C7: Energy Changes Flashcards
What is an exothermic reaction and give an example
e.g. wood burning (combustion), certain oxidation reactions, neutralisation, hand warmers, self heating cans.
A reaction in which energy is transferred to the surroundings, meaning their temperature increases.
What is an endothermic reaction and give an example
e.g. thermal decomposition, cold packs
A reaction in which energy is taken in from the surroundings, causing the temperature to drop.
What is the energy profile of an exothermic reaction?
Products have less energy than reactants, as energy has been transferred to surroundings
What is the energy profile of an endothermic reaction.
Products have more energy than the reactants, as energy has been taken in. Difference in energy tells us that energy has been taken in by the reaction.
What is activation energy?
Reactions can only occur if particles collide with each other with sufficient energy and frequency (collision theory).
The minimum amount of energy particles need to react is called the activation energy.
On energy profiles, it’s the distance between the reactant energy and the peak.
Bond breaking is…
Endothermic
Bond making is…
Exothermic
How can you determine whether a reaction is endothermic or exothermic using bond energies?
Negative value = exothermic
Positive value = endothermic
What are cells?
If we take 2 different metals and place them in an electrolyte (a solution that can conduct electricity e.g. an ionic compound) then we can produce electricity. If both metals are connected to a voltmeter, a p.d. will be shown and a current will flow through the wire. Chemical reactions take place between the electrolyte and metals, and this setup is called a cell.
What are the properties of cells?
Can only produce electricity for a certain amount of time. Eventually the reactants run out and the reaction stops.
Cells only produce electricity if metals with different reactivities are used.
How does the reactivity of metals used influence the cell?
The bigger the difference in reactivity between the metals, the bigger p.d. is produced.
(Electrolyte also affects p.d.)
What is a battery?
2 or more cells connected in series, producing a greater voltage.
What is a rechargeable cell/ battery?
In alkaline batteries, the reactants run out at some point - this reaction is non-reversible, so the battery is non-rechargeable.
Rechargeable batteries can be recharged because the chemical reactions can be reversed when an electrical current is supplied.
What is a fuel cell?
Reacting a fuel (such as hydrogen) with a reactant (such as oxygen or air). This produces an electric current.
What are the waste products of hydrogen fuel cells?
Water
What is the reaction at the negative electrode of a fuel cell?
2H₂ → 4H+ + 4e-
What is the reaction at the positive electrode of a fuel cell?
O₂ + 4H+ + 4e- → 2H₂O
What is the overall equation for the hydrogen fuel cell?
2H₂ + O₂ → H₂O
Hydrogen is oxidised
What are the advantages of using hydrogen fuel cells?
Will provide electricity for as long as you provide hydrogen, whereas rechargeable batteries will run out and need to be recharged.
Do not get the less efficient the more they run, whereas rechargeable batteries store less electricity the longer they run, and eventually need to be replaced.
They can be a source of drinkable water e.g. in spacecraft
What are the disadvantages of using hydrogen fuel cells?
Hydrogen is highly flammable and is difficult to store.
Do not produce a large p.d. so several are needed together.
