Energy Changes Flashcards
what is an exothermic reaction
transfers energy to the surroundings e.g. combustion and neutralization reactions
what is an example of a use of an exothermic reaction
hand warmers
self heating cans
what is an endothermic reaction
takes in energy from the surroundings e.g. thermal decomposition
what is a use of endothermic reactions
a cold pack
how is the overall energy change shown on a reaction profile
difference between the energy of the reactants and energy of the products
what does an endothermic reaction profile look like
the product is higher than the reactant
why does the line go up to begin with on a reaction profile
this is the activation energy. energy required to break the bonds of the reactants.
what is activation energy
the minimum energy needed for the reactants to react when they collide
how many joules are there in a kilojoule
1000
what is the unit of energy transfer usually measured in
kJ/mol
is breaking bonds endothermic or exothermic
endothermic cus energy must be supplied to break the bonds
is bond forming exothermic or endothermic
exothermic because energy is released
if the energy required for bond breaking is more than the energy is released in bond making, will the reaction be exothermic or endothermic
endothermic
energy change = energy of bond breaking - ?
energy of bond making
what is a cell
a system that contains chemicals which react and produce electricity
how do cells work
it has two different electrodes and an electrolyte which is a liquid that can conduct electricity and contains ions that react with the electrodes
the reactions between the electrodes and electrolyte set up a charge difference between the electrodes . if the electrodes are connected by a wire the charge is able to flow and electricity is produced
how can you maximize the voltage of a cell
the bigger the difference in reactivity of the electrodes, the bigger the voltage
the concentration of ions in the electrolyte also impacts on the voltage
how is a battery formed
connecting two or more cells together in series
the voltage of a battery is what of the cells that it is made of
equal to the voltages of all the cells added together
are alkaline batteries rechargeable
no
what is a fuel cell
a cell that works on a fuel and oxygen. it efficiently transfers energy as electricity
how does a fuel cell work
fuel enters the cell at one side where it becomes oxidised and sets up a potential difference which powers the device. the air enters the cell at the other side where it reacts to form water
what is the electrolyte of a a hydrogen oxygen fuel cell
a solution of potassium hydroxide
in hydrogen fuel cells, what goes to the cathode (positive) and what goes to the anode (negative)
hydrogen goes to the anode
oxygen goes to the cathode
what happens at the anode (negative electrode) in a hydrogen fuel cell
oxidation.
hydrogen loses electrons to produce H+ ions and then the H+ ions move to the cathode
what happens at the cathode (positive electrode) in hydrogen fuel cells
reduction.
oxygen gains electrons, then reacts w H+ ions to make water
in a hydrogen fuel cell the electrons flow from the anode to the cathode through an external circuit
the overall reaction is redox reaction
2H2 + O2 = 2H2O
what is the half equation for the reaction at the anode of a hydrogen fuel cell
h2 = 2H + 2e-
what is the half equation for the cathode in a hydrogen fuel cell
o2 + 4H+ + 4e- = 2H2O
advantages of hydrogen fuel cells
don’t produce as many pollutants as other fuels- even electric cars cus there batteries are polluting to dispose of
fuel cells are cheaper than batteries and last longer
batteries need to be recharged more often
disadvantages of hydrogen fuel cells
hydrogen takes up more space to store
hydrogen is explosive when mixed with air so hard to store safely
hydrogen fuel is often made from hydrocarbons or by electrolysis of water which uses electricity
