MCAT gen chem real

MCAT CONCEPT CHECK 7.4 1. Define endothermic and exothermic processes. • Endothermic: • Exothermic:

1. Endothermic reactions involve an increase in heat content of a system from the surroundings (ΔH > 0), while exothermic reactions involve a release of heat content from a system (ΔH < 0)answer

MCAT CONCEPT CHECK 7.1 1. A person snaps an ice pack and places it on his or her leg. In terms of energy transfer, what would be considered the system and what would be the surroundings in this scenario? • System: • Surroundings: 2. What is unique about each of the following types of processes? • Isothermal: • Adiabatic: • Isobaric: • Isovolumetric (isochoric):

1. The boundary between system and surroundings could be placed anywhere. Most commonly, the ice pack would be considered the chemical system using up energy, and the person (and the remainder of the universe) constitutes the surroundings that are providing the heat for the ice pack to function. 2. • Isothermal: no change in temperature; ΔU = 0, Q = W • Adiabatic: no heat exchange; Q = 0, ΔU = −W • Isobaric: no change in pressure; line appears flat in a P-V graph • Isovolumetric (isochoric): no change in volume; W = 0, ΔU =Q

MCAT CONCEPT CHECK 6.2 Before you move on, assess your understanding of the material with this question. 1. Describe what would happen in the following situations: • In the reaction H2SO4 (aq) ⇋ H+ (aq) + HSO4 − (aq), the pH has been increased: • In the reaction 2 C (s) + O2 (g) ⇋ 2 CO (g), the pressure of the reaction vessel is decreased: • In the reaction CH4 (g) + 2 O2 (g) ⇋ CO2 (g) + 2 H2O (l) + heat, the reaction vessel is warmed: • In the reaction H3PO4 (aq) + H2O (l) ⇋ H3O+ (aq) + H2PO4 removed (without changing temperature):

*Tricky! remember, things with (s) behind them don't count

MCAT CONCEPT CHECK 8.1 1. Name some characteristics that make the gas phase unique: 2. A mercury barometer is primarily affected by atmospheric pressure. What would happen to the level of the mercury in the column if: • the barometer was moved to the top of a mountain? • the barometer was placed ten meters under water? 3. What are the conditions for STP? 4. What are the standard conditions

1. Gases are compressible fluids with rapid molecular motion, large intermolecular distances, and weak intermolecular forces. 2. At the top of the mountain, atmospheric pressure is lower, causing the column to fall. Under water, hydrostatic pressure is exerted on the barometer in addi-tion to atmospheric pressure, causing the column to rise. 3. STP: T = 273 K (0°C), P = 1 atm 4. Standard conditions: T = 298 K (25°C), P = 1 atm, concentrations = 1

MCAT CONCEPT CHECK 6.3 1. What conditions favor formation of a kinetic product? A thermodynamic product? • Kinetic product: • Thermodynamic product: 2. On a reaction coordinate diagram, how would the kinetic pathway appear as compared to the thermodynamic pathway?

1. Kinetic products are favored at low temperatures with low heat transfer. Thermodynamic products are favored at high temperatures with high heat transfer. 2. Kinetic pathways require a smaller gain in free energy to reach the transition state. They also have a higher free energy of the products, with a smaller difference in free energy between the transition state

MCAT CONCEPT CHECK 7.2 1. What are standard conditions? When are standard conditions used for calculations? 2. What is the definition of a state function? A process function? • State function: • Process function: 3. List at least five common state functions: • • • •

1. Kinetics, equilibrium, and thermodynamics calculations use standard conditions, which are 25°C (298 K), 1 atm pressure, and 1 M concentrations. 2. State functions are properties of a system at equilibrium and are independent of the path taken to achieve the equilibrium; they may be dependent on one another. Process functions define the path between equilibrium states and include Q (heat) and W (work). 3. State functions include pressure (P), density (ρ), temperature (T), volume (V), enthalpy (H), internal energy (U), Gibbs free energy (G), and entropy (S)

MCAT CONCEPT CHECK 7.5 1. Rank the phases of matter from lowest to highest entropy. 2. Describe entropy in terms of energy dispersal and disorder.

1. Solids have the lowest entropy, followed by liquids, with gases having the highest entropy. 2. Entropy increases as a system has more disorder or freedom of movement, and energy is dispersed in a spontaneous system. Entropy of the universe can never be decreased spontaneously

MCAT CONCEPT CHECK 9. 1. Describe the process of solvation. 2. Describe the differences between solubility and saturation: • • 3. What is one way in which solubility of a compound can be increased? 4. Name two ions that form salts that are always soluble: • •

1. Solvation refers to the breaking of intermolecular forces between solute particles and between solvent particles, with formation of intermolecular forces between solute and solvent particles. In an aqueous solution, water is the solvent. 2. Solubility is the amount of solute contained in a solvent. Saturation refers to the maximum solubility of a compound at a given temperature; one cannot dissolve any more of the solute just by adding more at this temperature. 3. Solubility of solids can be increased by increasing temperature. Solubility of gases can be increased by decreasing temperature or increasing the partial pres-sure of the gas above the solvent (Henry's law). 4. Group I metals, ammonium, nitrate, and acetate salts are always soluble

MCAT CONCEPT CHECK 7.3 1. Contrast temperature and heat. • Temperature: • Heat: 2. Contrast specific heat and heat capacity. • Specific heat: • Heat capacity: 3. Contrast constant-volume and constant-pressure calorimetry. • Constant-volume: • Constant-pressure: 4. What is the specific heat of liquid water (in calories)

1. Temperature is an indirect measure of the thermal content of a system that looks at average kinetic energy of particles in a sample. Heat is the thermal energy transferred between objects as a result of differences in their temperatures. 2. Specific heat (c) is the energy required to raise the temperature of one gram of a substance by one degree Celsius. Heat capacity (mc) is the product of mass and specific heat and is the energy required to raise any given amount of a substance one degree Celsius. 3. A constant-pressure calorimeter (coffee cup calorimeter) is exposed to constant (atmospheric) pressure. As the reaction proceeds, the temperature of the contents is measured to determine the heat of the reaction. A constant-volume calorimeter (bomb calorimeter) is one in which heats of certain reactions (like combustion) can be measured indirectly by assessing temperature change in a water bath around the reaction vessel. 4. cH2O (l) = 1 cal /g·K

Concept check 5.1 1. Describe in words what is occurring in the following two-step mechanism: Step 1: A2B + A2B → A4B2 Step 2: A4B2 → 2A2 + B2 2. What does it mean for a step in a mechanism to be the rate-determining step? 3. What is activation energy? 4. How does the transition state theory compare with the collision theory of chemical kinetics? • Transition state theory: • Collision theory

1. Two molecules of A2B come together in a combination reaction to form an intermediate, A4B2, which subsequently decomposes to produce the final prod-ucts, two molecules of A2 and one molecule of B2. 2. The rate-determining step is the slowest step of a reaction. It determines the overall rate of the reaction because the reaction can only proceed as fast as the rate at which this step occurs. 3. The activation energy is the minimum energy needed for a chemical reaction to occur. 4. Both theories require a certain activation energy to be overcome in order for a reaction to occur (therefore not all reactions will occur). The transition state theory focuses on forming a high-energy activated complex that can then proceed forward or backward, forming the products or reverting to the reac-tants, respectively. The collision theory focuses on the energy and orientation of reactants, and considers each potential reaction to be "all-or-nothing" (either there is enough energy to form the products, or there is not)

4. Identify the triple point and critical point on the diagram below. What is the definition of the triple point? The critical point? • Triple point: • Critical point

4. The triple point is the specific combination of temperature and pressure at which all three phases are in equilibrium. The critical point is the temperature and pressure above which the liquid and gas phases are indistinguishable and the heat of vaporization is zero.