Chemistry Unit 10
what is the specific heat capacity of a substance that realeased 150 J of heat as 10g of it goes from 38 c to 21 c?
-150=10(-17)x .88J/g*c
how many moles of product are made when 985kJ of heat is released? 4Fe (s) +3O2 (g) -->2Fe2O3 (s) +1625kJ
-1625 kJ:1 mol rxn -985kJ: x mol rxn (.606) ---- 1 mol rxn:2 mol .606 :x (1.21 mol Fe2O3)
endothermic
-absorbs energy -"entering" -energy change is positive
specific heat capacity
-different for every substance because it is a set value for how much what is needed to change (raise or lower) the temp of 1 g of he substance by 1 c -the Cp of water is 4.19 J/gc -when dealing with a solution, us the Cp of water as the specific heat capacity
What happens during a phase change?
-energy is gained or lost -temp remains constant because all the ehat is used to change the state of the substance
what is the difference between phase change questions and reaction questions?
-equation vs phase change -both asking for heat
why is one side of the equation negative?
-heat is a flow of energy -as one substance gains energy another loses energy -one is positive and the other is negative
expand on: Did the water act endothermically or exothermically? what is a dissolving process?
-heat went into the dissolving process -system is absorbing=endothermic solute is system
Did the conversion of solid ammonium chloride to aqueous ammonium chloride directly involve a change in kinetic energy OR potential energy? -During the dissolving of ammonium chloride, is the sign of ΔHwater "+" or "-"?
-potential -ionic bonds broken and it takes energy to break bonds (endothermic reaction=change in energy/flat line) -breaking forming atractive forces involes changes in PE -
exothermic
-releases energy "exiting" -energy change is negative
what can be said about masses and the time it takes to change their temp? -smaller mass
-smallest mass=fastest heat up +cools off the fastest -largest mass=slowest to heat up, but remains hot for long time
During the dissolution (dissolving) of sodium hydroxide the temperature of the surrounding water increased -Did the surrounding water absorb or release heat? -Did that affect the water's overall amount of kinetic energy OR potential energy? -Did the surrounding water act endothermically or exothermically?
-surrounding water ABSORBED heat -affected the waters KINETIC energy -Water acted endothermically (bc absorbed)
why did the metal's temp change so drastically while the water temp changed so little?
-the specific heat capacity of the metal was lower than the water -lower heat capacity=higher change in temp
since heat flower you need to identify the "target" substance -the target is the ________ and everything else is the _______
-the target is the system and everything else is the surroundings
when you place a heated rod (from a boiling pot of water) in cool water what is the starting temp?
-the temp of the boiling water=the metal temp
During the dissolution (dissolving) of ammonium chloride, the temperature of the surrounding water decreased -Did the surrounding water absorb or release heat? -Which changed for the water: kinetic energy OR potential energy? _________________________ Explain. -Did the water act endothermically or exothermically? -Based on your answer, was the dissolving process endothermic or exothermic?
-water RELEASED it -Kinetic energy changed for the water -change of temp not state -water acted EXOTHERMICALLY -dissolving process was ENDOTHERMIC
the enthalpy amount is?
1 mol rxn
use the following thermochemical equation to calculate the mass of oxygen needed to produce 1500kg of heat? 4Fe +3O2-->2Fe2o3 ^H-1625kJ
1 mol rxn: -1625kJ x mol rxn: -1500 x=.92 mol rxn 3 : 1 mol rxn x: .92 x=2.77 mol of oxygen =88.62g
how much ehat is released when 125g of octane is burned? (-5471kJ) formula: ^H=mol(^Hcombustion)
1 mol=-5471kJ ^H=1.094(-5471) ^H=-5987.7kJ
How many joules of heat must be added to 1 gram of water to raise its temperature from 25°C to 26°C?
4.18 J
there are two ways to right the thermochemical equation: 4Fe (s) +3O2 (g) -->2Fe2O3 (s) ^H=-1626 what is the other way?
4Fe (s) +3O2 (g) -->2Fe2O3 (s) +1625kJ
how much eat will be released when .25 moles of iron reacts completely: 4Fe (s) +3O2 (g) -->2Fe2O3 (s) +1625kJ
4Fe (s) +3O2 (g) -->2Fe2O3 (s) +1625kJ 4 mol Fe:1 mol rxn 1 mol Fe: x (.25) ----- 1 mol Fe: x (.25) .25 mol Fe: x (.0625 mol rxn) ----- 1 mol rxn = -1625 k J .0625=x (-101.56 kJ)
P (g) +3Cl (g) ----> PCl3 (g) changein energy= -966.7 what does that statement mean?
996.7 kj was released when the bonds were formed
200 J of heat is added to Beaker A; 1000 J of heat is added to Beaker B. What is true of their relative temperature changes? (same amount of water)
Beaker B will have a larger temperature change because the masses are the same and they absorbed different amounts of heat
Beaker A contains water (Cp = 4.184 J/g•°C) Beaker B contains isopropanol (Cp = 2.6 J/g •°C) If 1000 J of energy are added to both beakers, what is true of their relative temperature changes? -same amount of water
Both beakers will have an increase in temperature, but Beaker B will show a greater temperature change because it takes less energy (per gram) to raise the temperature (per degree).
Assume that the same amount of heat is added to both beakers, what will be true of their relative temperature changes? -Beaker A has a smaller volume than beaker be
Both will have an increase in temperature, but the temperature in Beaker A will change more because there is less water (fewer water molecules) to absorb the heat.
ways to find the heat
DURING REACTION -mol to mol ratio (reacting) -formula (state change) non reaction: q=mcp^T
what is the specific heat capactiy
It is an intensive physical property (meaning it does not depend on changing amounts) that has a different characteristic value for every substance
Mass and the amount of heat absorbed have a/an _________________ relationship.
Mass and the amount of heat absorbed have a/an direct relationship.
a potof boiling water is _____ energy (heat)
absorbing
temperature and heat...
are NOT the same (one is energy the other is measurement)
bond breaking= bond forming=
bond breaking=endothermic bond forming=exothermic
when a chemical reaction occurs..... when products form...
bonds in the reactant moleciles use (absorb) energy to break -new bonds are made and energy is released
example prblm: how much heat will be released when .25 moles of iron reacts completely? 4Fe +3O2-->2Fe2o3 ^H=-1625 k
compare the iron to the reaction: 4:1 mol rxn=.25 mol:x 4x=.25 x=.0625 mol rxn 1 mol rxn: -1625 =.0625: x x=-1625(.0625) x=-101.56kJ
the overall energy change of a reaction is calculated by...
comparing the bonds broken with the bonds formed
Compare the amount of the energy value from 19(a) with that of Experiment 2, Trial D in Model 2. What accounts for the difference in the amount (not the sign)?
difference of mass
change in temp and energy relationship
direct (when mass is held constant)
energy and mass relationship?
direct (when temp is constant)
determine ^Hrxn for the reaction shown: CH4 +H2O -->CO +3H2
draw out lewis structure -write table for (bond, #, KJ) for both sides then mult the # of bonds with the Kj find a total -the reactant side=pos, product side=neg add together + +(-) ^Hrxn=220 kJ
what are the endothermic phase changes vs the exothermic phase changes?
endo: melting, vaporization, sublimation (s-g) exo: freezing, condensation, deposition (g-s)
8. The reaction between sodium bicarbonate and vinegar was ( endothermic / exothermic ). Explain.
endothermic -the vinegar absorbed the heat -water temp decreased, the water lost heat, heat went into the reaction
Heat is a form of_____
energy
what does specific heat mean?
energy required to change the temperature of a substance heat capacity
what is another name for heat?
enthalpy (H)
^H means
enthalpy change
what does the negative mean when it comes to the ^H -what would happen if it was positive
exothermic reaction (heat is given off as a product) -it would be on the left side of the equation
Based on your answer was the dissolving process endothermic or exothermic? Did the conversion of solid sodium hydroxide to aqueous sodium hydroxide) directly involve a change in kinetic energy OR potential energy?
exothermically, kinetic energy bc the temp increased
how do you find the change in temp?
final temp - initial temp
heat=
flow of energy from a warmer object to a coller object
Based on what happened to the water, did the chemical reaction between the sodium bicarbonate and the vinegar gain or lose energy? _________________________________ Explain. (water got colder)
gain -when water reacted with sodium bicarbonate it got colder, energy is decreasing -know that the water lost heat BC temp went down (where did the heat go?) only thing present is the reaction so heat went to reaction
what does it mean if the entahlpy change is on the right side?
heat is being given off as a product -exothermic
or each container, include whether the temperature is expected to increase, decrease, or remain the same after heating. _____________________________________________________________________ Explain your reasoning based on kinetic energy and particle movement
increase more particle movement causes temp to increase so smaller volume will gain more energy -the smaller pot gained more because it had less volume -smaller volume more collisions
mass and change in temp relationship
inverse (when energy is held constant)
what is the SI unit of heat?
joules (q)
since the temp of the water changed, there was a change in (potential/kinetic) energy
kinetic
A 23.8g piece of unknown metal is heated to 100c and is placed in 50g of water at 24c water. If the final temp of the water is 32.5, what is the heat capacity of the metal?
mCP(x-Ta)=mCP(x-Ta) (23.8)(x)(32.5-100)=-(50)(4.18)(32.5-24) x=1.107J/gc
temperature is used to:
measure heat and heat changes
what does the enthaly amount, amount to?
one mole of reaction
a 10g of aluminum (Cp is .9) at 95c is placed in a container of 100g of water at 25c. Whats the final temp?
(10)(.9)(x-95)=-(100)(4.18)(x-25) x=26.47 c (it does NOT matter which side the negative goes on)
if the specific heat capacity of aluminum is .9 J/gc what is the final temp when 437 J is added to 30 g sample at 15c?
(437)=30(.9)(x-15) 437=27(x-15) 437=27x-405 842=27x x=31.15 c
what type of enthalpy? -chocolate (system) melting in my hand (surroundings) -water (surroundings) gets warm as calcium chloride (system) dissolves -aqueous hydrogen peroxide (system) releases heat to the water (surroundings) that makes up the solution
+^H -^H -^H
hint to when do you use the equal equation?
WHENEVER PLACED IN WATER
^H vaporization vs. ^H fusion
^H vaporization -change between liquid and vapor -(liquid to vapor (+), vapor to liquid (-)) ^H fusion -change between solid and liquid -(solid to liquid= pos, liquid to solid=neg)
When the same amount of energy is added to water samples of different mass, the change in temperature gets ( smaller / larger ) as the mass of the water increases.
smaller
stoichiometry with heat uses a balanced equation that has the entahlpy change for the reaction (__________ )
stoichiometry with heat uses a balanced equation that has the entahlpy change for the reaction (thermochemical equation)
a 10g of aluminum (Cp is .9 J/gc) at 95 c is placed in a container of 100g of water at 25 c. What is the final temp?
system: 10g, .9 CP, 95 ta surroundings: 100g, 4.18 CP, 25 ta -mcp(a-ta)=mcp(a-ta) 10(.9)(x-95)=-100(4.18)(x-25) x=26.47 d celcius
what are the two ways to write a thermochemical reaction?
the ^H =-1625 off to the side +1625 in the equation
complete the following to describe the relationship between the observed temperature change and the energy added. When the same mass of water is being heated, __________________________________________________
the increase of the enrgy added also causes an increase in change in temp
Complete the following to describe the relationship between the amount of energy added and the mass of water. To achieve the same change in temperature, ____________________________________________________
the mass has to increase as the energy is added
How much heat is released when 25g of methanol freezes? how do you solve this problem?
turn into moles because of kj/mol use fusion bc liquid to solid (loosing heat=exothermic=negative)
how much energy is needed to melt 30g of ice at 0c and then increase the temp of the liquid water to 75c?
two equations then add to find total energy ^T=(334)(30)+ ^T=(4.18)(30)(75) 19,434J
how much heat is released when 125g of octane is burned? what do you do in this scenario?
use combistion reaction formula
calorimetry
uses the heat change measured in the surroundings to find the energy change of the system -heat is a flow of energy, as one substance gains energy another loses energy -one is positive and other other negative but the AMOUNT is the same
the ending temp of the calorimeter of the water and of the metal
when the temp of the water in the cup stops going up this is the ending temp of the water
endothermic enthalpy
(+) -heat is absorbed from the surroundings and enters the system
exothermic enthalpy
(-) -heat is released from the system and enters the surroundings
how many moles of product are made when 985kJ of heat is released? 4Fe +3O2-->2Fe2o3 + 1625 k
1625: 1 mol rxn = 985: x x=.606 mol rxn (.606 applies to all of the molar amts for the equation) 1mol rxn : 2mol .606: x x=1.21 mol Fe2O3
How much heat is evolved when 1255 g of water con- denses to a liquid at 100°C? Hcond 40.7 kJ/mol
2834.54 kJ pos bc heat INVOLVED
How much heat is liberated by the combustion of 206 g of hydrogen? Hcomb 286 kJ/mol
29,200kJ bc its asking for number not the equation
How much heat is absorbed by a 2000 kg granite boul- der as energy from the sun causes its temperature to change from 10°C to 29°C?
30,000 kJ -if mass is given in KILO leave it UNLESS its a compound problem that uses regular grams
enthalpy amount is always...
ONE mole of reaction
what is the diff between: how many moles of product are made when 985kJ of heat is released? How much heat is released when 25g of methanol freezes?
PHASE CHANGE -no change=stochionomety -change=specific formula
what can you infer about the final temp when comparing substances in the same container?
SAME final temp
Enough heat is added to each beaker to increase the temperature of the water inside by 10°C. Did each beaker of water absorb the same amount of heat or did one absorb more heat? If so, which absorbed more?
Since Beaker B has more water (greater mass), it has to absorb more heat to show the same temperature change of 10o C
key word for having to do mole to mole ratios
THERMOCHEMICAL EQUATION
Temperature and mass have a/an _____________ relationship bc the larger the mass the smaller/larger the temperature change.
Temperature and mass have a/an inverse relationship bc the larger the mass the smaller the temperature change.
Enough heat is added to each beaker to increase the temperature of the water inside by 10°C. Has the PE or the KE of the water changed?
The KE of the water changed since temperature and kinetic energy are directly proportional.
how do you know which side the enthalpy change goes on?
positive left negative=right
expand on: What is the sign of ΔHdissolving process?
positive: the water itself has a change of energy, water released energy and got cold, dissolving procces (heat went into the solute and water got colder)
otential energy can be viewed as "stored energy." b. Did the reaction between sodium bicarbonate and vinegar (reactants) create a product with more stored energy (increased potential energy) or did it create a product with less of stored energy (decreased potential energy)? Explain.
products had more potential energy than reactants
what happens to an equation when it looses j's of heat?
put a negative in-front of q
which variables can be positive/negative -q, m, Cp, ^T
q, ^T=both Cp, m=positive ONLY
define the variables: -q, Cp, m, ^T
q=heat/energy Cp=specific heat copacity (Joules/gc) m=mass ^T=change in temp
how much heat must be removed from 25g of water at 75 c to drop the temp to 30c? -how do you check your answer simply?
q=mCp^T x=26(4.18)(-45) q=-4702.5 J -negative in the end (REMOVED HEAT) =same thing
the container of a reaction between acids and bases becomes hot because they are both ______
releasing energy (heat)
Did the liquid in one container gain more energy than the other or did both gain the same amount? (heated for 5 minutest at a rate of 30 J per min)
same amount (don't matter the size they both got that much energy)
when diagram phase changes: (heat is on bottom, temp is y) -what do the slanted lines mean? -flat lines mean -moving to the right vs left (endo vs exo?)
slanted lines mean: increase in heat and temp (designated state) -kinetic energy (bc temp is increasing) flat lines: increase in heat (for phase change) but not temp -potential energy bc no temp change right: endo, left: exo
When adding the same amount of energy to two similar massed samples, the substance with the larger specific heat capacity will have a ( larger / smaller ) temperature change?
smaller
