Energetics Flashcards
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.
Recall the 3 main sources of error in calorimetry.
Heat loss
Incomplete combustion
Heat absorption by the calorimeter
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 method for measuringΔHsoln of MgCl2 . [8]
Measure ___ cm3 water using _________________
Into polystyrene cup
Record initial temperature (min 2 measurements)
Add ___ g of MgCl2 (weigh by difference)
Record T every minute for ____ mins
Plot T vs time
Extrapolate lines to when solid added
Find ΔT
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)
State the meaning of the term mean bond enthalpy for a bond: MS
The enthalpy change to break1 moleof covalent bonds
Arranged over a range of compounds/molecules
State why the value for the standard enthalpy of formation of Mn(s) is zero: MS
Because it is an element
(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 forΔH for exothermic / endothermic?
- ΔH is POSITIVE for endothermic as the SYSTEM gains energy
- ΔH is NEGATIVE for exothermic as the system LOSES energy
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.
What’s important to note about the definition of mean bond enthalpy?
It only applies whent he substances are in the gaseous state
Define mean bond enthalpy
The enthalpy needed to break a covalent bond, averaged over different molecules
Define mean bond enthalpy
The enthalpy needed to break a covalent bond, averaged over different molecules
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
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
Describe a simple calorimetry method
- wash equipment
- put polystyrene cup in a beaker for insulation and support
- clamp thermometer into place
- place first reactant into the cup, measure initial temperature of both solutions
- transfer second reagent to the cup
- stir to ensure all solution is at the same temperature
- record temperature every minute after addition
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.
Give 2 examples of endothermic processes
thermal decomposition of calcium carbonate, melting/boiling
Give 2 examples of exothermic processes
combustion of fuels
oxidations of carbohydrates such as glucose in respiration
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
