energy changes Flashcards
What is an exothermic reaction?
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. Making bonds is exothermic.
Give three examples and two everyday uses of exothermic reactions.
exothermic
- combustion, neutralisation, some oxidation reactions
- uses are self heating cans and hand warmers
What is an endothermic reaction?
An endothermic reaction is one that takes energy in from the surroundings so the temperature of the surroundings decreases. Breaking bonds is endothermic.
Give an example and an everyday use of endothermic reactions.
endothermic
- thermal decomposition
- uses are sports injury packs
What is activation energy?
Chemical reactions can only occur when reacting particles collide with each other with sufficient energy. The minimum energy required for the reaction to occur is the activation energy.
What are cells?
Cells contain chemicals which react to produce electricity.
What is the voltage produced by a cell dependent upon?
The voltage produced by a cell is dependent upon the electrode and electrolyte.
How can a simple cell be made?
A simple cell can be made by connecting two metals in contact with an electrolyte. They must be two different reactivities and the greater the difference, the greater the voltage produced.
What is a battery?
A battery is two or more cells connected in series to produce a greater voltage.
What happens in non-rechargeable cells and batteries? Give an example.
In non-rechargeable cells and batteries, the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are an example.
What happens in rechargeable cells and batteries?
In rechargeable cells and batteries, the chemical reactions can be reversed when supplied with an external current.
What is a fuel cell?
A fuel cell is when we react an external source of fuel (like hydrogen) with oxygen or air.
What is the half equation at the anode and cathode in a hydrogen fuel cell?
hydrogen fuel cell:
anode - 2H2 –> 4H+ + 4e-
cathode - O2 + 4H+ + 4e- –> 2H20
overall - 2H2 + O2 –> 2H20
What are four advantages of fuel cells?
advantages of fuel cells:
- produce electricity as long as H is provided
- don’t get less efficient as used, whilst batteries need to be replaced
- source of drinking water
- no greenhouse gases
What are two disadvantages of fuel cells?
disadvantages of fuel cells:
- H is explosive gas so it is difficult to store safely, but no dangerous chemicals in batteries
- produce relatively low V so several needed, but batteries produce greater V
