c7 Flashcards
what is an exothermic reaction
reaction that transfers energy to surroundings. results in raise of temp of surroundings
what is an endothermic reaction
Reaction that takes in energy from surroundings
Examples of exothermic reactions
Combustion, self-hating cans, hand warmers
Examples of endothermic reaction
Sports injury packs
What is bond making
It is an exothermic reaction
What is bond braking
It is an endothermic reaction
Bond energy calculations - overall energy change
If overall energy change is negative then it is an exothermic reaction
If overall energy change is positive then it is an endothermic reaction
How do cells work
Contain chemicals which react to produce electricity
Simple cells
Made by connecting two different metals in contact with an electrolyte. The greater the difference in reactivity of the two metals the greater the voltage.
Why do non-rechargeable cells stop working
One of the reactants has been used up so the chemical reaction stop. Alkaline batteries are an example
Why can rechargeable cells and batteries be recharged
Because the chemical reactions are reversed when an external current is supplied
How do fuel cells work
The fuel enters the cell. It becomes oxidized at the negative anode and gains electrons. This sets up a potential difference across the cells.
Why is water produced in hydrogen fuel cells
The hydrogen becomes oxidized to produce water.
Pros of hydrogen fuel cells
They only require oxygen and hydrogen. They do not produce any toxic waste. They are less polluting. They last longer than batteries.
Cons of hydrogen fuel cells
Hydrogen is a gas so it requires more space to store than a fossil fuel or batteries. Explosive when mixed with air so it is dangerous to store.
