chemistry topic 5 energy changes Flashcards
whats an exothermic reaction ?
a reaction which transfers energy to the surroundings usually by heating. this is shown by a rise in temperature
what reactions are exothermic?
combustion, neutralisation and oxidisation
what everyday objects provide exothermic reaction?
hand warmers and self heating cans
what is an endothermic reaction?
one which takes in energy from the surrounding. this is shown by a fall in temperature
whats an example of an endothermic reaction?
thermal decomposition
what everyday objects provide an endothermic reaction?
sports injury packs
how can you measure energy transfer in a practical?
taking the temperature of reagents, putting them in a polystyrene cup, mixing them and then measuring the temperature at the end
what is the biggest problem when measuring energy transfer?
energy can be lost in the surroundings
how can you reduce energy loss when measuring energy transfer?
put a lid on the cup and put more insulation in the cup e.g. cotton wool
what is bond breaking ? exothermic or endothermic?
endothermic
why is bond breaking an endothermic reaction?
energy must be supplied to break existing bonds
what is bonds formed? endothermic or exothermic?
exothermic
why is bond formation an exothermic reaction?
energy is released when new bonds are formed
in exothermic reactions the energy released by forming bonds is ?
greater than the energy used to break them
in endothermic reactions the energy used to break the bonds is?
greater than the energy released by forming them
in an exothermic reaction profile where are the products compared to the reactants?
the products are at a lower energy than the reactants
in an exothermic reaction profile what does the difference between products and reactants mean?
the overall energy change in the reaction
in an exothermic reaction profile what does the first rise in the line represent?
the energy needed to break the old bonds and start the reaction
in an exothermic and endothermic reaction profile what is the first initial rise called?
the activation energy
in an exothermic and endothermic reaction profile what is the activation energy?
the minimum amount of energy the reactants need to collide with each other and react
in an endothermic reaction profile where are the products compared to the reactants?
the products are at a higher energy than the reactants
in an endothermic reaction profile, what does the overall energy change mean?
the overall energy taken in
what is the equation for overall energy change?
overall energy change= energy required to break bonds-energy released by forming bonds
when finding bond energies what do you do?
add up the energies of your products and reactants then take them away reactants - products
what is an electrcochemical cell?
a basic system made up of two electrodes in contact with an electrolyte
exothermic endothermic practical: what do you prepare in separate beakers?
25 cm cubed of hydrochloric acid (o.25 mol/dm3) and sodium hydroxide
exothermic endothermic practical: what do you do with your beakers at the start?
place the beakers in a water bath set to 25 degrees until they are the same temperature
exothermic endothermic practical: what do you do after the beakers are the same temperature?
add the HCl followed by the NAOH to a polystyrene cup with a lid
exothermic endothermic practical: how often do you take the temperature and what do you record?
every 30 seconds and record highest temperature
exothermic endothermic practical: what do you repeat the practical using?
0.5 mol/dm3 and 1 mol/dm3 of hydrochloric acid
what must the two electrodes be able to do in an electrochemical cell?
conduct electricity
what is the electrolyte in a electrochemical cell?
a liquid that contains ions which react with other electrodes
what do the chemical reactions between the electrodes and the electrolyte set up in an electrochemical cell?
charge differences between the electrode
if the electrochemical cell has electrodes connected by wire, what can charge do?
flow and electricity Is produced
what can be connected to an electrochemical cell to measure volts?
voltmeter
the bigger the difference in reactivity of the electrodes, the bigger the what?
voltage of the cell
how will the electrolyte used affect the voltage?
different ions will react differently with the electrodes used
how is a battery formed?
by connecting two or more cells together in series
what kind of reaction happens in a non rechargeable battery?
irreversible one
what happens over time to reacting particles in a non rechargeable cell?
the ions in the electrolyte and electrode get used up and turn into products for the reaction
once the reactants are all used up in non rechargeable batteries what happens?
reaction cant happen and no electricity is produced as reaction cant be reversed
how do rechargeable batteries work again?
the reaction can be reversed by connecting it to external electric current
what is the chemical name for a non rechargeable battery?
alkaline battery
what is a fuel cell?
an electrical cell supplied with fuel and oxygen- it uses energy from the reaction between them to produce electrical energy efficiently
when fuel enters a fuel cell what happens?
it becomes oxidised and sets up a potential difference within the cell
whats an example of a fuel cell?
hydrogen- oxygen fuel cell
what happens in a hydrogen oxygen fuel cell?
it combines hydrogen and oxygen to produce nice clean water and energy
how are hydrogen-oxygen fuel cells used in vehicles?
conventional fuels like petrol are finite and very polluting therefore vehicles use electrical energy to power them
advantages of hydrogen-oxygen fuel cells: what don’t they produce?
many pollutants (carbon monoxide carbon dioxide etc)
advantages of hydrogen-oxygen fuel cells: what are the only by-products of them?
water and heat
advantages of hydrogen-oxygen fuel cells: electric vehicles car cells don’t produce many pollutants however what is it like to dispose of them ?
more polluting because they’re made from highly toxic metal compounds
hydrogen-oxygen fuel cells: what is bad about the rechargeable electric batteries in cars?
theres a limit on how many times you can recharge them before they need replacing
of hydrogen-oxygen fuel cells: what is the cost like of a rechargeable cell?
they are more expensive than fuel cells
hydrogen-oxygen fuel cells: what don’t batteries store as much compared to fuel cells?
less energy- they need to be recharged more often
in a hydrogen-oxygen fuel cell what is the electrolyte often?
potassium hydroxide
where does hydrogen and oxygen go in a fuel cell?
hydrogen goes in the anode compartment and oxygen goes in the cathode comnpartment
what happens at thenegative electrode in a fuel cell?
hydrogen loses electrons to produce H+ ions. this is oxidation
where do the H+ ions move in a fuel cell?
they move to the cathode
what happens at the positive electrode in a fuel cell?
oxygen gains electrons from the cathode and react with h+ ions to make water. this is reduction
where do electrons flow in a fuel cell?
through an external circuit from the anode to cathpde (the electric current)
what is the overall reaction for a hydrogen- oxygen fuel cell?
hydrogen + oxygen = water
how is a fuel cell a redox reaction?
theres reduction at cathode and oxidation at anode
when something is oxidised where are the electrons in the half equation?
last thing
when something is reduced where are the electrons in the half equation?
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