3.1.4 Energetics Flashcards
What is enthalpy change?
heat energy change at a constant pressure
What happens in an exothermic reaction?
➤ Energy is transferred from the system to the surroundings. The products have less energy than the reactants.
➤ ΔH is negative
PIC ON GOOGLE DOC
What happens in an endothermic reaction?
➤ Energy is transferred from the surroundings to the system
➤ The require an input of heat energy
➤ Products have more energy than the reactants
➤ ΔH is positive
Give 2 examples of exothermic processes
combustion of fuels
oxidations of carbohydrates such as glucose in respiration
Give 2 examples of endothermic processes
thermal decomposition of calcium carbonate, melting/boiling
What is the standard enthalpy change of formation?
The enthalpy change when 1 mole of the compound is formed from its elements under standard conditions, with all reactants and products being in their standard states.
What is the standard enthalpy change of combustion?
The enthalpy change that occurs when one mole of a substance is combused completely in oxygen under standard conditions, with all reactants and products being in their standard states.
What are standard conditions?
➤ 100kPa pressure
➤ 298K temperature
➤ Solutions at 1 mol/dm^3
➤ All substances at their standard state
What is the VALUE for the enthalpy change of formation of an element?
0kJ mol-1
What’s important to note about the balancing of standard enthalpy change equations?
It’s important to note that it’s only 1 mole being formed / combusted.
Hence, in the equation, you can’t have any big numbers in front of the products (during formation) or the combustion reactant (in combustion)
How do you calculate the energy change in a reaction?
q = mcΔt
energy change = mass of solution x heat capacity x temperature change
Describe a simple calorimetry method
- wash equipment for e.g. cup and pipettes with the solutions to be used
- put polystyrene cup in a beaker for insulation and support
- measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
- clamp thermometer into place - ensure bulb of therm is in the solution
- measure initial temperature of both solutions every minute for 2-3 minutes
- At the 3rd min transfer second reagent to the cup
- stir to ensure all solution is at the same temperature
- record temperature every minute after addition for several mins
In a simple calorimetry method, what are the sources of error?
Heat Loss
Incomplete Reaction
Approximation in Heat capacity
Density taken to be the same as Water
How do we calculate enthalpy change of a reaction?
- q=mcΔt c= 4.18 m = mass of solution
- Work out moles
- convert q to kJ
- -q/n = enthalpy change
What does Hess’s law state?
total enthalpy change for a reaction is independent of the route
Define mean bond enthalpy
The enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
What’s important to note about the definition of mean bond enthalpy?
It only applies whent he substances are in the gaseous state
In general, what is the formula for enthalpy change when working with bond energies?
DH = Σ bond energies broken - Σ bond energies made
Describe the trends in enthalpies of combustion in a homologous series
Constant rise as the number of carbon atoms rise.
Tihs is because there is a constant amount and type of extra bonds being broken and made.
(ESQ) State why the enthalpy change of formation may not be 0 for an element
It may not be in it’s standard state
What are the signs for ΔH for exothermic / endothermic?
ΔH is POSITIVE for endothermic as the SYSTEM gains energy
ΔH is NEGATIVE for exothermic as the system LOSES energy
Recall the method for measuring ΔHsoln of MgCl2 . [8]
-Measure ___ cm3 water using __________ (10-200cm^3/g)(e.g. pipette)
-Into polystyrene cup
-Record initial temperature (min 2 measurements)
-Add ___ g of MgCl2 ( then weigh by difference)
-use the ‘before and after’ weighing method
- Record initial temperature
-Record T every minute for __(5+) mins
-Plot T vs time
-Extrapolate lines to when solid added to find initial and final T
-Find ΔT (Tfinal-Tinitial = ^T)
Recall the method to measure ΔHc of an alcohol.
-Weigh spirit burner before and after combustion
-Measure mass (or volume) of water
-Water in calorimeter
-Burn alcohol to heat the water
-Measure temperature change in water
-Use q = mcDt and DH = - q / n
Recall the 3 main sources of error in calorimetry.
Heat loss
Incomplete combustion
Heat absorption by the calorimeter
When calculating enthalpy change during the reaction fo two solutions, what moles do you use?
The moles of the limiting reagent.
Might have to use the stoichiometry.
State why the value for the standard enthalpy of formation of Mn(s) is zero: MS
Because it is an element
Give the meaning of the term enthalpy change: MS [1]
Heat energy change at constant pressure
Define the term standard enthalpy of formation: MS [3]
-The enthalpy change at constant pressure when 1 mole of a compound
-Is formed from its constituent elements
-With the reactants and products in standard states
Define the term standard enthalpy of combustion: MS [3]
-The enthalpy change at constant pressure when 1 mole of a compound
-Is completely burned in oxygen
-Under standard conditions of 298K and 100kPa
State the meaning of the term mean bond enthalpy for a bond: MS
-The enthalpy change to break 1 mole of covalent bonds
-Arranged over a range of compounds/molecules
Give one reason why titanium cannot be extracted directly from titanium(IV) oxide using carbon: MS [1]
Ti is not produced, TiC (carbide) will be produced (brittle)
