components  A solute is a substance who particles are dissolved in a solution  A solvent is the substance in which the solute dissolves in. There are three ways that substances can dissolve into water: dissociation, dispersion, ionization
  • 26. DISSOCIATION  In order for a solution to form, the attractions that hold the solute together and the solvent together must be overcome. The process in which an ionic compound separates into ions as it dissolves is called dissociation.  Example: Sodium Chloride & Water
  • 27. DISPERSION Sugar dissolves into water by dispersion, or breaking into small pieces that spread throughout the water.  Example: Sugar & Water
  • 28. IONIZATION The process in which molecules gain or lose electrons is known as ionization.  Example: Ions are formed by the reaction of the solute and solvent particles.
  • 29. PROPERTIES OF LIQUID SOLUTIONS Conductivity: ability to conduct electric current Boiling Point: temperature needed for solution to change from liquid phase to gas phase Freezing Point: temperature needed for solution to turn from liquid phase to solid phase. Solutions can also be described as endothermic or exothermic depending upon whether energy is released or absorbed.
  • 30. 8.2: SOLUBILITY & CONCENTRATION The maximum amount of solute that dissolves in a given amount of solvent at a constant temperature is. called solubility  Depending upon the amount of solute in a solution, solutions can be described as either saturated, unsaturated or supersaturated.
  • 31. SATURATED A saturated solution is one that contains as much solute as the solvent can hold at a given temperature.
  • 32. UNSATURATED A solution that has less than the maximum amount of solute that can be dissolved is called an unsaturated solution.
  • 33. SUPERSATURATED A supersaturated solution is one that contains more solute than it can normally hold at a given temperature.  Usually very unstable.
  • 34. FACTORS AFFECTING SOLUBILITY Polarity of the solvent  “like dissolves like”  Solution formation is more likely to happen when the solute and solvent are either both polar or both nonpolar. Temperature  The solubility of a solids increases as the solvent temperature increases Pressure  Increasing pressure on a gas increases solubility in a liquid.
  • 35. CONCENTRATION The concentration of a solution is the amount solute dissolved in a specified amount of solution.  Can be expressed as percent by volume, percent by mass and molarity.
  • 36. SOLUBILITY CURVE  Each line on the graph is called a solubility curve for a particular substance.  You can use a solubility curve to figure out how much solute will dissolve at any temperature given on the graph.
  • 37. 10.1: RADIOACTIVITY Henri Becquerel  1896  left uranium salt in a drawer with a photographic plate  when he developed the plate, he found an outline of the clumps of the uranium salt  he hypothesized that the uranium salt emitted some sort of energy Marie and Pierre Curie  students of Becquerel  2 years later, they discovered Po and Ra while studying uranium ore “pitch blende”
  • 38. THEY DISCOVERED …  Radiation  release of matter and energy from nucleus  Light energy (electromagnetic spectrum)  all forms of radiation
  • 39. STRONG FORCES Protons are held together by strong forces short range force as the distance increases, the force weakens causes protons and neutrons to be attracted to each other
  • 40. STRONG FORCES  to hold a nucleus together tightly, the nucleus can decay and give off matter and energy  stable nucleus  stays together permanently  unstable nuclei  radioactive!!!!  nucleus does not stay together; emits matter and energy
  • 41. RADIOACTIVE ELEMENTS  radioactivity  the process of nuclear decay  elements after #83 are radioactive  all elements after #92 are synthetic and decay soon after they are created
  • 42. REVIEW !!!!!!!  Mass number = # protons + # neutrons  Atomic number = # of protons Example (12 = mass #) (6= atomic #) 12 6 C
  • 43. REVIEW: ISOTOPES  most elements have at least one radioactive isotope  isotope  same element with a different number of neutrons  Example: Carbon-12 (stable) Carbon-14 (unstable)
  • 44. RADIOACTIVE DECAY Transmutation  the process of changing one element into another through nuclear decay Radioactive decay  occurs until a stable nucleus is formed
  • 45. ALPHA DECAY alpha decay (α)  releases alpha particle (a helium nucleus)  a helium nucleus consists of 2 protons and 2 neutrons  atomic mass is 4  atomic number is 2 4 2He  Ex: 238 92 U 234 90 Th + 4 2He
  • 46. BETA DECAY Beta Decay (β)  release beta particle it occurs when a neutron breaks down into 1 electron and 1 proton the result is an atom with 1 more proton Ex: 14 6 C 14 7 N+ 0 e -1
  • 47. PRACTICE: ALPHA & BETA DECAY 214 84 222 86 214 82 234 92 Po Rn Pb U 210 82 Pb ______ ______ 4 2 ______ 0 1 234 93 Np He e ______
  • 48. NUCLEAR REACTIONS  FISSION  process of splitting a nucleus into several smaller nuclei
  • 49. NUCLEAR REACTIONS FUSION:  Two nuclei with low masses are combined to form one nucleus of larger mass
  • 50. HALF LIFE half-life  the amount of time it takes for half the nuclei in a sample of the isotope to decay the nucleus left after the isotope decays is called the daughter nucleus some half-lives are seconds, others are millions of years
  • 51. EXAMPLE: HALF LIFE  Assume a 20g sample of Ba-139 has a half-life of 86 minutes. how much Ba-139 remains after 86 minutes?  after 172 minutes?  how many ½ lives leave 1.25g of Ba-139? 