Topic 14 - Energetics Flashcards
What is enthalpy change represented as
ΔH
What are the units for enthalpy change
kJ/mol
define endothermic
Reactions that take in energy from their surroundings (+ΔH)
Define exothermic
Reactions that transfer heat energy to the surroundings (-ΔH)
How do you calculate ΔH from bond energies
Bonds broken - bonds made
Breaking bonds is an endothermic reaction because energy must be supplied
Making bonds is an exothermic reaction because energy is released
describe the method for calorimetry; combustion
1) put 50g water in a copper can
2) measure the initial temperature of the water, and the initial mass of the spirit burner and lid
3) put the spirit burner underneath the can and light the wick, burn for 30s while stirring constantly
4) put out the taken using the burner lid, measure the final temperature of the water and the final mass of the burner and lid
5) reduce heat loss to atmosphere by using a screen to act as a draught excluder, don’t do it next to an open window
What is change in energy represented as
ΔE
How do you calculate change in energy (J)
ΔE = mass of water x SHC x ΔT of the water
How do you calculate enthalpy change
-ΔE
ΔH = ———
moles
Why is calorimetry so inaccurate
- heat loss to atmosphere
* incomplete reaction
What is enthalpy change?
The overall change in energy in a reaction
Measured in kJ/mol
Exothermic reactions give negative enthalpy change
Endothermic reactions give positive enthalpy change
What is a reaction profile?
Diagrams which show the relative energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction
The difference in height between the reactants and the products show the energy given out/taken in
The initial rise in energy represents the activation energy (the minimum amount of energy the reactants need to collide with each other and react)
In exothermic reaction profiles, the products are at a lower energy than the reactants
In endothermic reactions the products are at a higher energy than the reactants
Describe the general method for calorimetry: dissolving, displacement and neutralisation reactions
*general theory- take the temperature of reactants (making sure they’re the same), mix them and measure the temperature of the products)
1) mix the reactants in a polystyrene cup
2) reduce energy loss to the surroundings by putting the polystyrene cup into a beaker of cotton wool to give it more insulation, and putting a lid on the cup to reduce energy lost by evaporation
Describe the calorimetry neutralisation reaction
1) put 25cm3 of hydrochloride acid and sodium hydroxide into separate beakers
2) place the beakers in a waterbath at 25°C until they both reach this temperature
3) add the HCl followed by the NaOH to a polystyrene cup with a lid
4) take the temperature of the mixture every 30s, record the highest temperature
