Topic 8 - Energetics I Flashcards
What is enthalphy change?
The heat change in a reaction at constant pressure.
What is the symbol for enthalpy change?
ΔH
What is the unit for enthalpy change?
kJ/mol
What is the symbol ΔH° used for?
- Enthalpy change under standard conditions and with the elements in their standard state
- The ° indicates this (a circle with a line through it).
What are standard states of elements?
Their physical states under standard conditions.
What are standard conditions?
- 100kPa (about 1 atm)
* Specified pressure (normally 298K)
What is an exothermic reaction?
One that gives out heat energy.
What happens to the temperature of the system in an exothermic reaction?
It goes up.
Give an example of an exothermic reaction.
Combustion of a fuel.
What is an endothermic reaction?
One that takes in heat energy.
What happens to the temperature of the system in an endothermic reaction?
It goes down.
Give an example of an endothermic reaction.
Thermal decomposition of CaCO3.
What is an enthalpy level diagram?
A diagram that shows the relative energies of the reactants and products in a reaction.
Describe what an enthalpy level diagram looks like.
- Enthalpy on the y-axis, no x-axis
- Horizontal line for reactants and products -> No line joining the two
- Arrow between the two lines, showing ΔH
What is another name for an enthalpy level diagram?
Energy level diagram
How does enthalpy relate to how stable a substance is?
The lower a substance’s enthalpy, the more stable it is.
Describe the enthalpy level diagram for an exothermic reaction.
- Reactants line above products line
* Downwards arrow between the two, showing ΔH
Describe the enthalpy level diagram for an endothermic reaction.
- Reactants line below products line
* Upwards arrow between the two, showing ΔH
What is a reaction profile diagram?
A diagram used to show how enthalpy changes throughout a reaction.
Describe what a reaction profile diagram looks like.
- Enthalpy on y-axis, Progress of reaction on x-axis
- Horizontal line for reactants and products
- Curve between the two -> Peak up to activation, then falls to products
- Arrow up to peak = Activation energy (Ea)
- Arrow from reactants to products = ΔH
What is the activation energy of a reaction?
The energy that must be supplied to break bonds in the reactants and start a chemical reaction.
What is the symbol for activation energy?
Ea
What is the ΔH value for an exothermic reaction?
Negative
What is the ΔH value for an endothermic reaction?
Positive