Enthalpy Changes (9) Flashcards
Enthalpy
Heat energy in a chemical system that is made up of the atoms, ions and molecules in chemicals. It can be thought of as the energy in the bonds. It cannot be measured so we use enthalpy changes
Enthalpy change ΔH
Enthalpy of the products - Enthalpy of reactants
Law of Conservation of Energy
This states that energy cannot be created or destroyed, it can only be transferred from the system to the surroundings
Exothermic Change
Energy is transferred from the system to the surroundings. Enthalpy is negative
Endothermic Change
Energy is transferred from the surroundings to the system. Enthalpy will be positive
Exothermic Conservation of Energy
As energy is transferred from the system and it cools down, it will be balanced by the surroundings gaining that energy and increasing its temperature
Endothermic Conservation of Energy
As energy is transferred from the surroundings to the system, the surroundings lose energy so cool down and the system will gain energy so increase temperature.
Activation Energy (Ea)
It’s the minimum amount of energy needed for a reaction to take place. A certain amount of energy is needed for the bonds in the reactants to break in order for the reaction to begin.
How does the activation energy affect the speed of the reaction?
If the activation energy is low, the reaction will happen very fast. If the activation energy is high, the reaction will be very slow.
A standard enthalpy change ( ΔHº)
An enthalpy change that takes place under standard conditions
Standard Conditions
Standard temp - 298K or 25 degrees celsius
Standard pressure - 100 kpa
Standard conc - 1moldm-3
Standard state - what state your substance is in standard conditions
Enthalpy change of reaction ΔrH⦵
The enthalpy change when reactants, as shown in the reaction equation, react under standard conditions to give products in their standard states.
Enthalpy change of formation ΔfH⦵
The enthalpy change when ONE MOLE of a compound is formed from one mole of its elements in their standard states under standard conditions.
If it’s the ΔfH⦵ of an element, then it will be zero as there is no change.
Enthalpy change of combustion ΔcH⦵
The enthalpy change when ONE MOLE reactant reacts completely with oxygen under standard conditions when both reactants and products are in their standard states.
Enthalpy change of neutralisation ΔneutH⦵
Enthalpy change when an acid is neutralised by a base under standard conditions to form ONE MOLE of water where the reactants and products are in their standard states. The value will be the same for all neutralisation reactions
What is 0 kelvin?
-273 degrees celsius
What is the difference between temperature rise in kelvin and degrees?
There is no difference, the temperature change will be the same.
How to calculate enthalpy change
Look at the mass, specific heat capacity and temperature change.
Specific Heat Capacity
The amount of energy it takes to raise 1g of the substance by 1K. Conductive substances will have a small spec heat cap and non-conductive will have large
Mass
Just weigh in grams
Temperature change
Determine it by a thermometer by doing final temp-initial temp
The equation for energy change
Q= MCΔT (m=mass that changes temp) (c=spec heat cap) (ΔT= change in temp)
Experiment for the enthalpy change of combustion
Put water into a beaker and measure the temperature with a thermometer.
Put it on a stand
Weigh a spirit burner with methanol in it and then light it under the water.
Wait for three minutes while stirring with the thermometer
Extinguish the fire and record the temperature of the water and the mass of the spirit burner.
You can find out the enthalpy change of combustion
Why might we not get a maximum amount of heat transferred during the experiment for ΔcH⦵
Heat might be lost in the surroundings other than the water
It might be incomplete combustion
If the burner isn’t weighed as soon as possible, the methanol can evaporate from the wick
You might not have standard conditions.
This would mean the enthalpy change was less exothermic.
How could you minimise the limitations of the experiment for ΔcH⦵
You could put up draught screen and try and set the room to standard conditions. You could add excess oxygen to make sure it’s not an incomplete combustion.
Enthalpy changes between solutions
These take place between the solute and solution, as the solution is seen as the outside surroundings that will occur when the solute and solution collide and transfer energy.
Experiment for the enthalpy change of reaction
Add your solution to a polystyrene cup which will insulate it from the external environment. You then use a thermometer to calculate any energy change.
Determining the enthalpy change of neutralisation
It’s the same as the previous experiment, only it’s using two solutions instead of a solution and a solid
Average Bond Enthalpy
The amount of energy required to break one mole of a specific bond in a gaseous molecule
Properties of bond enthalpy
They always occur in gaseous molecules
Energy is always required to break bonds, so energy is taken from the surrounding and transferred to the system so it’s endothermic always.
They will always have a positive value
What is bond making?
It is exothermic so will release energy
What is bond breaking?
It is endothermic and requires energy
Energy diagram for exothermic
The energy released for making the bonds will be greater than the energy needed to break the bonds
Calculating bond enthalpies
After figuring out the total bond enthalpy for the products and reactants separately, always do:
Reactants-Products
Limitations of bond enthalpy
Because it’s an average bond enthalpy, the enthalpies of each bond could be slightly different because of a different environment. It is also not a standard enthalpy change because the products formed are all gases, so some might not be in their standard state.
Hess’ Law
It states that if a reaction takes place by two roots and the start and finish conditions are the same, the enthalpy change will be the same for each route.
Route A + Route B = Route C
