5: Energy Change Flashcards
what happens in exothermic reactions?
energy is given out to the surroundings so the temperature of the surroundings increases
give examples of where exothermic reactions are used in everyday life.
hand warmers; self-heating cans of food and drinks
give examples of typical exothermic reactions.
combustion, oxidation and neutralisation
what happens in endothermic reactions?
energy is taken in from the surroundings so the temperature of the surroundings decreases
give examples of typical endothermic reactions.
electrolysis, thermal decomposition reactions and the first stages of photosynthesis
give examples of where endothermic reactions are used in everyday life.
cold packs used in sports injury treatment
describe the practical to perform a calorimetric study of the reaction between HCl and NaOH.
- using a measuring cylinder, place 25 cm3 of the NaOH into the calorimeter.
- measure and record the temperature of the solution.
- add 5cm3 of the dilute HCl and stir, lid should be placed on the calorimeter as soon as the acid is added to minimise heat loss to the surroundings
- measure and record the highest temperature reached by the mixture.
- repeat steps 1 – 4 increasing the amount of acid added by 5cm3 each time.
conc. the larger the difference in the temperature the more energy is absorbed or released.
what is activation energy?
the minimum quantity of energy required to start a reaction - the amount of energy needed to break the bonds of the reactants
what does a reaction profile for an exothermic reaction look like?
energy increases when bond breaking then decreases by more than it increased when bond making - reactants are at a higher energy than products
more energy is released when new bonds are formed than energy required to break the bonds in the reactants
what is enthalpy change?
the difference in energy between reactants and products
what is the enthalpy change like in an exothermic reaction?
always negative bc heat/energy is given out to surroundings
what does a reaction profile for an endothermic reaction look like?
energy rises as bonds break, energy decrease when making bonds, but the decrease is lower than the increase, so the energy of the products is higher than the energy of the reactants
more energy is required to break the bonds than that gained from making the new bonds
what is the enthalpy change like in an endothermic reaction?
always positive bc energy is taken in from surroundings; this increase in energy is also known as the heat input
what kind of reaction is bond breaking?
endothermic - energy is needed so is absorbed from the surroundings
what kind of reaction is bond making?
exothermic - energy is transferred back to the surroundings
how do you calculate energy change?
energy change = energy taken in (add together all the bond energies for all the bonds in the reactants) - energy given out (add together the bond energies for all the bonds in the products)
what is bond energy?
the amount of energy required to break the bond or the amount of energy given out when the bond is formed
what is the structure of a simple cell?
take two different metals (electrodes) with different reactivities, place them in an electrolyte (solution that can conduct electricity) and connect them to an external voltmeter by wire, creating a complete circuit.
why can a cell only produce electricity for a certain period of time?
eventually, the chemicals in the cell will run out and reaction will stop - ions in the electrolyte and the metal ions at the electrodes are converted into products
how do simple cells work?
the more reactive electrode will form ions for easily and release electrons; the electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes; electrons then flow around the circuit to the other electrode which is now the more positive electrode; difference in ability of electrodes to release electrons causes a voltage to be produced; the greater the difference in the metals reactivity, greater the voltage produced; the electrolyte used also affects the voltage - different ions react with electrodes in different ways
what are batteries?
contains 2 or more cells connected in series to produce a greater voltage
how do rechargeable batteries work?
the reactions are reversed by connecting the cells to an external electrical supply, recharges the battery which can then be used again
what are the adv. and dis-adv. of a rechargeable battery?
adv. : saves money; decreases waste, lasts longer
dis-adv. : preparation required; not versatile; classified as hazardous waste
what are the adv. and dis-adv. of a non-rechargeable battery?
adv. : convenient; cost less per battery; ready to use; high energy density; relatively versatile
dis-adv. : waste and pollution
what is a fuel cell?
an cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
how does a fuel cell work?
the fuel enters the cell becoming oxidised which sets up a potential difference or voltage within the cell
what is produced in a hydrogen-oxygen fuel cell?
energy, water
at the anode in a fuel cell, what reaction takes place (in acidic conditions)?
4H+ + O2 + 4e– -> 2H2O
at the cathode in a fuel cell, what reaction takes place (in acidic conditions)?
H2 → 2H+ -> 4e–
at the anode in a fuel cell, what reaction takes place (in alkaline conditions)?
H2 + 2OH -> 2H2O + 2e-
at the cathode in a fuel cell, what reaction takes place (in alkaline conditions)?
O2 + 2H2O + 4e- -> 4OH
what are the adv. of a hydrogen fuel cell?
do not produce pollution; easy to maintain as there are no moving parts; small size; water is the only chemical product
what are the dis-adv. of a hydrogen fuel cell?
very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas; high pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle
