C3f Flashcards
What is an exothermic reaction?
One in which energy is transferred into the surroundings (releases energy).
What is an endothermic reaction?
One in which energy is taken from the surroundings (absorbs energy).
How can you recognise exothermic and endothermic reactions using temperature changes?
An exothermic reaction is shown by a rise in temperature and an endothermic reaction is shown by a fall in temperature.
What type of reaction is bond making?
An exothermic process.
What type of reaction is bond breaking?
An endothermic process.
Why is a reaction exothermic or endothermic, depending on the energy changes that occur during bond breaking and bond making?
In an exothermic reaction, the energy released in bond formation is greater than the energy used in breaking old bonds. In an endothermic reaction, the energy required to break old bonds is greater than the energy released when new bonds are formed.
What is a simple calorimetric method for comparing the energy transferred in combustion reactions?
1) Put some fuel into a spirit burner (or use a bottled gas burner if the fuel is a gas) and weigh the burner full of fuel.
2) Measure out 200cm3 of water (this is the same as 200g of water) into a copper calorimeter.
3) Take the initial temperature of the water - then put the burner under the calorimeter and light the wick.
4) When the heat from the burner has made the water temperature rise by 20-30 degrees Celsius, blow out the spirit burner and make a note of the highest temperature the water reaches.
5) Reweigh the burner and fuel.
6) If comparing two fuels, repeat the procedure with the second fuel.
How do you make the calorimetric method a fair test?
When comparing the energy content of different fuels, you need to do the same experiment several times, but using a different fuel in the burner each time.
To be fair, everything but the fuel should be the same. So you should use the same apparatus, the same amount of water each time, and the water should start and finish at the same temperature each time.
For the results to be reliable, you would have to repeat the experiment several times and discount any anomalous results.
What is the formula for the amount of energy transferred to the water each time?
Energy transferred = m x c x triangle T
m = mass of water (g)
c = specific heat capacity of water (4.2J/g degrees C)
triangle T = temperature change (degrees C)
What is the formula for the energy given out per g of fuel?
energy given out per gram (J/g) = energy released (J)/ mass of fuel burned (g)
