Topic 5 - Energy Changes Flashcards
How can you tell, using a reaction profile, that a reaction is exothermic?
The energy of the products is less than that of the reactants / a negative enthalpy change
What does BENDOMEX mean?
- Breaking bonds is endothermic
2. Making bonds is exothermic
How do you calculate overall energy change in a reaction?
a) Calculate the energy required to break the bonds in the reactants
b) Calculate the energy required to make the bonds in the products
c) Overall change = Energy to break - Energy to make
(If the overall change is positive, it is an endothermic reaction where as if it is negative, it is an exothermic reaction)
Give an example of an exothermic reaction:
Oxidation Neutralisation (Acid + Alkali) Displacement MASH Combustion
Give an example of an endothermic reaction:
Electrolysis
Decomposition
Citric Acid and Sodium Hydrogen Carbonate
T/F: The overall amount of energy increases in an exothermic recation
False: Energy can never be created or destroyed (Law of the Conservation of Energy) so whilst the energy of the products increases the overall amount remains the same
T/F: Exothermic reactions are much less commmon then endothermic reactions
True
How do you measure the amount of energy released by a chemical reaction?
1) Take the temperature of the raegents
2) Mix them in a polystyrene cup
3) Measure the temperature again
What is a reagant?
An organic or inorganic substance added to a system which is designed to provoke a change in said system
Why do you insulate a reaction where you are measuring a temperature change?
To minimise the amount of energy lost to the surroundings and to get the most accurate measurements and results
Label the parts of a reaction profile (find one in your book)
- Energy of products
- Activation energy
- Overall energy change
- Energy released or gained
- Energy of reactants
x axis = Progress
y axis = Energy
What do you need to calculate the overall energy change in the reaction?
Known bond energies for the products and reactants - these allow you to work out how much energy was needed to break the bonds in the reactants and to make the bonds in the products
T/F: In endothermic reactions energy goes from a chemical store to a heat store
False: Endothermic = heat -> chemical
Exothermix = chemical -> heat
What does enthalpy change refer to?
The change in energy
Is a positive enthalpy change a sign of an exothermic or endothermic reaction?
Endothermic reaction - the energy has increased as it has ‘entered’
