Evaluative Flashcards
Standard Enthalpy Change of Combustion
The enthalpy change that takes place when 1 mole of a substance reacts completely in oxygen under standard conditions where all reactants and products are in their standard states
Standard Enthalpy Change of Reaction
The enthalpy change that accompanies the reaction in the molar quatities expressed in a chemical reaction under standard conditions where all reactants and products are in their standard states.
Standard Enthalpy Change of Formation
The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
4 reasons why the standard value from a data book of delta Hc is more exothermic:
- There may have been incomplete combustion
- There may have been heat loss to surroundings
- Water may have evaporated - steam escapes and mass of water decreases
- There may have been non standard conditions
these reduce amount of heat to water
what can be done to reduce errors from heat transfer to surroundings and how?
Use bomb calorimeter:
- fuel burns using pressured oxygen so complete combustion
- Heat is transferred to the water which is well insulated to reduce heat loss to surroundings
- Sealed container and air jacket to prevent loss of water through evaporation
Indirect Enthalpy change:
Why may it not be possible to measure the enthalpy change of a reaction directly:
- A high activation energy
- A slow reaction rate
- More than 1 reaction taking place
Hess’s law:
If a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route
If the molar mass/RFM increases what happens to the enthalpy change of combustion?
Increases as the number of moles decreases so deltaH increases so reaction is more exothermic
What would happen to enthalpy change of combustion if more water (increased m) but the same amount of fuel was burnt?
No effect on the enthalpy change of combustion as although m increases, temp rise is smaller as there is more water to heat up in the same amount of time
Why is deltaHc less exothermic when the initial temperature is higher and the final temperature is above 100 with the same mass of fuel used?
- the max temp is unknown specifically so deltaT is less = less exothermic
- After 100degrees, water boils to steam and energy is needed to break the bonds for evaporation which comes from the fuel - so the mass of water also decreases meaning Q decreases
If the water is heated for longer why is the enthalpy change of combustion value vey similar to before?
Heating water for longer increases deltaT and also more fuel is burnt so Q increases and moles increases - cancel each other out
percentage error:
error / measurement x 100
why would using a smaller measuring cylinder of 20cm3 with an error of 0.5cm3 increase the error of measuring 140cm3 compared to a 140cm3 cyclinder of 1c 3 error?
Using a smaller measuring cylinder doesnt reduce percentage error of measuring the large volume of warer as it can actually increase the error since it needs to be used more often (repeats) to get overall volume so more chances of error
