Energy Changes Flashcards
State the law of conservation of energy.
Energy can neither be created nor destroyed, only transfered
What is an exothermic reaction?
A reaction which transfers energy to its surroundings, in turn increasing the temperature of its surroundings
Greek: exo - outside, therme - heat
What is an endothermic reaction?
A reaction which takes in energy from its surroundings, in turn increasing the temperature of its surroundings
Greek: exo - outside, therme - heat
Give two everyday examples of exothermic reactions
Self heating cans, hand warmers
Give two examples of endothermic reactions
Thermal decomposition, reaction of citric acid and sodium hydrogencarbonate
What is activation energy?
The minimum amount of energy required that the particles must have to react
What is the unit of measurement for activation energy?
kJ/mol
How do you calculate the overall energy change of a reaction?
sum of energy needed to break the bonds in the reactants - sum of energy released when the bonds in the products are formed
How can you make a simple chemical cell?
By connecting two different metals contact with an electrolyte
What is a battery?
A battery is two or more cells together in series to provide a greater voltage
Are alkaline batteries rechargeable or non rechargeable?
Alkaline batteries are non rechargeable
Why/how can rechargeable cells and batteries be recharged?
They can be recharged because chemical reactions are reversed when an external current is supplied
How do fuel cells work
Fuel cells are supplied with a fuel (eg hydrogen) and oxygen or air. The fuel is then electrochemically oxidised to produce a potential difference
What is the overall reaction in a hydrogen fuel cell?
hydrogen + oxygen → water
2H2(g) + O2(g) → 2H2O(l)
What are the two half equations of a hydrogen fuel cell?
At the negative electrode: 2H2 + 4OH- → 4H2O + 4e-
At the positive electrode: O2 + 2H2O + 4e- → 4OH-
