C5 - Energy Changes (PMT) Flashcards
What is the conservation of energy principle?
Energy is conserved in chemical reactions. The amount of energy at the end of the chemical reaction is the same as before the reaction takes place.
What is an exothermic reaction? Give examples
A reaction where the energy is transferred to the surroundings so the surrounding temperature increases. - eg. combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change
What is an endothermic reaction? Give examples
A reaction where the energy is taken in from the surroundings so the surrounding temperature decreases. - e.g thermal decomposition, reaction of citric acid and hydrogencarbonate. Positive sign of energy change
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 the activation energy of the reaction.
What occurs in a chemical reaction, in terms of bond energies? Describe exothermic and endothermic reactions in terms of bonds breaking/forming.
Energy is supplied to break bonds and energy is released when bonds are made
exothermic: energy released from forming bonds is greater than energy needed to break the bonds.
endothermic: energy needed to break bonds is greater than the energy released to make them
What is the equation to find the 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, to produce a greater voltage
What determines the voltage obtained from a cell?
Identities of metals used as electrodes and the identity and concentration of the electrolyte.
(the difference of reactivity between the metals - the greater the difference of reactivity between the metals, the greater the voltage of the cell)
State the advantages and disadvantages of using cells and batteries
+ more or less cheap, some are rechargeable, a convenient source of electrical energy
- harmful chemicals
Describe rechargeable and non-rechargeable cells
Rechargeable: chemical reactions are reversed when an external current is applied
Non-chargeable: reactants are used up, cannot be recharged (alkaline batteries)
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?
2H2 + O2 -> 2H2O
What is the half-equation at the negative electrode? in a hydrogen fuel cell
Negative electrode (cathode): 2H2 -> 4H+ + 4e-
