C7 Energy Changes Flashcards
What is an exothermic reaction
A reaction which gives out heat energy (so surrounding temperature goes up)
Exothermic-exit
What is an Endothermic reaction
Taking in heat energy (surrounding temperature goes down)
Examples of exothermic reactions
Combustion, neutralisation, oxidation
Examples of endothermic reactions
Photosynthesis, thermal decomposition
What percentage of reactions are exothermic
90%
What percentage of reactions are endothermic
10%
Step 1 of investigating temperature change practical
Use the first measuring cylinder to measure 25 cm3 of the sodium hydroxide solution and pour this into the polystyrene cup.
Step 2 of the investigating temperature change practical
Stand the polystyrene cup in the 250 cm3 beaker.
Step 3 of the investigating temperature change practical
In the second measuring cylinder, measure 25 cm3 of hydrochloric acid.
Step 4 of the investigating temperature change practical
Using the thermometer, measure the temperature of the sodium hydroxide every 30 seconds whilst gently stirring.
Step 5 of the investigating temperature change practical
After exactly 2 minutes add the hydrochloric acid and continue to stir and to record the temperature of the solution every 30 seconds for 10 minutes.
Step 6 of the temperature change practical
Repeat the experiment twice:
• with 25 cm3 of copper(II) sulfate and iron filings
• with 25 cm3 potassium hydroxide and nitric acid.
What can exothermic changes be used for
Hand warmers and self heating cans
What is used in reusable hand warmers
Crystallisation of a supersaturated solution
What can endothermic changes be used for
Instant cold packs for injuries in sport
What is activation energy
The minimum energy required to cause a process to occur
What do reaction profiles show
The relative difference in the energy of reactants and products
What does endothermic reactions have to do with bonds
-Bond breaking is endothermic
-If the bonds broken are stronger than the ones being made it is an endothermic reaction
What does exothermic reactions have to do with bonds
-Bond making is exothermic
-If the bonds broken are weaker than the ones being made it is an exothermic reaction
What does a negative number mean when calculating bond energy
It is an exothermic reaction as more energy is required to make the bonds than to break them.
What does a positive number mean when calculating bond energy
It is an endothermic reaction as less energy is required to make the bonds than to break them
Calculate: H—H=436. Br—Br=193. H—Br=366.
H—H + Br—Br —> 2 H—Br
(436)+(193)=629=bonds broken
2(366)=732=bonds made
Bonds made-bonds broken=629-732=-103.
Exothermic reaction as more energy is needed to make the bonds than to break them
