Exothermic and endothermic reactions (5.1) (M) Flashcards
Energy is (…) in chemical reactions
conserved
The amount of energy in the universe at the end of a chemical reaction is the (…) as before the reaction takes place
same
Why would energy be transferred to the surroundings after a reaction?
the product molecules must have had less energy than the reactants, by the amount transferred
What is an exothermic reaction?
one that transfers energy to the surroundings so the temperature of the surroundings increases
What happens to the temperature of the surroundings after an exothermic reaction?
It increases
3 needed
What are examples of exothermic reactions?
- Combustion
- Many oxidation reactions
- Neutralisation
2 needed
What are some everyday uses of exothermic reactions?
self-heating cans and hand warmers
What is an endothermic reaction?
one that takes in energy from the surroundings
What happens to the temperature of the surroundings after an endothermic reaction?
It decreases
2 needed
What are examples of endothermic reactions?
- Thermal decompositions
- The reaction of citric acid and sodium hydrogencarbonate
What is an everyday use of endothermic reactions?
Some sports injury packs
The temperature of the surroundings has decreased, the reaction therefore must be…
endothermic
Chemical reactions can occur only when what happens?
reacting particles collide with each other and with sufficient energy
What is activation energy?
The minimum amount of energy that particles must have to react
3 needed
What do reaction profiles show?
- the relative energies of reactants and products
- the activation energy
- the overall energy change of a reaction
Draw the reaction profile for an exothermic reaction
Draw the reaction profile for an endothermic reaction
2 things
What must happen to bonds in a chemical reaction?
- energy must be supplied to break bonds in the reactants (endothermic)
- energy is released when bonds in the products are formed (exothermic)
What is bond energy?
the energy needed to break bonds
or
the energy released when bonds are formed
How do you calculate overall energy change of a reaction?
the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed
For an exothermic reaction, the overall energy change will always be (…), whereas in an endothermic reaction, the overall energy change will always be (…)
negative
positive
In an exothermic reaction, the energy released from forming new bonds is (…) than the energy needed to break existing bonds
greater
In an endothermic reaction, the energy needed to break existing bonds is (…) than the energy released from forming new bonds
greater
How can you tell from a reaction profile that a reaction is exothermic?
the products are at a lower energy level than the reactants
What might a bond in the product being weaker mean for the energy released?
Less energy is released when forming bonds (more positive overall energy change - so more in endothermic direction) as less energy is required to form the bonds
6 marker
Use the above statement to predict and explain how the overall energy change for the reaction of ethene with chlorine will differ from the overall energy change for the reaction of ethene with bromine.
Size and strength
- chlorine atoms have fewer electron energy levels
- chlorine atoms form stronger bonds
- Cl-Cl bond stronger than Br-Br
- C-Cl bond stronger than C-Br
Energies required
- more energy required to break bonds with chlorine
- more energy given out when making bonds with chlorine
- Overall energy change depends on size of energy changes
Conclusions
- if Cl-Cl bond changes more the more/less exothermic
- cannot tell how overall energy change will differ as do not know what changes more
2 points
The bond energy of the reactants is calculated to be 6000kJ/mol and the bond energy of the products is 7148kJ/mol.
Explain why the reaction is exothermic (use values)
More energy is released (7148kJ/mol) when bonds in the products are formed than the energy used (6000kJ/mol) to break bonds in the reactants.
So energy is released to the surroundings
