Chemistry Test #2 chapter 5 and 6
Mn+2HCl --> MnCl2 + H2 when 0.625 g Mn is combined with enough hydrochloric acid to make 100.0 mL of solution in a coffee-cup calorimeter, all of the Mn reacts, raising the temperature of the solution from 23.5 C to 28.8 C. find triangleH(rxn) for the reaction as written (assume that the specific heat capacity of the solution is 4.18 J/gC and the density is 1 g/mL)
-195 kJ
chapter 6: how to measure the change in enthalpy for many aq reactions?
-calorimetry at constant pressure! -for many aq reactions, can measure change in Hrxn by using coffee-cup calorimeter. q(reaction)=-q(solution) =-(mass(solution) x C(s, solution) x change in T) Change in H(reaction)=q(constant pressure) = q(reaction) To get change in Hreaction per mol, divide by the number of moles Summarizing calorimetry: Bomb calorimetry occurs at constant volume and measures change in internal energy (change in E) for reaction Coffee-cup calorimetry occurs at constant pressure and measures change in H for reaction
use standard enthalpies of formation to determine triangleHo(rxn) for the reaction: Fe2O3 + 3CO --> 2Fe + 3CO2
-24.8 kJ
when a 3.80 g sample of liquid octane (C8H18) is burned in a bomb calorimeter, the temperature of the calorimeter rises by 27.3 degrees C. the heat capacity of the calorimeter, measured in a separate experiment, is 6.18 kJ/C. determine the triangleE for octane combustion in units of kJ/mol octane
-5.07 x 10^3 kJ/mol
Chapter 5: what is graham's law of effusion? how will heavier gases diffuse?
-Graham's law of effusion: for two different gases at the same temperature, the ratio of their rates of effusion is given by the following equation: ratea/rateb = (square root)(molar mass B/molar mass A) -heavier gases will diffuse more slowly
Chapter 5: example of the molar volume of argon at differing pressures (low pressure? as pressure increases? high pressure?)
-at low pressures, molar volume of argon is nearly identical to that of an ideal gas. -as the pressure increases, the molar volume of argon becomes greater than that of an ideal gas -At higher pressures, the argon atoms themselves occupy a significant portion of the gas volume, making the actual volume > than that of an ideal gas (predicted by ideal gas law)
chapter 6: bonds in relation to exothermic and endothermic reactions? Which is exothermic? Melting ice cubes, formation of snow in clouds, conversion of frost to water vapor, separating ion pairs, splitting a gas molecule apart.
-breaking bonds always absorbs energy. -exothermic: weak bonds break and stronger bonds form (only takes small amount of energy to break weak bonds and greater amount given off when strong bonds form, resulting in net energy production) -endothermic: strong bonds break and weak ones form, nuclei and electrons reorganize into arrangement with higher potential energy, absorbing thermal energy in the process. -formation of snow in clouds!
Chapter 6: exothermic vs endothermic reactions? are chemical heat packs examples of exothermic or endothermic reactions? what about chemical cold packs? what does the value of the change of heat in a chemical reaction determine?
-endothermic: chemical reaction with +change in H, indicating that heat flows into the system as the reaction occurs. System absorbs heat from surroundings Chemical heat packs --> example exothermic -chemical cold packs → example of endothermic reaction. -exothermic: chemical reaction with -change in H, gives off heat to its surroundings. Summary: Value of change of H for chemical reaction=amount of heat absorbed or evolved in a reaction under conditions of constant pressure -endothermic reaction has +change in H and absorbs heat from reaction. Endothermic reaction feels cold to touch -exothermic reaction has -change in H and gives off heat to surroundings. Exothermic reaction feels warm to touch.
Chapter 6: what is energy? what is heat? KE? Thermal energy? what are the two different ways that an object can exchange energy with other objects?
-energy- anything that has the capacity to do work (work=force acting over distance). Energy=work=forcexdistance -heat: flow of energy caused by a difference in temperature -energy can be exchanged/transferred between objects through contact (ex: through collisions)... ex billiard balls colliding; one transfers energy to other, second ball absorbs energy. -heat and work are two different ways that an object can exchange energy with other objects (either into it or out of it) -energy: object/set of objects possesses. Heat and work=ways that objects exchange energy. Kinetic energy- energy of motion or energy that is being transferred. Energy associated with motion -transferred in collisions. Vibration in solids and molecular rotation and vibration Thermal energy- energy associated with temperature (thermal energy=random KE bc arises from motion of atoms/molecules within substance)
Chapter 6: elements --> formation of compound= + or - HoF? compound --> elements=+ or - HoF? Question: Find triangleH(rxn) for N2O + NO2 --> 3NO given the following: 1) 2NO+O2 --> 2NO2 changeH=-113.1 kJ 2) N2+O2 --> 2NO changeH=182.6 kJ 3) 2N2O--> 2N2+O2 changeH=-163.2 kJ
-formation of compound from its constituent elements in their standard states: elements → compound (change in H0f positive) -decomposition of compound into its constituent elements in their standard states: compound → elements (change in H0f negative) N2O--> N2+O2/2 changeH=-163.2 kJ/2=-81.60 NO2--> NO+O2/2 changeH=113.1/2=56.6 N2+O2-->2NO changeH=182.6 157.6 kJ!
Chapter 6: what is heat? when does heat exchange occur? what is temperature? how does heat flow?
-heat: exchange of thermal energy between a system and surroundings caused by temp difference. Heat exchange occurs when system and surroundings have a difference in temp. -temp=measure of thermal energy within sample of matter. heat=transfer of thermal energy -heat flows from matter with high temp to matter with low temp until both objects reach the same temperature (thermal equilibrium is reached)
Chapter 5: how would gases behave if gas molecules exerted no intermolecular forces? what is an ideal gas? when are gases most ideal?
-if gas molecules exerted no intermolecular forces, all gases would behave like a point particle. Each molecule would have its mass (m) and momentum (p=mv) and energy (E=.5mv2) where E=(3/2)kT -an "ideal gas" is assumed to exert no intermolecular forces (except when molecules collide) -at 1 atm pressure and room temp, most gases are almost ideal
Chapter 6: what is the change in system energy if the reactants have higher internal energy than the products? -what is the change in system energy if the reactants have lower internal energy than the products?
-if reactants have higher internal energy than products, change in system energy - and flows out of system into surroundings -if reactants have lower internal energy than products, change in system energy + and flows into the system from teh surroundings.
Chapter 5: what are intermolecular forces? what is the affect of weak attractions (which occur at lower temperatures?) how does this relate/compare to an ideal gas?
-intermolecular forces=attractions between atoms/molecules that compose any substance. Typically small, dont matter much in high or low temperatures. -at lower temperatures, collisions can occur with less KE, and weak attractions can affect collisions. Effect of weak attractions=decrease in # of collisions with surfaces of container and decrease in pressure compared with that of an ideal gas.
Chapter 5: what is diffusion and effusion? what are the rates of diffusion and effusion related to?
-process of a collection of (gas) molecules spreading out from high concentration to low concentration=diffusion. Process by which gas particles spread out in response to concentration gradient. -process by which a collection of molecules (gas) escapes from a container through a small hole into a vacuum=effusion -rates of diffusion and effusion of a gas both related to its rms average velocity -for gases at same temp, rate of gas movement inversely proportional to square root of molar mass
chapter 6: what is a standard enthalpy change? standard enthalpy of formation?
-standard enthalpy change, triangleHo: enthalpy change for a process when all reactants and products are in their standard states. Degree sign indicates standard states. -Standard enthalpy of formation, triangleHfo: enthalpy change for the reaction forming 1 mole of a pure compound from its constituent elements. -for pure compound: change in enthalpy when 1 mol of the compound forms from its constituent elements in their standard states. -the elements must be in their standard states and the triangleHfo for a pure elements in its standard state=0 kJ/mol
Chapter 6: what does the conservation of energy require? how does the energy transfer compare for surroundings and system?when energy flows into the surroundings, is the change in energy in the surroundings + or -? what about the change in energy in the system?
-sum of the energy changes in the system and the surroundings must be 0. Change in energy(universe)=change in energy(system)+change in energy(surroundings) -surroundings gain the exact amount of energy lost by the system -when energy flows out of a system, must all flow into surroundings. When energy flows out of a system, change in energy in system=negative. When energy flows into surroundings, change in energy in surroundings is positive. So, -(change in energy(system)=change in energy(surroundings) Conservation of energy- amount of energy gained/lost by system=amount of energy lost/gained by surroundings. -when energy flows into system, must come from surroundings/ change in energy of system=+ -when energy flows out of surroundings, change in energy of surroundings=-
which sample is most likely to undergo the smallest change in temperature upon the absorption of 100 kJ of heat? 15 g water, 50 g water, 15 g lead, 50 g lead
50 g water
write the formation reaction for CO(g)
-the formation reaction=reaction between elements in the compound (C and O)..C + O → CO(g) -the elements must be in their standard state: there are several forms of solid C, but the one with triangleHfo =0 is graphite. Oxygen's standard state is the diatomic gas: C(s, graphite)+O2(g) ⇒ CO2(g) -the equation must be balanced, but the coefficient of the product compound must be 1. Use whatever coefficient in front of the reactants is necessary to make the atoms on both sides = without changing the product coefficient. C(s, graphite) + ½ O2(g) → CO (g)
Chapter 6: why do change in H (change in enthalpy) and change in E tend to be similar in value? when is the difference between the change in H and change in E largest? smallest? difference between change in E and change in H? Change in H is negative... Change in H positive...
-usually change in H and change in E are similar in value, and both represent changes in a state function for the system; difference is largest for reactions that produce/use large quantities. Difference smallest with chemical reactions that do not exchange much work with surroundings. Change in E(reaction)=q(reaction) at constant volume -change in E=measure of all of the energy (heat and work) exchanged with the surroundings. Change in H=measure of only the heat exchanged under conditions of constant pressure. When change in H=negative, heat is being released by the system (exothermic reaction) When change in H=positive, heat is being absorbed by system (endothermic reaction)
Chapter 6: when 2 objects at different temperatures are placed in contact, how does heat flow? when thermal energy transfers heat from a metal to water, what does the exact temperature change depend on?
-when 2 objects at different temperatures are placed in contact, heat flows from the material at higher temp → material at lower temp. Heat flows until both materials reach the same final temperature. Amount of heat energy lost be hot material=amount of heat gained by cold material. -thermal energy transfers heat from metal → water. Exact temperature change depends on the following: mass of metal, mass of water, specific heat capacities of the metal and of water. -qmetal=-qwater m(metal) x C(s, metal) x change in T(metal) = -m(water) x C(s, water,) x change in T(water)
Chapter 6: when a system absorbs heat.... how is the increase in temperature related to the amount of heat absorbed? equation for the heat absorbed by a system? the larger the heat capacity of the object being studied, the smaller the... what is an extensive property?
-when a system absorbs heat, its temperature increases. The increase in temperature is directly proportional to the amount of heat absorbed. -heat absorbed by system and corresponding temperature change are proportional -proportionality constant is called the heat capacity, C (measure of system's ability to absorb thermal energy with undergoing a large change in temperature). units C=J/degree C or J/K. q=C x change in T. -heat capacity of system=quantity of heat required to change its temperature by 1 degree C -the larger the heat capacity of the object being studied, the smaller the temperature rise will be for a given amount of heat. -extensive property: depends on amount of matter being heated.
natural gas burns in air to form carbon dioxide and water, releasing heat: CH4+O2 --> CO2 + H2O triangleHo(rxn)=-802.3kJ what minimum mass of CH4 is required to heat 55 g of water by 25 degrees C?
0.115 g
a cylinder with a moving piston expands from an initial volume of 0.250 L against an external pressure of 2 atm. the expansion does 288 J of work on the surroundings. what is the final volume of hte cylinder?
1.67 L
hydrogen gas reacts with oxygen to form water: 2H2+O2 --> 2H2O triangleH=-483.5 kJ determine the minimum mass of hydrogen gas required to produce 226 kJ of heat
1.88 g
how much heat must be absorbed by a 15.0 g sample of water to raise its temperature from 25 degrees to 55 degrees (for water, Cs=4.18 J/gC)
1.88 kJ
a chemical system produces 155 kJ of heat and does 22 kJ of work. what is triangleE for the surroundings?
177 kJ
Chapter 5: how did van der waals modify the ideal gas equation? what is the effect of intermolecular attractions?
1873: johannes van der waals modified ideal gas equation to fit the behavior of real gases at high pressure -molar volume makes the real volume larger than the ideal gas law would predict -van der waals modified the ideal gas equation to account for the molecular volume -b=van der waals constant, different for every gas because their molecules are different sizes (V=nRT/P) → V=(nRT/P)+nb Effect of intermolecular attractions: -at high temp, pressure of the gases nearly identical to that of an ideal gas. At lower temperatures, the pressure of gases is less than that of an ideal gas. -at lower temps, gas atoms spend more time interacting with each other and less time colliding with walls, making actual pressure less than that predicted by ideal gas law Effect of intermolecular attractions: -van der waals modified ideal gas equation to account for intermolecular attractions -a=another constant, different for every gas because molecules have different strengths of attraction.
Chapter 6: what is the first law of thermodynamics? when energy is transferred between objects or converted from 1 form to another, the total amount of energy present at the beginning must be...
1st law of thermodynamics: law of conservation of energy: energy can neither be created nor destroyed, so you can never design a system that will continue to produce energy without some other source of energy. Energy can be transferred between objects, and energy can be transformed from 1 form into another. -when energy transferred between objects or converted from 1 form to another, total amount of energy present at beginning must be present at the end. E=mc2
a 12.5 gram sample of granite initially at 82 degrees C is immersed into 25 g of water initially at 22 degrees C. what is the final temperature of both substances when they reach thermal equilibrium? (for water, Cs=4.18 J/gC and for granite, Cs=0.790 J/gC)
27.2 C
chapter 6: what is the effect of the combustion of products on our environment? what is the problem with burning fossil fuels?
: because of additives and impurities in the fossil fuel, incomplete combustion, and side reactions, harmful materials are added to the atmosphere when fossil fuels are burned for energy. -therefore, fossil fuel emissions contribute to air pollution, acid rain, and global warming. Reactions for combustion of components of fossil fuels and associated enthalpies of reaction: Coal: C(s)+O2(g) → CO2(g) change in H0rxn=-393.5 kJ Natural gas: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) change in Horxn=-802.3 kJ Petroleum: C8H18(l) + 25/2 O2(g) → 8CO2(g) + 9H2O(g) change in Horxn=-5074.1 kJ -problem with burning fossil fuels: finite amount and products of combustion cause environmental problems (air pollution, acid rain, global climate change)
two substances A and B of = mass but at different temperatures come into thermal contact. the specific heat capacity of substance A is twice the specific heat capacity of substance C. which statement is true of the temperature of the two substances when they reach thermal equilibrium? a) the final temp of both substances is closer to the initial temp of substance A than that of substance B b) the final temp of both substances is closer to the initial temp of substance B than that of substance A c) the final temp of both substances is exactly midway between the initial temperatures of substance A and substance B d) the final temperature of substance B is greater than the final temperature of substance A
A
chapter 6: air pollution, global warming, and renewable energy
Air pollution: sulfur oxides created, leads to acid rain and affect respiratory system. Carbon monoxide created, displaces O2 in blood and forces heart and lung to work harder, nitrogen oxides eye/lung irritants and precursors of acid rain, ozone pollutant and irritant. Global warming: CO2=greenhouse gas. Allows light from the sun to reach earth, but does not allow the heat/infrared light reflected off earth to escape into outer space. Acts like a "blanket" -CO2 levels in the atmosphere have been steadily increasing. -Some models predict that the result will be more severe storms, more floods and droughts, shifts in agricultural zones, rising sea levels, and changes in habitats. Renewable energy: New technologies are being developed to capture the energy of sunlight (parabolic troughs, solar power towers, dish engines concentration the sun's light to produce electricity.) -solar energy used to decompose water into H2(g) and O2(g); the H2 can then be used by fuel cells to generate electricity. -other examples of renewable energy: hydroelectric power and wind power
Chapter 6: whats a bomb calorimeter? what is the heat capacity of the calorimeter? if no heat escapes from the calorimeter, then the amount of heat gained by the calorimeter=
Bomb calorimeter: used to measure the change in internal E because it is a constant volume system -heat capacity of the calorimeter is the amount of heat absorbed by the calorimeter for each degree rise in temperature... called calorimeter constant=C(cal), (units kJ/degrees C). Reaction occurs in sealed container known as a bomb, which ensures that the reaction occurs at constant volume. -if no heat escapes from calorimeter, amount of heat gained by calorimeter=that released by reaction. So qcal=-qrxn
Chapter 6: what is the change in the energy of a reaction? enthalpy? How does H (enthalpy) relate to the energy, pressure, and volume of a system? GO BACK OVER THIS! CONFUSING!
Change in Erxn=total energy change (both heat and work) that occurs during the reaction. Enthalpy(H): enthalpy of a system=sum of the internal energy of the system and the product of pressure and volume. -H is a state function: H=E+PV. Change in H=change in E+change in PV -for ideal gas, change in PV=RT. Therefore at constant T for ideal gas, change in PV=RT(change in n) → change in n=change in number of moles from reactants to products. -enthalpy change, change in H, of a reaction is the heat evolved in a reaction at constant pressure: change in Hreaction=qreaction at constant pressure
Chapter 6: electrical energy? translational energy? heat or thermal energy? potential energy? chemical energy? light/radiant energy? nuclear energy?
Electrical energy=Ke associated with the flow of electrical charge Translational energy= KE association with traveling molecules or objects Heat or thermal energy= KE associated with molecular motion in materials Potential energy= energy associated with position or composition of object. -energy stored in an object, or energy associated with the composition and position of the object. Ex: compressed spring Chemical energy= energy associated with relative positions of e-s and nuclei in atoms and molecules. PE in the arrangement of atoms in molecules or in the locations of electrons in atoms Light/radiant energy- electromagnetic energy associated with particles called photons Nuclear energy- PE in the nucleus of atoms
Chapter 5: what is the mean free path? what happens to the mean free path as pressure increases?
Mean free path: average distance that a molecule travels between collisions -molecules in a gas travel in straight lines until they collide with another molecule or the container. The average distance a molecule travels between collisions=mean free path. Mean free path decreases as pressure increases
Chapter 6: how does the total amount of internal energy of one mol of C and 1 mol of O2 differ from the amount of internal energy of 1 mol CO2 at same temp and pressure? -example of a reaction where energy absorbed from surroundings into reaction what is an energy exchange? how is energy exchanged between system and surroundings?
Energy flow in a chemical reaction: -total amount of internal energy in 1 mol of C(s) and 1 mol O2(g) > internal energy in 1 mol CO2(g) at the same temperature and pressure. -in C(s)+O2(g) → CO2(g), will be a net release of energy into the surroundings. Change in energy of reaction=change in energy of surroundings -in reaction CO2(g) → C(s) + O2(g), there will be an absorption of energy from the surroundings into the reaction. Change in energy of reaction= -(change in energy of surroundings). -energy exchanged between system and surroundings through heat (heat/thermal energy, w) and work (w). Q and w are NOT state functions, and their values depend on the process (the path... think back to mountain example) Change in internal energy=q+w
chapter 6: what is the enthalpy of a reaction? it is an intensive or extensive property? -the more reactants you use, the larger... example of how the change in Hrxn reflects the stoichiometric amounts of reactants and products for a reaction?
Enthalpy of reaction (change in Hrxn): the enthalpy change in a chemical reaction is an extensive property. The more reactants you use, the larger the enthalpy change. By convention, we calculate the enthalpy change for the number of moles of reactants in the reaction as written. - amount of heat generated/absorbed when chemical reaction occurs depends on amounts of reactants that actually react. -magnitude change in Hrxn reflects the stoichiometric amounts of reactants and products for the reaction as written: C3H8(g) + 5O2 (g) → 3CO2 + 4 H2O (g) change in Hrxn=-2044 kJ 1 mol C3H8(g)=-2044 kJ or 5 mol O2(g)=-2044 kJ With this, 2044kJ of heat evolves when 1 mol of C3H8 and 5 mol O2 completely react
Chapter 6: example of energy exchange between two balls colliding on a smooth table? -what does the change in internal energy depend on? what does heat and work depend on? kinetic energy of ball #1: 5.0 J heat lost: 0.5 J KE at collision: 4.5 J
Example: on a smooth table, most of Ke transferred from white ball to purple ball, with small amount lost through friction. (amount of work done on second ball depends on quality of billiard table. On smooth table, amount of energy lost to friction small. On rough table, ball loses much of initial KE as heat.) -change in internal energy only depends on ball's initial and final KE. heat and work depend on details of the ball's journey.) -on smooth table, w greater in magnitude than q. On rough table, q greater than w. But sum is always constant. -energy change for white ball → change in energy=KEfinal-KEinitial → 0J-5.0J=-5.0J -KE transferred to purple ball w=-4.5 J -KE lost as heat is q=-0.5 J. q+w=(-0.5 J) + (-4.5 J) = -5.0J=change in energy
Chapter 6: what are the equations for exchanging energy between system and surroundings?
Exchanging energy between system and surroundings: -exchange of heat energy: q=mass x specific heat x change in temp -exchange of work: w= -pressure x change in volume. -easiest way to obtain value of change in internal energy for chemical reaction is to force all energy change associated with a reaction to manifest itself as heat rather than work. equations: Heat(q)=m x Cs x change in T Work (w)=-P x change in V
Chapter 6: what factors affect the heat capacity?
Factors affecting heat capacity: heat capacity of an object depends on its amount of matter. Usually measured by its mass -200 g water requires twice as much heat to raise its temp by 1 degree C as does 100 g of water. -heat capacity of an object depends on the type of material: 1000 J of heat energy will raise the temp of 100 g of sand 12 degrees C, but only raise the temperature of 100 g of water by 2.4 degrees C
Chapter 5: Under what conditions would Cl2 be at least ideal?
High pressure, low temp this is for all elements/compounds! a real gas will be least ideal when at high pressure and low temperature
Chapter 6: chemical reaction relationships For Hrxn what is Hess's law?
If a chemical equation is multiplied by some factor, then change in Hrxn is also multiplied by the same factor: A+2B → C change in H1 2A + 4B → 2C change in H2=2 x change in H1 2) If a chemical equation is reversed, then change in Hrxn changes sign A + 2B → C change in H1 C → A + 2B change in H2=-(change in H1) 3) If a chemical equation can be expressed as the sum of a series of steps, then change in Hrxn for the overall equation is the sum of the heats of a reaction for each step: Hess's law! Relationships involving change in Hrxn Hess's law: the change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps. -if a reaction can be expressed as a series of steps, then the change in Hrxn for the overall reaction is the sum of the heats of reaction for each step. A + 2B → C (triangle H1) C → 2D (triangle H2) A + 2B → 2D (triangle H3=triangle H1+triangle H2) C's cancel! This is a direct consequence of change in Hrxn being a state function
a sample of Xe takes 75 seconds to effuse out of a container. an unknown gas takes 37 seconds to effuse out of the identical container under identical conditions. what is the most likely identity of the unknown gas? a) He b) O2 c)Br2 d) Kr
O2
Chapter 6: what is pressure-volume work? when gases expand, is the change in volume positive or negative? understand this equation!
Pressure- volume work: change in E=q+w. Occurs when the force is caused by a volume change against an external pressure. Pv work = work caused by a volume change against an external pressure. -When gases expand, change in V=+, but the system is doing work on the surroundings, so work gas=negative -as long as the external pressure is kept constant, Work(gas)=F x d=external pressure x change in volume(gas) → w=-P x change in V -to convert the units of J use 101.3 J=1 atm x L
Chapter 6: what is the specific heat capacity? molar heat capacity? are specific heat capacity and molar heat capacity extensive or intensive properties?
Specific heat capacity: measure of a substance's intrinsic ability to absorb heat. -the amount of heat energy required to raise the temperature of 1 gram of a substance 1 degree C. Cs, units J/(g x degrees C) -molar heat capacity=the amount of heat energy required to raise the temperature of 1 mole of a substance 1 degree C → units J/mol degrees C -total heat capacity of an object C(J/degrees C)= m Cs -specific heat capacity and molar heat capacity are intensive properties, so depend on kind of substance being heated (not amount)
Chapter 5: what does a plot of PV/RT versus P for 1 mole of a gas show when comparing real and ideal gases?
Real gases: -Reveals a curve that shows the PV/RT ratio for a real gas is generally lower than ideal for "lower pressures"- the most important factor is intermolecular attractions -reveals a curve that shows the PV/RT ratio for a real gas is generally higher than ideal for "high" pressures- the most important factor is molecular volume
Chapter 5: real gases? what are the long range forces? what are the short range forces?
Real gases: all molecules exert electrical forces on other molecules because they contain + nuclei and - electrons -at long range (2-3 molecular diameters,) forces always attractive -at short range, forces always become repulsive because electron-electron repulsion dominates
Chapter 6: what is thermochemistry? how do hand warmers connect to thermochemistry? what factors relate to the amount of temperature change in your hands? what is a calorimeter? why was the conservation of energy finally realized?
Thermochemistry: the study of the relationships between chemistry and energy Conservation of energy is a fundamental law that wasn't evident until it was realized that heat is energy. Calorimeter- tool that measures the heat flow in a combustion reaction. Chemical hand warmers: most work by using the heat released from the slow oxidation of iron: 4 Fe(s) + 3O2(g) → 2Fe2O3(s) -the amount your hand's temperature rises depends on factors: size of hand warmer, size of glove etc. Mainly, the amount of heat released by the reaction. Size of temp increase proportional to amount of heat released by a reaction
Chapter 6: KE formula? how much is 1 J of energy? 1 calorie? kcal?
Units of energy: amount of KE an object has directly proportional to its mass and velocity. KE=.5mv2 When the mass is in kg and velocity is in m/s, the unit for KE is (kg x m2)/s2 1 J of energy=amount of energy needed to move a 1 kg mass at a speed of 1 m/s → J=(kg x m2)/s2 watt=J/s J: amount of energy needed to move a 1 kg mass a distance of 1 meter. Calorie: amount of energy needed to raise the temperature of 1 g of water 1 degree C. -kcal=energy needed to raise 1000 g water 1 degree C -food calories=kcals 1 calorie=4.184 J, 1 calorie or kcal → 1000 cal=4184 J, 1 kWh=3.6x106J
Chapter 5: what is van der waals equation?
Van der Waals's equation: used for real gases! Combining equations to account for molecular volume and intermolecular attractions we get the equation: [P+a(n/v)2]x(V-nb)=nRT
Chapter 5: when can attractive intermolecular forces be ignored? when does the volume of gas particles begin to become important? how do real gases behave?
When can attractive intermolecular forces between gas molecules be ignored? At 1 atm, the distance between molecules is roughly 3-5 times greater than the diameter of the molecule. -same # molecules in each flask, P1<P2 Effect of the Finite Volume of gas particles: becomes imp at higher pressures because the V of the particles themselves occupies a significant portion of the total gas volume. -Real gas behavior: because real molecules take up space, the molar volume of a real gas is larger than predicted by the ideal gas law at high pressures
Chapter 6: how do you work to measure the change in internal energy in a system? what is calorimetry? what does the magnitude in temperature change in the surroundings depend on?
calorimetry- measure the thermal energy exchanged between the reaction (defined by system) and the surroundings by observing the temperature of the surroundings. Magnitude of temperature change in surroundings depends on magnitude of change in temp for reaction and on the heat capacity of the surroundings. -because change in E=q+w, we can determine change in E by measuring q and w -easiest to use a process where there is no change in volume so w=0 -at constant volume, change in E(system)=q(system) -in practice, cannot observe the temperature changes of the individual chemicals involved in a reaction, so we measure the temp change in the surroundings (using insulated and controlled surroundings) → q(system)=-q(surroundings) -the surrounding area=bomb calorimeter. Usually made of a sealed, insulated container filled w water. q(surroundings)=q(calorimeter)=-q(system)
Chapter 6: what is an example of a state function?
change in altitude depends only on the difference between the initial and final values, not the path taken (ex mountain). State specified by parameters like temp, pressure, concentration, and physical state. -ex: when going up a mountain, regardless of the trail you choose, when you reach the top you will be 10,000 ft above the base. -distance from base → peak=state function. Depends only on difference in elevation between base and peak, not how you arrived there. PE=state function
the standard enthalpy of formation for glucose=-1273.3 kJ/mol. what is the correct formation equation corresponding to this triangleHof? a) 6C(s,graphite)+6H2O(g) --> C6H12O6(s, glucose) b) 6C(s, graphite)+6H2O(l) --> C6H12O6(s, glucose) c) 6C(s, graphite)+6H2(l) +3O2(l) --> C6H12O6(s, glucose) d) 6c(s, graphite) +6H2(g)+3O2(g) --> C6H12O6(s, glucose)
d
which process is endothermic? a) evaporation of water from the skin b) burning of candle wax c) oxidation of iron in chemical hand warmer d) combustion of natural gas in stove
evaporation of water from skin
chapter 6: molecular view of endothermic reactions vs molecular view of exothermic reactions?
exothermic reaction: surrounding temperature rises due to a release of thermal energy by the reaction. Extra thermal energy comes from the conversion of some of the chemical potential energy in the reactants into KE in the form of heat. During the course of a reaction, existing bonds are broken and new bonds are made. The products of the reaction have less chemical potential energy than the reactants. So, the difference in energy is released as heat. Endothermic reaction: surrounding temperature drops due to absorption of some of its thermal energy by the reaction. During the course of a reaction, existing bonds broken and new bonds made. -products of reaction have more chemical potential energy than the reactants. To acquire this extra energy, some of the thermal energy of the surroundings is converted into chemical potential energy stored in the products.
Chapter 6: What are formation reactions? standard conditions? standard states? what is the standard state for a gas? for a liquid/solid? in a solution?
formation reactions: reactions of elements in their standard state to form 1 mole of a pure compound. -coeff of reactants may be fractions! Standard conditions: standard state, standard enthalpy change, and standard enthalpy of formation Standard state: the state of a material at a defined set of conditions: -for a gas: standard state for a gas is the pure gas at a pressure of exactly 1 atm -liquid/solid: standard state for liquid/solid is the pure substance in its most stable form at a pressure of 1 atm and at the temp of interest (often taken to be 25 degrees C) -substance in solution: standard state for a substance in solution is a concentration of exactly 1 M
Chapter 6: a block of CU of unknown mass has Ti=65.4 degrees C is immersed in 95.7g of water at ti=22.7 degrees. when equilibrium is reached Tf=24.2 degrees. what is the mass of Cu?
q(metal)=-q(water) m(metal)xC(s,metal)xchange in T(metal)=-m(water)xC(s,water)xchange in T(water) and then, just plug in your values!
Chapter 6: what is the heat capacity of an object proportional to? Question: consider the following specific heats of metals: copper=0.385, cobalt=0.418, chromium=0.447, gold=0.129, silver=0.237 100-g samples of each of the metals at 95 degrees C are added to water held at a constant temperature of 25 degrees C. Assuming these specific heats are constant, which element releases the most heat when cooled from 95 degrees to 25 degrees?
the heat capacity of an object is proportional to the following: its mass and the specific heat of the material. So, we can calculate the quantity of heat absorbed by an object if we know the mass, the specific heat, and the temperature change of the object.
Chapter 6: energy conservation. what is internal energy? what does the change in internal energy of a system depend on? what is a state function?
total energy of the universe is constant (with energy, can't get something from nothing.) Internal energy=sum of KE and PE of all the particles that compose the system. State function. -change in internal energy of system only depends on amount of energy in the system at the beginning and end. -state function- mathematical function whose result only depends on the initial and final conditions, not on the process used to get from the initial to the final state. Its value depends only on the state of the system, not how it arrived at that state. -change in energy=energy final-energy initial -change in energy=energy of products-energy of reactants
consider the reactions: A -> 2B triangleH1 A -> 3C triangleH2 what is triangleH for the reaction 2B -> 3C?
triangleH2-triangleH1