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- Thermochemistry with Equation Stoichiometry
Consider an exothermic combustion reaction. The amount of heat may be treated as a stoichiometric product, precisely dependent on the amount of hydrocarbon combusted.
Methane combustion drives our civilization to a very great degree:
#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O, DeltaH = -890# #kJ# #mol^-1# .
The quoted enthalpy of combustion is per mole of reaction as written. You don't have to know these; you do have to know how to balance the equation. Because this energy is associated with the combustion of 1 mol of methane, I could also treat the evolved energy as a reagent or product in the reaction:
i.e. #CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O + 890# #kJ# .
It would be on the reactant side if the reaction was endothermic. In other words the minus sign denotes evolution of heat. Alternatively, #890# #kJ# of heat are evolved from the above reaction, so I am treating energy as a product just as carbon dioxide and water are (as indeed it is; of course the heat is a consequence of the formation of water and carbon dioxide bonds). If less than 16 g (1 mol) methane are combusted, the heat evolved will diminish stoichiometrically. Does this address your question?
Heats of Reaction - Hess' Law
This activity provides a demonstration of Hess' Law using three reactions: the solubility NaOH in water, the solubility NaOH in HCl and the reaction of a solution of HCl and a solution of NaOH.
Chemical Potential- Staircase Demonstration
As we look for connection points across disciplines, we are increasingly drawn to chemical potential as the connection point. This simulation shows particles thermally distributed on a staircase. A linear increase…
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High School Chemistry : Stoichiometry
Study concepts, example questions & explanations for high school chemistry, all high school chemistry resources, example questions, example question #1 : stoichiometry.
Consider the following reaction:
Since we are given the mass of nitrogen gas that will be used, we will need to convert the amount into moles. We can then compare the molar ratios in the balanced reaction, and multiply the moles of ammonia by its molar mass. Using dimensional analysis, we can use a calculation that will allow us to end with "grams of ammonia" as the solved unit.
Example Question #2 : Stoichiometry
What is the percent composition by mass of oxygen in a molecule of glucose?
First, we must find the molecular weight of the entire molecule of glucose. We do this individually for each element. Using oxygen as an example, we multiply the atomic mass (16) by the number of atoms per glucose molecule:
We do similar calculations for carbon and hydrogen to find our total molecular weight for glucose.
Sum these values to find the molecular weight.
Next, to find percent composition by mass for oxygen, we are being asked to find how much of one molecule of glucose's weight comes from oxygen. To do this we divide the mass of oxygen by the molecular weight:
Thus 53.3% of the mass of glucose comes from oxygen alone.
Example Question #3 : Stoichiometry
What is the mass of 25 iron atoms?
First, convert the number of atoms to moles using Avogadro's number:
Next, convert moles of iron to grams of iron using the molar mass:
Example Question #4 : Stoichiometry
Next, use dimensional analysis to find the number of moles in the given mass of the sample.
Example Question #5 : Stoichiometry
Next, use dimensional analysis to convert grams to moles.
Use the correct number of significant figures.
Example Question #6 : Stoichiometry
A silicon microchip weighs 7.2mg. How many atoms of silicon are in the microchip?
First convert the 7.2mg of silicon into grams:
Then convert grams of silicon into moles of silicon:
Finally, convert moles of silicon into atoms using Avogadro's number:
Since 7.2mg of silicon is much less than 1 mole (which we know weighs 28.09g) our answer makes sense as it is much less than Avogadro's number.
None of these
Use Avogadro's number to convert from atoms to moles:
Example Question #8 : Stoichiometry
What is the atomic number of calcium?
The atomic number is equal to the number of protons in an atom and can be determined using the periodic table. The atomic number is located on the top left of the box containing the desired chemical symbol. The chemical symbol for calcium is Ca and the atomic number is 20.
Example Question #9 : Stoichiometry
How many neutrons does calcium-42 have?
The 42 in calcium-42 represents the mass number of calcium. The mass number equals the sum of the number of protons and the number of neutrons. Using the periodic table, the number of protons in calcium can be determined using the atomic number. The atomic number of calcium is 20.
Using all this information the number of neutrons can be determined by subtracting the number of protons from the atomic mass number. The number of neutrons is 22.
Atomic Mass Number = Protons + Neutrons
Atomic Mass Number - Protons = Neutrons
Example Question #10 : Stoichiometry
The mass number is equal to the number of protons plus the number of neutrons in an atom.
Protons + Neutrons = Mass Number
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