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Semester 2 Review

Semester 2 Review. May 30 and 31, 2013. Acids and Bases. 3 Standards 18 Questions. Temperature and Thermochemistry. 4 Standards 24 Questions. Reaction Rates. 3 Standards 15 Questions. Chemical Equilibrium. 2 Standards 10 Questions. Biochemistry and Organic Chemistry.

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Semester 2 Review

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  1. Semester 2 Review May 30 and 31, 2013

  2. Acids and Bases 3 Standards 18 Questions

  3. Temperature and Thermochemistry 4 Standards 24 Questions

  4. Reaction Rates 3 Standards 15 Questions

  5. Chemical Equilibrium 2 Standards 10 Questions

  6. Biochemistry and Organic Chemistry 2 Standards 10 Questions

  7. Nuclear Chemistry 2 Standards 8 Questions

  8. Dissociation of a strong acid: HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq) • (5b) Acids are hydrogen-ion-donating and bases are hydrogen-ion-accepting substances. Dissociation of a weak acid: CH3COOH(aq) + H2O(l) ↔ H3O+(aq) + CH3COO-(aq) Dissociation of a strong base: KOH(aq) → K+(aq) + OH-(aq) Dissociation of a weak base: NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq) Hint! Dissociate means separate. Acids dissociate to form a conjugate base. Bases dissociate to form a conjugate acid.

  9. Strong Acid: All the acid molecules have dissociated to form hydronium (H3O+) and the conjugate base. • (5c) Strong acids and bases fully dissociate and weak acids and bases partially dissociate. Weak Acid: Only some of the acid molecules have dissociated to form hydronium (H3O+) and the conjugate base. Other acid molecules remain. Strong Base: All the base molecules have dissociated to form hydroxide (OH-) and the conjugate acid. Weak Base: Only some of the base molecules have dissociated to form hydroxide (OH-) and the conjugate acid. Other base molecules remain.

  10. (5d) Students know how to use the pH scale to characterize acid and base solutions. • The pH scale goes from 0-14. Water is neutral, and has a pH of 7. Acids have a pH below 7, and bases have a pH above 7. • Each value of pH means the concentration of H+ changes by a power of 10. • Example: pH 1 means [H3O+] = 10-1 and pH 2 means [H3O+] = 10-2 • As the pH decreases, the hydronium ion concentration [H3O+] increases and the hydroxide ion concentration [OH-] decreases. • As the pH increases, the hydronium ion concentration[H3O+] decreases and the hydroxide ion concentration [OH-] increases. Most Acidic Most Basic

  11. (7a) Describe temperature and heat flow in terms of the motion of molecules (or atoms). • Temperature is a measure of the average kinetic (movement) energy of particles in an object. • As kinetic energy increases, temperature increases. • Because molecular motion increases as a substance changes from solid, to liquid, to gas, temperature increases as well. System: The egg • Heat is a form of energy. Heat is transferred from high temperature (hot) to low temperature (cold). Surroundings: The pan, stove, etc. Heat flows: Into the egg

  12. (7b) Chemical reactions either release (exothermic) or absorb (endothermic) thermal energy.

  13. (7c) Energy is released when a material condenses or freezes and is absorbed when a material evaporates or melts. What parts of the phase change diagram represent a phase change? How do you know? What temperatures (C and K) are the melting point, freezing point, and boiling point of water? Does adding heat to water always increase its temperature? Why or why not? Which phase changes are endothermic? Which are exothermic?

  14. (7d) Solve heat flow and temperature change problems. Example Problem: How much heat is absorbed when 100 g of water is heated from 365 K to steam at 400 K? Step 1: The boiling point of water is 373 K ΔT = 400-365 = 35K phase change = evaporation Step 2: q=mcΔT q = (100g)(4.18J/gK)(35K) q = 14,630 J q = mΔHt q = (100g)(2260J/g) q = 226,000 J Step 3: 14,630 J + 226,000 J = 240,630 J Phase Change: Q = m × ΔHtranformation • Multi-Step Thermochemistry Problems: • The Basic Steps to Solve: • Step 1: Break down the problem into “temperature change” and “phase change” steps. • Use the boiling point and melting point as a guide. • Step 2: Solve the steps separately. • Use q=mcΔT for temperature change and q = mΔHt for phase change • Step 3: Add up the answers to get the total heat transferred.

  15. (8a) The rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time. Rate = change over time Example Test Question C3H8(g) + 5 O2(g) → 3 CO2 (g) + 4 H2O(g) + heat Which is an acceptable method to measure the rate of the reaction? (a) the rate of consumption of C3H8 (b) the rate of consumption of O2 (c) the rate of production of CO2 (d) All of the above Can you justify your answer? • Reaction rate is the rate at which a chemical reaction takes place. • It is measured by the rate of production of product or disappearance of reactant. • Usually, the rate decreases gradually as the reaction continues. The rate is zero when the reaction is complete.

  16. (8b) Reaction rates depend on such factors as concentration, temperature, and pressure. • To start a chemical reaction, molecules must collide with a certain amount of energy. • As reactant concentration increases, rate increases. As reactant concentration decreases, rate decreases. • As surface area increases, rate increases. As surface area decreases, rate decreases. • If working with gases, as pressure increases, rate increases. As pressure decreases, rate decreases. • As temperature increases, rate increases. As temperature decreases, rate decreases.

  17. (8c) What is the role a catalyst plays in increasing the reaction rate? • Activation Energy is the minimum amount of energy required to start a chemical reaction. • A catalyst is a substance that increases the reaction rate when it is added to a reaction mixture. • Catalysts increase the reaction rate by lowering the activation energy. • A catalyst is not a reactant or a product because it is not used up or changed during the reaction. • An enzyme is a protein that acts as a catalyst. It is a biological catalyst. The new reaction pathway should have the same “reactants” and “products” energy level. The hill of activation energy should be smaller.

  18. (9a) Use Le Chatelier's principle to predict the effect of changes on an equilibrium system. Le Chatelier’s Principle: For a reversible process at equilibrium, when conditions of concentration, temperature, or pressure are changed, the reaction shifts in a direction that will counteract the stress and restore equilibrium. Example #1 exothermic reaction: N2(g) + 3H2(g) ↔ 2NH3(g) + heat

  19. (9a) Use Le Chatelier's principle to predict the effect of changes on an equilibrium system. Le Chatelier’s Principle: For a reversible process at equilibrium, when conditions of concentration, temperature, or pressure are changed, the reaction shifts in a direction that will counteract the stress and restore equilibrium. Example #1 endothermic reaction: N2O4(g) + heat ↔ 2NO2(g)

  20. (9b) Equilibrium is established when forward and reverse reaction rates are equal. • Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of the products and reactants stop changing. • The amounts of products and reactants do not have to be equal. • Only reversible chemical reactions can reach equilibrium. Reversible reactions are shown with a double arrow: • Ex: H2SO4  2H+ + SO42 • When this reaction is in equilibrium, H2SO4 is produced and broken down at equal rates. Example Problems: 2N2(g) + 6 H2O (l) ç4NH3(g) + 3O2 (g) 1) Write the forward reaction: 2N2 (g) + 6 H2O (l) 4NH3(g) + 3O2 (g) 2) Write the reverse reaction: 4NH3(g) + 3O2 (g) 2N2(g) + 6 H2O (l) 3) At equilibrium, does the concentration of water have to equal the concentration of oxygen? Answer: No, the concentrations of products and reactants do not have to match. But, the rate of the forward and reverse reactions will be the same.

  21. (10a) Polymers are made of repetitive combinations of simple subunits. Condensation reactions connect monomers to make a polymer Hydrolysis reactions break down polymers into monomers

  22. (10b) Carbon’s bonding characteristics allow it to form a large variety of structures. • Carbon has four valence electrons. • Because carbon can form simultaneous covalent bonds with up to four partners, an enormous number of carbon compounds are possible. • Among the simplest organic compounds are those containing only hydrogen and carbon, or hydrocarbons. • Hydrocarbons include methane gas, liquid gasoline, and solid paraffin wax in candles.  • Carbon is the backbone of the macromolecules also. • Allotropes of carbon are diamond, graphite, and graphene. The different carbon structures lead to different properties.

  23. (11c) Radioactive isotopes may be man-made or found in nature. • Radioactivity is the spontaneous changing of the nucleus of an atom from one element to another. • Radioactive isotopes occur naturally. • They are also man-made. • Half-Life is the amount of time it takes for ½ of a radioactive substance to undergo radioactive decay. • Example: The half-life of Carbon-14 is 5700 years. A sample containing 100 g of carbon-14 today, will contain 50 g carbon-14 in 5700 years from now. • Nuclear fission is the process of splitting the nuclei of atoms, which releases energy from within those atoms. • Nuclear fusion is the process of joining, rather than splitting, such atomic particles with similar releases of energy.

  24. (11c) The three most common types of nuclear decay are alpha, beta, and gamma. Each changes the nucleus in a different way. Alpha (α) decay: The mass number decreases by 4 and the atomic number decreases by 2. 218 4 Po He + 84 2 214 Pb 82 Beta (β) decay: The mass number remains the same. The atomic number increases by 1. 14 0 C e + 6 -1 14 N 7 Gamma (γ): Gamma rays are released during electron capture. 51 0 Cr + e + γ 24 -1 51 V 23

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