energy changes Flashcards
each reactant all has
differents amount of chemical energy in their chemical energy store
what is on the y and x axis for a reaction profile diagram
the y axis is the energy
the x axis is the progress of the reaction
when the products have less energy than the reactants where is everything placed on the reaction profile and what type of reaction would it be
exothermic
1)reactants at the top left starting the graph but not touching y axis
2)products much lower down as they have lost a lot of energy but the start aligning with the end of the reactants
3)straight arrow line drawn down saying energy released
what is an example of exothermic reactions
combustion reaction
oxidation
neutralisation(base and acid)
what are examples of exothermic reactions
what would the reaction profile for an endothermic reaction look like
reactants would be low at the bottom left
products would be high starting at the end of reactants
there would be an up facing arrow drawn for energy absorbed
what is activation energy
the minimum amount of energy needed for the reactant particles to collide and react
how does the activation energy link to starting the reaction
the higher the activation the more energy required to start the reaction
how can we show the activation energy in the reaction profiles
by drawing a curve from products to reactants
the increase in energy from the reactants level to the highest point in our curve is called the activation energy
what is bond energy
the amount of energy needed to break one mole (6x10(23)of a particular covalent bond
is bond breaking endothermic or exothermic and why
it is endothermic as it requires energy from the surroundings
bond making is
exothermic-releases energy to surroundings
what is the equation for energy change
energy required to break bonds- energy released by forming bonds
or
energy of reactants - energy of products
what does a - energy change mean
the reaction is exothermic
as chemicals lose that value to surroundings
what 3 things happen in a chemical reaction
The bonds of the reactant molecules are broken
The atoms rearrange themselves
The bonds of the product molecules are formed
what are electrochemical cells
cell made by connecting 2 different electrodes and placing them in contact with an electrolyte all connected to a voltmeter
how do batteries compare electrochemical cells
they are 2 or more cells connected in series for a greater voltage
what 3 factors affect the voltage of a cell
-The metals used for the two electrodes. The greater the difference in reactivity of the two metals, the greater the voltage will be.
-The type and concentration of the electrolyte used.
-The conditions, such as temperature.
why are rechargable cells/batteries rechargable
Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied. These are the batteries used in laptops and mobile phones.
why are non rechargable cells not rechargable
chemical reaction stop when 1 reactant has been used up
eg:batteries used in smoke alarms or tv remotes
alkaline batteries are an example
why do electrochemical cells have their names
converts chemical energy to electrical
a fuel and oxygen into electrical energy which we can use as power
what do hydrogen fuel cells do
combine hydrogen and oxygen to form water while generating a lot of electrical energy
what is the structure of hydrogen oxygen cell
electrolyte(commonly potassium hydroxide) in between electrodes with anode (negative)on left and cathode(negative) at top connected by wire at the top
the electrodes are the opposite of electrolysis
what do fuel cells have/features
electrodes are made of porous carbon with catalyst
they have anode compartment outside electrode with an inlet at the top where hydrogen enters
cathode compartment inlet where oxygen enters
when the fuel cell reaction finishes what happens
the heat and water leave through the cathode compartment outlet
how do fuel cells work
hydrogen enters and is oxidised by anode and becomes a hydrogen ion
this equation is
h2—–> 2h+ +2e-
then the electrons flow around wire to the cathode
the hydrogen ions pass the electrode to the cathode
they react with the oxygen that comes in from the cathode inlet combining to make water
what is the fuel cell equation at the cathode
o2+ 4h+ +4e- ——-> 2h2o
when the electrons in a half equation are on the right /left
right is loss
left is gain
what is the overall equation
o2+ 2h2 —>2h2o
where does this electrical energy come from in a fuel cell
from the movement of electrons through the wire and is powered by the chemical reaction
how do fuel cells work
as fuel enters cell it gets oxidised setting up pd across cell
pros and cons of fuel cells
pros
-replace polluting fossil fuel engines and batteries we have in vehicles
-only need hydrogen and oxygen
-no waste pollutants
-simple devices
-last longer than batteries
-less polluting to dispose of
cons
-hydrogen takes up much more space than fossil fuels
-explosive in air
-requires energy to make which often comes from fossil fuels
