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AHSGE Review J. Pollock Spring 2007 Science Skills and Chemistry Concepts What is the scientific method? It is a framework (guidelines) for investigating a problem. There are five steps. – – – – – State the problem. Make an educated guess, or hypothesis. Design and conduct an experiment to test. Analyze the data collected during the test. State the conclusion. What skills are necessary in order to interpret data? To determine how data should be classified and interpreted, graphs and tables are used. Measurements are compared to standards. Hypotheses are tested and either accepted as theories or rejected. What are controls and variables? Variables are parts of an experiment that can be measured. – They CAN change. Controls are parts of the experiment that do no undergo change. – Controls are used for comparison. – Since the control does not change, the variable can be compared to it and the change measured. How is the metric system used to measure dimension and volume? The metric system is measurement based on powers of 10. It has standards for length, mass, time, electric current, temperature, amount of substance, and luminous intensity. Prefixes are used for very large or very small numbers. Conversions can be made by moving decimals. Common Metric Units Quantity Unit Symbol Length Meter m Mass Kilogram kg Time Second s Electric current Ampere A Temperature Kelvin K Amount of substance Mole mol Luminous intensity Candela cd What is a hypothesis? A hypothesis is an educated guess. You use what you already know to guess how an experiment is going to turn out. It must be tested with an experiment. What glassware is necessary for use in the laboratory? Top row: test tube holder, Florence flask, funnel, Erlenmeyer flask, test tube, graduated cylinder; Bottom row: triple-beam balance, beaker, thermometer What is mass, and what are its units? Mass is the measure of the amount of matter in an object. It is measured in milligrams, grams, kilograms, etc. Kilograms are the standard unit. Mass is NOT the same as weight. Weight is a force that measures the force of gravity on an object. What units are used for temperature in the metric system? Temperature is measured in degrees Celsius or Kelvins. Kelvin is the standard unit for scientific purposes. Calculations can be used to convert between the two units. – Kelvin = Celsius + 273 – Celsius = Kelvin - 273 Temperature Conversion Examples Convert 100C to Kelvin. K = 100+273 K = 373 Convert 200 K to Celsius. C = 200-273 C = -73 What is density? Density is the amount of matter in a specific volume. The formula is density = mass ÷ volume (d=m/v). Example: An object having a mass pf 8.40 g and a volume of 4.2 cm3 will have what density? D = m/v D = 8.40g / 4.2 cm3 D = 2.0 g/cm3 How is the periodic table arranged? The periodic table was first published in 1869 by Russian scientist Dmitri Mendeleev. The first periodic table was arranged according to the atomic mass of elements. Mendeleev’s table was revised by Henry Moseley in 1911. Moseley’s table was arranged by atomic number, since this number does not change. How is the periodic table arranged? The periodic table has seven periods (horizontal rows) and eighteen groups (vertical columns). Groups are together because the elements in them have similar properties and react in the same manner. Across periods (left to right), atomic radius (size) decreases, ionization energy (ease of losing an electron) increases, and electronegativity (ability to attract electrons) increases. What are valence electrons, and how do they work? Valence electrons are found in the outer shells of atoms. These are the only electrons involved in chemical reactions. The most valance electrons an atom can have is 8. This is called the octet rule. The number 8 is possible by gaining, losing or sharing electrons. The octet rule explains why atoms tend to form compounds. What are valence electrons, and how do they work? What are valence electrons, and how do they work? How is the size of an atom measured? Atoms are too small to be seen or measured with the naked eye. Scanning electron microscopes can be used to detect them, but we generally see the EFFECT of the atom, not the atom itself. Atoms can be measured in moles. A mole is 6.02 X 1023 atoms. The mole is just a unit. What is chemical reactivity? Reactivity is the ability of an atom to undergo a reaction and the speed of the reaction. It refers to the ability of the element to react with other substances. The element’s position on the periodic table determines its reactivity. Compare and contrast metals, nonmetals, and metalloids. Metal Nonmetal Metalloid Shiny Dull Dull Hard, malleable, and ductile Brittle Intermediate ability to break Excellent conductor of heat and electricity Poor conductor of heat and electricity Intermediate conductor of heat and electricity Mostly solid at room temperature Mostly gas at room temperature Mostly solid at room temperature Left side of periodic table Right side of periodic table Along zig-zag line of periodic table Compare and contrast metals, nonmetals, and metalloids. What are noble gases? The noble gases are found in group 18 of the periodic table. They are generally unreactive and were not discovered until 1962. They can react with other elements when they are treated. Noble gases are very useful because they are stable. Noble Gases What is the difference between a family and a period? Families are the vertical columns, or groups, of the periodic table. Periods are the horizontal rows of the periodic table. Families are arranged because of similar properties. Each element in a group will react and appear much the same. Coded Periodic Table What is the atomic number and how is it used? The atomic number is the number of protons in an atom of a particular element. The number of protons cannot change, so the atomic number of an element will always remain the same. Atomic number is useful for organizing elements on the periodic table. They are listed in order of increasing atomic number. What affects the mass number of an atom? The mass number is the number of protons and neutrons in the nucleus of an atom. Since the number of protons cannot change, the number of neutrons determines the mass number of a particular atom. The formula for determining mass number is # of protons + # of neutrons = mass number. It is possible for an element to have several different mass numbers. Mass Number Calculation Example An atom of uranium has 92 protons and 143 neutrons. What is its mass number? 92 + 143 = 235 What are isotopes? Isotopes are atoms of the same element that have different numbers of neutrons. The number of protons and electrons remains the same. For example, carbon can be carbon-12, carbon-13, or carbon-14. All of these isotopes have 6 protons, but their neutrons are 6, 7, and 8, respectively. What are isotopes? Isotope Protons Electrons Neutrons Carbon-12 6 6 6 Carbon-13 6 6 7 Carbon-14 6 6 8 Compare and contrast physical and chemical changes. Chemical changes – One or more substances converted into different substances – Cannot be reversed back to original substances by physical means Physical changes – Observed or measured without changing the identity of the substance – Melting point, boiling point – Phase – Color, size, shape What are the characteristics of chemical changes? Chemical changes have specific properties. You can tell a chemical change has occurred, if you observe the following. – – – – Heat or light Gas given off Formation of precipitate Color change Compare endothermic and exothermic reactions. Endothermic – Absorbs energy – Temperature of surroundings lowers – Example: photosynthesis – Bonds formed Exothermic – releases energy – Temperature of surroundings rises – Example: creation of salt from sodium and chlorine – Bonds broken – Released energy from chemical energy What is the difference in solute and solvent? Solute is the substance in the larger quantity in a solution. It is said to do the dissolving. Solvent is the substance in the smaller quantity in a solution. It is said to be the substance being dissolved. A solution is a homogeneous mixture of two or more substances uniformly spread throughout a single phase. What is solubility? Solubility involves the amount of solute needed to form a saturated (maximum amount of solute dissolved) solution with a specific amount of solvent at a certain temperature. It is affected by temperature, agitation (stirring), phase (surface area), pressure, and polarity of the substances. What factors affect solubility? Polarity temperature surface area stirring pressure What are units of concentration? Units of concentration serve to show how much solute is contained in a certain volume of solvent. – Molarity (moles per liter) – molality (moles per kilogram) How do I calculate concentration? You have 3.50 L of solution that contains 1.55 mol of sodium chloride, NaCl. What is the molarity of that solution? M = mol/L M = 1.55/3.50 M = 0.44 How do I calculate concentration? A solution was prepared by dissolving 0.05 mol of sucrose (table sugar, C12H22O11) in 0.125 kg of water. Find the molal concentration of this solution. m = mol/kg m = 0.05/0.125 m = 0.4 What is conductivity? Conductivity is the ability of a substance to conduct heat, electricity, or sound. Metals and ionic compounds (acids, bases, and salts) have high conductivity. Nonmetals and molecular compounds have low conductivity. Compare saturated, unsaturated, and supersaturated solutions. Saturated Unsaturated Contains Less solute than maximum of a saturated dissolved solute solution under same conditions Supersaturated More dissolved solute than a saturated solution under same conditions What is an electrolyte? An electrolyte is a substance that dissolves in water to give a solution that conducts electric current. – – – – Ionic compounds highly polar molecular compounds strong acids strong bases Compare ionic and covalent bonds. Ionic – metal and nonmetal – electrons gained or lost – good conductor – contains highly electronegative element Covalent – two nonmetals – electrons shared – poor conductor – similar electronegativities among bonded elements What is an electron dot diagram? Also known as Lewis structures (molecules) formulas in which atomic symbols represent nuclei and inner-shell electrons and dots represent outer-shell electrons For example –H He B Ne How are ions used to form bonds? Ions gain or lose electrons to form stable outer shells when they bond. Example Na + Cl Na+ Cl - How are compounds named? Ionic compounds are named by stating the elemental name of the cation (positive ion), removing the ending of the cation (negative ion), and replacing it with -ide. Example: sodium and chlorine sodium chlorine sodium chloride How are compounds named? Molecular compounds are named using Greek prefixes. Example: arsenic and oxygen (As2O5) diarsenic pentoxide Greek Prefixes Used for Compound Naming mono - 1 hexa - 6 di - 2 hepta - 7 tri - 3 oct - 8 tetra - 4 nona - 9 penta - 5 deca - 10 How are chemical equations balanced? There are steps for balancing chemical equations. First, you must list the atoms on both sides of the equation. Then, you must count and tally the atoms on both sides of the equation. Balance one type of atom at a time. First balance the atoms of elements that appear only once on both sides of the equation. Balance polyatomic ions as single units. Balance H and O last. Balancing Equations Example HNO3 + Mg(OH)2 Mg(NO3)2 + H2O See next slide for list and tally. Mg is balanced. Balance NO3. 2 HNO3 + Mg(OH)2 Mg(NO3)2 + H2O Balance OH. 2 HNO3 + Mg(OH)2 Mg(NO3)2 + 2 H2O Check tally for balance. Balancing Equations Example 21 H 21 NO3 2 NO3 1 Mg 1 Mg 2 OH 21 OH 21 H What is the Law of Conservation of Matter? The Law of Conservation of Matter states that matter can neither be created nor destroyed in a chemical reaction. This law is essential to balancing chemical equations. The mass on both sides of the equation must be equal. Conservation applies to mass and matter as well. LaVoisier proved the law completing his reactions on a balance with a bell jar to contain the gases. How do electrons affect chemical reactions? Electrons in the outer shell ONLY are involved in chemical reactions. The number of electrons being transferred determines the properties of the substances being combined, as well as that of the resulting substance. The location of the electrons involved in the reaction determines the amount of energy involved in the reaction. What are the types of chemical reactions? There are five types of chemical reactions. – Decomposition: breaking down of substances – synthesis: creation of new substance – single replacement: exchanging of one element for another – double replacement: two substances react to form two new substances – combustion: reaction of oxygen and a hydrocarbon What are the types of chemical reactions? Synthesis: 2 Mg + O2 2 MgO Decomposition: 2 H2O 2 H2 + 2 O2 Single Replacement: 2 N2 + 2 H2O 2 NaOH + H2 Double Replacement: 2 KI + Pb(NO3)2 PbI2 + 2 KNO3 Combustion: C3H8 + 5 O2 3 CO2 + 4 H2O