ENERGY CHANGES Flashcards
In an exothermic reaction heat and energy is…..
given out
an exothermic transfers energy to its ,, how is this shown
surroundings this is shown by the increase in temperature
examples of exothermic reaction
combustion
neutralisation
oxidation
hand warmers(example of oxidation)
in an endothermic reaction heat and energy is….
taken in
an endothermic reaction takes in energy from….. and this is shown…
takes energy in from its surroundings and this is shown by a fall in temperature
examples of endothermic reactions
sports injury pack
thermal decomposition
in an exothermic energy profile where are the reactants and the products
products are lower than the reactants
in an endothermic energy profile where are the reactants and the products
the reactants are lower than the products
what is activation energy
activation energy is the minimum amount of energy the reactants need to collide with each other and react and to break the bonds
the greater the activation energy….
the more energy needed to start the reaction
what does the difference in height show in an energy profile
energy change during the reaction
during a chemical reaction….
old bonds are broken and new bonds are formed
energy must be supplied to break existing bonds so bond breaking is…
endothermic
energy is released when new bonds are formed so bond forming is….
exothermic
bond breaking=
endothermic
bond making=
exothermic
in exothermic reactions the energy _______ is ______ than the energy used to break them
in exothermic reactions the energy released is greater than the energy used to break them
in an endothermic reaction the energy _____ to break the bonds is________ than the energy released by forming them
in an endothermic reaction the energy used to break the bonds is greater than the energy released by forming them
BOND ENERGIES
NEGATIVE=
exothermic
BOND ENERGIES
POSITIVE=
endothermic
how to work out bond energies
- write out equation and balance it
- draw out everything you have
- work out the bond energies
- reactants minus products
- see if it is endo or exothermic
how do catalysts increase the rate of reaction
decrease the amount of activation energy needed
what is a cell
a cell is a basic system made up out of two different electrodes in contact with an electrolyte
what material are the electrodes
metal as they need to conduct electricity
what is an electrolyte
is a liquid that contains ions which react with the electrodes
chemical reactions in a cell produce…..
electricity
different materials will react ……. with the same electrolyte
differently
what effects the voltage of a cell
the difference In reactivities between the electrodes
the electrolyte
the bigger the difference in reactivity of the electrodes
the higher the voltage
how can the electrolyte used in a cell affect the size of voltage
the different ions in solution will react differently with the metal electrodes used
NON rechargeable batteries
over time the reacting particles, (the ions in the electrolyte and the metal ions in the electrode) gets used up. once the reactants are used up the reaction cannot happen and so no electricity is produced
the products cant be turned back into reactants so the cell cannot be recharged
example of non rechargeable batteries
Alkaline batteries- contains cells which use irreversible reactions
in a rechargeable cell
the reaction is reversible so once all the reactants have been used up when you connect it to a external electric current electricity will pass through reversing the reaction, recharging the reactants.
what do fuel cells use to produce electrical energy
fuel and oxygen
what is a fuel cell
a fuel cell is an electrical cell that’s supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
what happens when the fuel enters the cell
it becomes oxidised and sets up a potential difference within the cell
one type of fuel cell
hydrogen oxygen fuel cell
what type of reaction does a hydrogen oxygen fuel cell involve
redox reaction
experiment hydrogen fuel cells
- the electrolyte is often a solution of potassium hydroxide, the electrodes are often porous carbon with a catalyst
- Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
- at the anode hydrogen loses electrons to produce h+ ions (THIS IS OXIDATION)
- H+ ions in the electrolyte move to the cathode
- at the cathode oxygen gains electrons from the cathode and reacts with H+ ions from the electrolyte to make water (THIS IS REDUCTION)
- the electrons flow through an external circuit from the anode to the cathode - that is the electric current
advantages for using hydrogen oxygen fuel cells in cars
fuel cell vehicles don’t produce as many pollutants as other fuels as their only by products are water and heat
batteries are more expensive to make than fuel cells
theres a limit on how many times the batteries can be recharged before they need replacing
fuel cells store more energy than batteries do
batteries are more polluting to dispose of as they are made of highly toxic metal compounds
disadvantages for using hydrogen fuel cells in cars
hydrogen is a gas so it takes up loads more space to store than a rechargeable battery
hydrogen is explosive when mixed with air so its hard to store safely
half equation at the anode for hydrogen oxygen fuel cells
H2— 2H+ (+) 2e-
half equation at the cathode for hydrogen oxygen fuel cells
O2 + 4H+ (+) 4e- ——- 2H2O
in fuel cells the anode is ……
negative
in fuel cells the cathode is……
positive
What type of energy is released by hydrogen fuel cells
Electrical energy
Describe what must happen to the molecules of X and Y to allow them to react.
T
why can alkaline batteries not be recharged
the reaction is not reversible
a student used test tubes when he investigated the volume of gases produced during electrolysis its wrong
what should be changed
use measuring cylinders instead of test tubes because test tubes cannot measure volume
