energy changes Flashcards
what is an exothermic reaction?
a reaction in which heat energy is given off- more energy is released when new bonds are formed than energy required to break the bonds in the reactants. has a negative enthalpy value.
what is an endothermic reaction?
a reaction in which heat energy is taken in from the surroundings- more energy is needed to break bonds than is released in making bonds. has a positive enthalpy value
enthalpy change (ΔH) = …?
energy taken in - energy given out
what are some uses of endo and exothermic reactions?
endothermic- sports injury packs
exothermic- self heating food, hand warmers
what are some examples of exothermic reactions?
combustion, neutralisation, oxidation, displacement
what are some examples of endothermic reactions?
therman decomposition, alkane cracking, sodium hydrogencarbonate + citric acid
what is the link between reactivity and energy changes?
the bigger the difference in reactivity in a displacement reaction, the more exothermic the reaction
what is activation energy?
the minimum amount of energy required to start a chemical reaction.
describe an energy profile diagram for an exothermic reaction
reactants are at a higher energy level than products
describe an energy profile diagram for an endothermic reaction
products are at a higher energy level than reactants.
what is the definition of bond energy?
the average amount of energy that will break/make a bond between two atoms
what is a catalyst + how do they work?
a substance that speeds up the rate of a chemical reaction without being used up- provides an alternate reaction pathway with lower activation energy
what is an electrochemical cell?
a device containing chemicals that react to produce electricity- 2 or more cells together in series form a battery
what factors determine a cell’s voltage?
type of electrode, concentration of electrolyte, type of electrolyte
how are some cells rechargeable?
the chemical reaction is reversed when a current is applied
what is a fuel cell?
an electrochemical cell that can be refilled with fresh chemicals
how do hydrogen fuel cells generate electricity?
hydrogen and oxygen react and produce electricity, water and heat
what are some advantages and disadvantages of hydrogen fuel cells?
advantages: only product is water, can be refilled quickly
disadvantages: hydrogen obtained using fossil fuels (a finite resource), it is flammable and difficult to store
what is the half equation at the negative terminal?
2H2 + 4OH- -> 4H2O + 4e-
what is the half equation at the positive terminal?
O2 + 2H2O + 4e- -> 4OH-
