DF - Enthalpy changes Flashcards
What is the chemistry word for energy?
Enthalpy.
What does enthalpy mean?
Energy.
What is the definition of enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What are the units for enthalpy change?
KJmol^-1
What does it mean if enthalpy changes are under standard conditions?
The are done at:
25 degrees celsius/298K
1 atmosphere pressure/100 kPa
A concentration of 1moldm^-3
How do you convert from Celsius to Kelvin?
Add 273 as the scale is the same but 0 degrees celsius is equal to 273 Kelvin.
What happens to energy in an exothermic reaction?
It is given out.
What happens to energy in an endothermic reaction?
It is absorbed.
What happens to temperature in exothermic reactions?
Increases.
What is the enthalpy change like in an exothermic reaction?
Negative.
What type of reaction is exothermic?
Oxidation.
What type of reaction is oxidation?
Exothermic.
What are 2 examples of exothermic reactions?
Combustion and oxidation.
What happens to the temperature (of the surroundings) in endothermic reactions?
Decreases.
What are examples of endothermic reactions?
Thermal decomposition and photosynthesis.
What type of reaction is bond breaking?
Endothermic
Why is bond breaking endothermic?
It requires energy.
What type of reaction is bond forming?
exothermic
Why is bond forming exothermic?
It releases energy.
How can enthalpy changes be calculated using average bond enthalpies?
Enthalpy change of reaction = total energy absorbed to break bonds - total energy released in making bonds.
In terms of bond enthalpies, what is enthalpy change for a reaction?
The overall effect of the changes in energy when bonds are formed and broken.
What is the enthalpy change like if you need more energy to break bonds than is released when bonds are made?
The enthalpy change is positive.
What is the enthalpy change like if you need less energy to break bonds than is released when bonds are made?
The enthalpy change is negative.
What is bond enthalpy?
The energy needed to break a bond, or the energy given out when a bond forms.
What is the correlation between the strength of the bond and its bond enthalpy?
The stronger the bond, the higher the bond enthalpy.
Are bond enthalpies exact?
No.
For example, water has two O-H bonds but it takes different amounts of energy to break them both. The first bond takes more energy to break than the second one so the average bond enthalpy is the amount of energy needed to break both of the bonds divided by 2.
What is average bond enthalpy?
The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds.
What is a bond enthalpy related to?
The length of the bond.
What is bond length?
The distance between two nuclei is the distance where the attractive and repulsive forces balance each other. This distance is the bond length.
What is the correlation between attraction between atoms and bond enthalpy and bond length?
The stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length. If there’s more attraction, the nuclei are pulled closer together.
Which bond has a greater bond enthalpy:
C=C or C-C?
C=C
Which has a shorter bond length:
C=C or C-C?
C=C
In which is the electron density greater:
C=C or C-C?
C=C as four electrons are shared rather than just 2 in C-C.
As bond length increases what happens to average bond enthalpy?
Decreases.
