Equilibrium I Flashcards
(20 cards)
What are standard conditions
100kPa and a specified temperature
usually 298K
What is enthalpy change
heat energy change measured at constant pressure
Define the standard enthalpy change of combustion
enthalpy change when 1 mole of a fuel is combusted completely in oxygen under standard conditions
Define the standard enthalpy change of formation
enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
Define the standard enthalpy change of neutralisation
the enthalpy change when and acid and an alkali react to form 1 mole of water under standard conditions
Define standard enthalpy change of a reaction
the enthalpy change when substances react under standard conditions in quantities given by the equation of the reaction
Give three assumptions when calculating enthalpy change
specific heat capacity of the solution is equal to the shc of water
solution has a density of 1gcm-3
mass of solids are not factored into the mass of Q because the mass is insignificant compared to the rest of the solution
How can you use a graphical method to determine the maximum temperature in the displacement prac
plot points on graph with temperature against time
extrapolate backwards from after zinc is added to when it was to find the change in temperature
use this to calculate E
Give four sources of error in enthalpy practicals
heat transferred to air and equipment rather than water
incomplete combustion
conditions not standard e.g H20 gas formed not water
takes time to heat - heat transferred away from water in this time
Describe how to find enthalpy of combustion practically
measure 100cm3 of water into a can
accurately weigh burner containing ethanol
place burner underneath clamped can
take temperature of water before heating using a thermometer
heat until a reasonable temperature change
reweigh burner
Describe how to investigate the energy change in a displacement reaction
using a 25cm3 pipette, put 50cm3 of copper sulfate into a polystyrene cup held in a beaker
take the initial temperature of the copper sulfate
continue stiring gently and take the temperature every 30 seconds
at 3.5 minutes add zinc powder and continue stiring whilst measuring the temperature from minute 4 to 9 every 30 seconds
Give 3 improvements to the displacement practical
use an electric stirrer
use lid on cup to reduce heat loss
use pipette to make volumes more accurate to make Q more accurate
Describe the method for enthalpy of neutralisation
fill a burette with the acid solution
clean and rinse a 25cm3 pipette and pipette 25cm3 of alkali into polystyrene cup within a glass beaker
record initial temperature of alkali solution
add 5cm3 of acid to alkali
stir solution and record temperature
repeat until 50cm3 acid has been added to alkali
Describe how to use a graphical method to find Q in a neutralisation reaction
plot volume of acid against temperature
draw two lines of best fit (one increasing and one decreasing)
record the volume of acid when the lines of best fit cross
use this value + 25cm3 (alkali in cup) as M to find Q
What is Hess’ law
the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs
What is bond enthalpy
enthalpy change when 1 mole of bonds is broken in the gaseous state
What are bond dissociation enthalpies
enthalpy change of breaking one bond in a diatomic molecule
- always endothermic (positive)
Why might bond enthalpies differ for the same molecule
each successive bond that is broken will have a slightly different bond enthalpy as the remaining bonds have a slightly different electronic environment
bond enthalpies also differ in different covalent environments
Equation for bond enthalpy of a reaction
break - make
