Chemistry - Thermochemistry Notes
Specific Heat Capacity (__):
(c): The amount of heat required to raise the temperature of 1.0g of a substance 1.0 oC.
What is a thermochemical equation?
A chemical reaction with the heat change given
___________ is a process used to calculate changes in heat. It measures _______________ accurately.
Calorimetry; changes in temperature
Surrounding:
Everything else in the universe.
Define exothermic and endothermic. Which feels hot? Which feels cold? Explain the sensations in terms of the system and surroundings (which one are you?).
Exothermic = heat is lost by the system to the surroundings. This would feel hot to touch because we would be a part of the surroundings and the heat lost by the system would be absorbed by us.
How are joules and calories related.
First, the calorie you are used to using, the food calorie, is actually a kilocalorie or 1000 calories. 4.18J = 1 calorie 4108J = 1 food calorie.
Which way did heat move?
From you, the warmer substance, to the stands, the colder substance. Heat always moves from hot to cold. Never the reverse. At least, not on it's own.
Enthalpy: Variable - Unit -
H J/mol
______ will always be exchanged between any two objects that are at different temperatures until the objects are the same temperature.
Heat
Explain the difference between heat and temperature.
Heat is the total kinetic energy in a system whereas the temperature is the average kinetic energy in a system
Suppose you have three particles each with an energy, or heat, of 5. Don't worry about units here as they are not necessary to make the point. -Now ask the questions: What is the heat of the particles? What is the temperature? -What if we double the number of particles but keep the energy of each the same? What is the heat now? What is the temperature now?
Heat it the total - so the heat is 15 Temperature is the average - so the temp is 5 Now it is 30 It is still 5.
Heat is a noun; it is some thing. Cold is an adjective, it describes the absence to heat. Why is this important? Can a description move? Can redness move? Can tallness move? (Hardin wishes). Can any description move?
Nope, but heat can. Heat is a thing. Things move.
Why did the stands stop feeling cold?
Remember that heat flows when there is a difference in temperature. If the objects in contact are the same temperature heat stops flowing. You are no longer losing heat to the stands so they no longer feel cold.
Temperature:
The average kinetic energy; It uses the variable T and has units of oC or K
In calorimetry, what do we know to be true about the heat released by the system and the heat absorbed by the surroundings?
The heat lost must equal the heat gained.
System:
The part of the universe that we are focused on.
calorimetry
The precise measurement of heat flow out of a system for chemical and physical processes
On a sunny day, why does a sewer lid get so hot when a puddle of water that has the same mass stay cool? Explain in terms of specific heat.
The sewer lid is made of iron, a metal, which has a relatively low specific heat when compared to that of water. Its lower specific heat means that it takes less energy to change the temperature of the iron than it does the water. If both substances were exposed to the same amount of radiant heat from the sun, the sewer lid will get hotter.
Heat:
The total amount of kinetic energy; It uses the variable q and has units of either joules (J) or calories (cal) (Note that heat is a form of energy)
The _________ (Ea) is the energy required to initiate the process or reaction. The ___________ is when the reactants have gained the activation energy and this is the point at which the reaction begins making products. If you look you will see that the system started with higher energy than it ended with. So the system ______ energy - exothermic. (__ is the energy change - lost in this case.)
activation energy; activated complex; lost; ΔH
thermal expansion
an increase in the size of a substance in response to an increase in the temperature of the substance
First, we pour some water into the ___________. We need to measure the mass of this water and its initial temperature. After this, we will heat a beaker of water until it is rapidly boiling. Why? Because the temperature of ________ water is constant. Why does that matter? It matters because we need to heat the metal in question to a constant measurable temperature. Next, we will measure the _____ of the metal Once the mass is known, place the metal into the boiling water for 5 - 10 minutes. Why? If we do this, we know that the temperature of the metal will be ________ as the boiling water and we can measure the temperature of the water to know the temperature of the metal. Once the metal has reached the time in the boiling water specified by the procedure, we quickly, using tongs, transfer the metal from the boiling water to the water in the ___________. Why? The hot metal will now transfer heat to the water until the two substances are the same temperature. Record the __________ temperature reached by the water in the calorimeter. This will be the final temperature for the water and the metal.
calorimeter; boiling; mass; the same; calorimeter; highest
If we can calculate the heat gained by the water in the ________, we know that it will equal the heat lost be the metal. -q_lost = q_gained (why is one side ___________ - because when heat is lost the sign of q will be negative - this will always be multiplied by a negative temperature change, so the negatives end up canceling out) -q_metal = q_water (remember that _________. Now substitute this in for both q's. The subscripts are m for metal and w for water) -m_m c_m ΔT_m = m_w c_w ΔT_w
calorimeter; negative; q=mcΔT
This method makes use of a piece of equipment called a _________. This is a very well insulated container that minimizes the transfer of heat to the surroundings. If we were to do this experiment in class, we would use _______________ Styrofoam cups. Styrofoam is a great insulator of heat.
calorimeter; two nested
The equation to use when using data from a ________ experiment: -m_m c_m ΔT_m = m_w c_w ΔT_w Only use the above equation if you have data for both the _______ and the _______. Sometimes you are told that the data you are given is from a calorimetry experiment, but only given information about one of the substances. If that happens use the equation _______.
calorimetry; metal; water; q = mcΔT
Note that there are 5 regions on this curve. In sections 1, 3, and 5, there is a ___________. So, we would use q=mcΔT for each section In sections 2 and 4, there is ____________ as we are changing states. Here we will use q=nH for each section. To calculate the _______ needed to go through the entire process shown we would need to do 5 different calculations and then add up the various heats calculated.
change in temperature; no temperature change; heat
When heat flows from the surroundings to the system we say that the process is __________. For reasons opposite of the above, these processes feel cold. As the _________ is absorbing heat from the _________, us.
endothermic; system; surroundings
ΔH, is the change in _______. What? I know, we haven't talked about that but I also said that this is the change in energy. Here is what you need to know, it is the change in heat (energy) per mole.
enthalpy
Triangle H = change in ________ def. -
enthalpy - The heat content of a system at constant pressure
The _______ value per mole according to the stoichiometric coefficients in the balanced chemical equation. What does it mean? We are going to use the _________ with the enthalpy. For example, if I want to connect how much heat is generated from C2H5OH, 1 mole = -1235kJ. If I wanted to connect how much heat is generated from O2 it would be 3molO2 = -1235kJ.
enthalpy; coefficients
When heat flows from the system to the surroundings we say that the process is _____________
exothermic.
Scientists often make potential energy diagrams to describe this flow of energy. Let's look at one for an ________ process. Note the axis - energy on the y (______) and reaction pathway on the x (think of like _______).
exothermic; heat; time
We can use the equation to solve for heat ______ or _____, or for the specific heat of a substance. (You could also use it to solve for ___________, but why would we - only in academic chem would you do this. Not is real chem)
gained; lost; temperature
________ is dependent on the amount of the substance you have. Temperature is not.
heat
ΔH can have many subscripts. When discussing changes of state you will most commonly see ΔH_f, for the _____________ and ΔH_v for ____________. Heat of _______ is used for the melting/freezing change of state and the heat of _________ is used for the boiling/condensing change of state.
heat of fusion; heat of vaporization; fusion; vaporization
We can predict how much _____ is absorbed or released using stoichiometry. Let's try one shall we.... If 15.0g of C2H5OH is combusted in excess O2, how much heat is produced according to the reaction below? C2H5OH + 3O2 → 2CO2 + 3H2O ΔH=-1235 kJ We have a new conversion factor. Usually in stoich we convert between two different substances. Here we will convert from one substance to _________. Pay attention to the last conversion factor in each problem as that is the new part. Note the number of _______ used on bottom and then look at the balanced equation. Do you see where that number comes from? The coefficients. The new conversion factor allows us to change moles to ___.
heat; energy; moles; kJ
The formula for calculating heat or specific heat: q = mcΔT q (_____) = m (____) c (_________) ΔT (___________) ΔT =________ T - ______ T If q is ___________, the process is exothermic If q is ________, the process is endothermic
heat; mass; specific heat cap; Change in temperature; final; initial; negative; positive
Note that almost everything here is the same except that this process starts with _____ energy and ends with ______ energy. There is still an ____________, still an _________, and still a change in energy.
low; higher; activation energy; activated complex
A hot object will have ________ kinetic energy which means _______ moving particles. A cold object will have ________ kinetic energy which means _______ moving particles.
more; faster; less; slower
Heat: Variable - Unit -
q J
If 200.0J of heat are absorbed by 13.0g of an unknow metal at 15.0 oC and the temperature rises to 75.0 oC, what is the specific heat of the metal?
q = mcΔT 200.0J = (13.0g)(c)(60.0 oC) c = 0.256 J/g oC Note that none of the units cancel out here.
How much heat is required to heat 5.00g of iron from a temperature of 22.0 oC to a temperature of 95.0 oC? c_Fe = 0.450J/g oC
q = mcΔT To get change in temp we take the final temp and subtract the initial temp. The sign is important here. q = (5.00g)(0.450J/g oC)(73.0 oC) q = 164J Positive q means this is an endothermic process Note how the units cancel out when you do the math leaving us with J.
What is the equation to calculate heat when there is a temperature change? Label all of the variables and indicate their units.
q = mcΔT q = heat lost or gained, unit Joules, J m = mass, unit grams g c = specific heat, unit J/g oC of cal/g oC ΔT = change in temperature, unit oC
What is the equation to calculate heat when there is no temperature change? Label all of the variables and indicate their units.
q = nH q = heat lost or gained, unit kilojoules, kJ H = enthalpy, unit kJ/mol
Heat is just heat and has the symbol __. Enthalpy is the heat change per _____ and had the symbol __.
q; mole; H
If we try to use the _________ equation we would always get a ΔT of 0, and thus a heat value of 0. Not good. There must be another way.... When changing states and there is no change in temperature, we use the equation q = nH, where q=______, n=______, and H is the ________ for the change of state occurring.
q=mcΔT; heat; moles; enthalpy
How much heat is required to turn 15.0g of ice at 0 oC to water at 0 oC? ΔH_f =6.02 KJ/mol First we convert g of ice to moles. 15.0g/18.0g/mol = 0.83mol Then, _________ q=(0.83mol)(6.02kJ/mol) = 5.0kJ Which equation you use is based on whether or not there is a change in ___________. Let's look at a heating curve of water. The following shows ice at a temp ________ 0 to a temp ________ 100.
q=nH; temperature; below; above
To calculate heat we need to discuss something called ______________. In simple terms, this tells us how easy or hard it is to change the _________ of something. The higher the ____________, the harder it is to change the ____________.
specific heat capacity; temperature; specific heat capacity; temperature
Why does it need to be well insulated? So that we can assume that the heat lost/gained by the _______ will ________ the heat lost/gained by the _____________. To use this method we must put together our understanding of how __________ with our understanding of how heat is calculated. Suppose we have a piece of metal that we would like to know how it absorbs heat, its specific heat.
system; equal; surroundings; heat flows
We like to think that we are the center of the universe and that we are the ________. Here is the problem, when we touch something to determine if it is hot or cold, we are always the ___________. If we touch an _________ process it feels hot. Here is why this can be a problem or confusing. If a process is ________ and is losing heat to the ________, we would likely say that it will get colder or that it would feel colder as it is losing heat. The _________ is losing heat, but where is it losing heat to? The ___________, to us. That is why it feels hot. We are the recipient of the lost heat.
system; surrounding; exothermic; exothermic; surroundings; system; surroundings
the ________ is what we are looking at (usually a reaction) and the __________ are everything immediately surrounding it.
system; surroundings
Earlier it said that if there was a _________ change, we calculate heat using the equation we just looked at. That must mean that there is a formula or method to use when there is no change in temperature. When substances undergo a change of state (melt, freeze, etc) there is __________ in temperature. Think about it, what is the temperature of boiling water? 100 oC right? Why does it not get hotter, after all, the stove element is hotter than 100 oC? Because ________ is a change of state and substances do not change temperature as that happens. All of the energy is being used to alter ___________ forces.
temperature; no change; boiling; intermolecular
thermometers are in ___________ def. -
thermal equilibrium - The state of two or more objects or substances in thermal contact when they have reached a common temperature
A _____________ equation is a chemical equation that has the enthalpy (remember this is just ______ per _____) change indicated. See example below: C2H5OH + 3O2 → 2CO2 + 3H2O ΔH=-1235 kJ
thermochemical; heat; mol
The specific heat capacity for _______: c_H2O = 4.18 J/g oC This tells us that to ______ the temperature of 1.0g of water 1.0 oC the water would need to absorb 4.18J of _________. (For point of reference, this is a very high specific heat capacity, most metals are ____ than 1.0)
water; raise; heat energy; less
