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process is one that can be reversed by an infinitesimally small change in a variable. While a(n) ______ process involves expansion against an external pressure that differs by a finite amount from the pressure of the system.
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The enthalpies of fusion and vaporization affect the appearance of the ______ curve of a substance
heating
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Heating curve is a graph showing the variation in ______ of a sample as it is heated at a constant ______/ at a constant ______ and therefore at a constant ______ of ______ _____.
- temperature
- rate
- pressure
- rate of increasing enthalpy
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Most pure substances have similar heating curves to that of ____ ____. Name two potential differences
- cold ice
- Superheating
- Supercooling
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superheating
boiling does not begin until a few degrees over the boiling point, but once it does begin, the temperature falls back to that of the boiling point
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supercooling
when a sample is cooled, the temp may fall below the freezing point briefly before freezing point before freezing begins
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The steeper the slope of a heating curve, the _____ the heat capacity. The high heat capacity of liquid water is due largely to the extensive ______-______ network in the liquid.
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Every chemical reaction is accompanied by a change in _____, and commonly that energy is released or absorbed as _____. For example, complete reaction with oxygen is called ______ and all such reactions are _______
- energy
- heat
- combustion
- exothermic
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Explain the difference between heat and enthalpy
Heat is always the energy in transit, i.e, the energy which 'crosses' the system boundaries. Whereas Enthalpy refers to total heat content in a system.
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An expression including a chemical reaction/equation as well as a reaction enthalpy (ΔH) is called a ______ equation.
- for ex:
- CH4(g) + 2O2(g)→CO2(g) + 2H2O(l) ΔH=-890
- thermochemical equation
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The stoichiometric coefficients in a thermochemical equation indicate the number of ______ that react to give the reported change in ______. If we doubled the value of the coefficients, the change in ______ would be doubled
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According to the 1st law of thermodynamics, because enthalpy is a ______ ______, the enthalpy change for the reverse of a process/ chemical reaction is the _______ of the enthalpy change of the forward process/chemical reaction.
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Once we know the reaction enthalpy, we can calculate the _____ _____, therefore the ______ released or required, for any amount, mass, or volume of ______ consumed or ______ formed, even if that reaction cannot actually be carried out
- enthalpy change
- heat
- reactant
- product
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State the equation used in order to calculate the amount of a substance that reacts in a thermochemical equation.
What other two values would we need to calculate overall enthalpy? What is the equation?
Moles (reacting) equals mass(g) divided by molality (reacting) or n=m/M
We would need: (q) and the number of moles of the coefficient that is reacting.
Equation: ΔH = (ncoefficient) * (q)/ (nthat react)
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The heat released by the reaction at constant pressure is calculated from the _____ _____ multiplied by the _______ ______ of the calorimeter
- (temp change) x (heat cap. of calorimeter)
- aka qcal = (Ccal)(ΔT)
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During a reaction, the temperature of the calorimeter dropped 5.5°C, is our reaction endothermic or exothermic
endothermic
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Standard reaction enthalpy, ΔH°
the reaction enthalpy when reactants in their standard states change into products in their standard states
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Standard state
a substance's pure form at 1 bar and usually 298.15K
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The standard enthalpy of combustion
ΔHc°, is the change in enthalpy per mole of a substance that is burned in a combustion reaction under standard conditions.
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Standard reaction enthalpies refer to reactions in which the ______ & ______ are in their _____ state, the pure form at ___ bar; they are usually reported for a temp of ______K
- reactant & product
- standard state
- 1 bar
- 298.15K
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Hess's law states that a reaction enthalpy is the ____ of the enthalpies of any sequence of reactions (at the same ______ & _____) into which the overall reaction can be divided.
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reaction sequence
a series of reactions in which the products of one reaction take part as reactants in another reaction.
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State the 5 necessary steps in using Hess's law to combine reaction enthalpies (story)
- Select one of the reactants in the overall reaction and write down a chemical equation in which it also appears as a reactant
- Select one of the products in the overall reaction and write down a chemical equation in which it also appears as a product.
- Add this equation to the equation written in step 1 and cancel species that appear on both sides of the equation.
- Cancel unwanted species in the sum obtained in step 2 by adding an equation that has the same substance or substances on the opposite side of the arrow
- Once the sequence is complete, combine the standard reaction enthalpies
- **keep in mind you may need to reverse the equation or multiply it by a factor
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If we multiply the stoichiometric coefficients by a factor, we must multiply the _____ _____ by the same factor
reaction enthalpy
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Standard enthalpy of formation (___), of a substance is the standard reaction enthalpy per ____ of ______ units for the formation of a substance from its elements in their most ______ form
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The chemical equation that corresponds to the standard enthalpy of formation of a substance has the substance as the _____ product with a stoichiometric coefficient of ____. This implies the formation of ____ ____ of substance and it may require coefficients that are
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If you wanted to report that the standard enthalpy of formation of liquid ethanol (C2H5OH) is -277.69kJ⋅mol-1, how would you do so?
ΔH°f (C2H5OH,l)= -277.69kJ⋅mol-1
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The standard enthalpy of formation of an element in its most stable form is _____ but the standard enthalpy of formation of an element in any other form is a ______. Explain
- zero
- it takes no energy to keep a substance in its most stable form, but it does take energy to make it depart
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What is the equation for calculating standard reaction enthalpies, specifically when given standard enthalpies of formation?
- ΔH°f= ΣnΔH°f(products)-ΣnΔH°f(reactants)
- **sum of product enthalpies (kJ⋅mol-1) minus sum of reactant enthalpies (kJ⋅mol-1)
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When calculating standard enthalpy of formation of a specific species in a thermochemical equation. If given an enthalpy of combustion, what formula would you use?
ΔH°f(species)= ΣnΔH°f(products)-ΣnΔH°f(reactants)/n(species)
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For a given ionic solid, the difference in molar enthalpy between the solid and gas states of widely separated ions is called the ______ _____ of the solid (___)
lattice enthalpy (ΔHL)
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What is the formula for lattice enthalpy
ΔHL = Hm(ions,g) - Hm(solid)
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Lattice enthalpy can be identified with the heat required vaporize the ions from ____ to ____ state at ______ pressure. The greater the lattice enthalpy, the ______ the heat required.
- solid
- gaseous
- constant
- greater
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Heat equal to the _____ _____ is released when the solid forms from gaseous ions
lattice enthalpy
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Give a rough 6 step estimate of what to do when devising a born haber cycle for calculating lattice enthalpy.
- if you are gives species in solid state, present a statement converting it to gaseous state list the energy of conversion which will be a sum of relevant enthalpies of formation
- Then find which species will be losing electrons and which would be gaining
- Then find the ionization energy for the one that would be losing
- Then write the gain enthalpy as the negative of the species' listed electron affinity
- Then find the negative (product written as reactant vice versa) standard enthalpy of formation for the thermochemical equation created
- Lastly sum up all enthalpy values from all relevant steps.
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