Energy Changes (5) Flashcards
(31 cards)
What is the conservation of energy principle?
Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place
What is an exothermic reaction?
A reaction where energy is transferred to the surroundings so that the surroundings temperature increases
What is an example of an exothermic reaction
combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change.
What is an endothermic reaction?
A reaction where energy is taken in from the surroundings so the surroundings temperature decreases
What is an example of an endothermic reaction?
Melting ice cubes.
Melting solid salts.
Evaporating liquid water.
What is activation energy?
Minimum amount of energy that particles need to react
What is a reaction profile?
Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
What occurs in a chemical reaction in
terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.
Bond breaking is an endothermic process, because it requires energy.
Bond forming is an exothermic process, because it releases energy.
What is the equation to find enthalpy change in terms of bond energies?
Energy of reaction = sum of bonds broken – sum of bonds made
What is a cell?
A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
What is a battery?
A battery consists of two or more cells connected in series.
What determines the voltage obtained from a cell?
Identities of metals used as electrodes and the identity and concentration of an electrolyte.
State the advantages of using cells and batteries.
(+) more or less cheap, some are rechargeable, a convenient source of electrical energy
State the disadvantages of using cells and batteries.
(-) harmful chemicals
Describe rechargeable cells
Rechargeable – chemical reactions are reversed when an external current is supplied
Describe non-rechargeable cells
Non-rechargeable – reactants are used up, cannot be recharged
What is a fuel cell?
Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
What is the overall reaction in a hydrogen fuel cell? What are the half equations?
What are the advantages of fuel cells
Advantages: no pollutants, no recharging
What are the disadvantages of hydrogen fuel cells?
What is 🔺(delta)H
Overall energy change
What is -🔺(delta)H
Exothermic (hot)
What is +🔺(delta)H
Endothermic feels cold
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