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.
