energy changes Flashcards
define an exothermic reaction
a reaction that transfers energy to the surroundings, so the temperature of the surroundings increases. therefore, the product molecules have less energy than the reactants
examples of exothermic reactions
- combustion
- many oxidation reactions
- neutralisation
describe reaction profile of exothermic reactions
- reactants are at a higher energy than the products because energy has been transferred from the molecules to the surroundings (energy is lost to the surroundings)
- negative energy change
everyday uses of exothermic reactions
- self-heating cans
- hand warmers
define an endothermic reaction
a reaction that takes in energy from the surrounds so the temperature of the surroundings decrease. therefore, the reactant molecules have less energy than the products
examples of endothermic reactions
- thermal decompositions
- reaction of citric acid and sodium hydrogencarbonate
describe reaction profile of endothermic reactions
- reactants are at a lower energy than the products because energy is taken in from the surroundings to the molecules (temperature of surroundings decrease)
- positive energy change
everyday uses of endothermic reactions
- sports injury packs (cold packs)
define activation energy
the minimum amount of energy that particles must have to react (for a reaction to occur)
what for reaction profiles show
- the relative energies of reactants and products
- activation energy
- if its exothermic or if its endothermic
- overall energy change
when is energy supplied and released during a chemical reaction
- energy must be supplied to break bonds in the reactants
- energy is released when bonds in the products are formed
what is bond energy
it’s the energy needed to break bonds and the energy released when bonds are formed
how do you calculate overall energy change
- calculate the bond energies for the reactants and for the products
energy change = energy of bond breaking - energy of bond making
i.e. bond energy of reactants minus bond energy of products
what are the units for overall energy change
kJ/mol
define overall energy change
the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed
describe making and breaking bonds in an exothermic reaction
the energy released from forming new bonds is greater than the energy needed to break existing bonds i.e. overall energy change will be negative
describe making and breaking bonds in an endothermic reaction
the energy needed to break existing bonds is greater than the energy released from forming new bonds i.e. overall energy change will be positive
define an electrolyte
a solution that can conduct electricity e.g. an ionic compound solution
how can you produce electricity using an electrolyte
by taking two different metals and placing them into an electrolyte
what do cells contain
chemicals which react to produce electricity
how is a simple cell made
it can be made by connecting two different metals in contact with an electrolyte
what is the potential difference produced by a cell dependent on
- type of electrode
- type of electrolyte
- concentration; the greater the concentration of ions in the electrolyte, the greater the potential difference produced
how do you provide a greater voltage
by connecting two or more cells together in series (batteries do this)
when will cells produce electricity
when there is a difference in reactivity between the two metals e.g., if they are the same metal, there will be no difference in reactivity, so no current will flow
how does difference in reactivity relate to potential difference
the greater the difference in reactivity between the two metals, the greater the potential difference produced by the cell. e.g., using magnesium and copper will produce a greater potential difference than using iron and copper
what happens in non-rechargeable cells and batteries
the reaction is irreversible, so the chemical reactions stop when one of the reactants has been used up (i.e. the limiting reactant), meaning no more electricity is produced
how do rechargeable cells and batteries work
the reaction is reversible, so when an external electrical current is supplied, the chemical reaction is reversed and the reactants are produced again
why does the more reactive metal transfer electrons to a less reactive metal. use the example of zinc and copper electrodes
because more reactive metals have a higher tendency to lose electrons to form positive ions e.g., the zinc electrode will lose electrons (oxidise) to the copper electrode which gains these electrons (reduced). because the zinc is the source of the electrons, the more reactive metal is the negative electrode, and the less reactive metal is the positive electrode
example of a non-rechargeable battery
an alkaline battery
what are fuel cells supplied by
an external source of fuel (e.g. hydrogen) and oxygen or air
how do fuel cells produce a potential difference
the fuel is oxidised electrochemically within the fuel cell to produce a potential difference
function of a hydrogen fuel cel
the overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water
what do hydrogen fuel cells offer
a potential alternative to rechargeable cells and batteries
cons of rechargeable batteries and cells in relation to fuel cells
- rechargeable batteries store less electricity the more charging cycles they go through, so eventually need to be replaced, however hydrogen fuel cells do not get less efficient the longer they run
- batteries store less energy than fuel cells, so need to be recharged more often
- recharging batteries takes a long time, whereas fuel cells take a very short amount of time to refill with hydrogen
pros of rechargeable cells and batteries in relation to fuel cells
- fuel cells run on hydrogen, which is an explosive gas if exposed to a flame, and is very difficult to store safely. no dangerous fuels are required with rechargeable batteries that are manufactured correctly
- hydrogen fuel cells produce a relatively low potential difference so several are needed together, whereas rechargeable batteries produce a greater potential difference
- hydrogen is a gas so takes up way more space to store than rechargeable batteries
fuel cell reaction of hydrogen and oxygen; what does this reaction do
2H₂ + O₂ → 2H₂O
it releases energy in the form of an electrical current and the only waste product it releases is water
what happens at the negative electrode of a fuel cell; mention the half equation
hydrogen molecules lose electrons (are oxidised) to form H⁺ ions; we can represent this using
2H₂ → 4H⁺ + 4e⁻
what happens to the electrons lost from the hydrogen molecules at the negative electrode of a fuel cell, and what happens to the ions formed
the lost electrons pass through the wire to form the electrical current. the H⁺ ions produced from the oxidation of hydrogen molecules are then passed through the electrolyte and to the cathode where they react with oxygen to form water
what does the electrolyte not/let through it
it doesn’t let hydrogen or electrons through, it only allows H⁺ ions (protons) through it
what happens at the positive electrode of a fuel cell; mention the half equation
oxygen molecules gain electrons (reduced) and react with the H⁺ ions to form water; we can represent this using
O₂ + 4H⁺ + 4e⁻ → 2H₂O
how are hydrogen fuel cells different from electrolysis, and how are they the same
DIFFERENCE: in electrolysis, the negative electrode is the cathode and the positive electrode is the anode. in hydrogen fuel cells, it is the opposite
SIMILARITY: the anode is always the site of oxidation and the cathode is always the site of reduction
where does each thing come from in a hydrogen fuel cell
- the H⁺ ions come from the oxidation of hydrogen at the anode and have travelled through the electrolyte
- the electrons have come from the electrical circuit
what is oxidised, what is reduced and where in a hydrogen fuel cell
- hydrogen is oxidised at the anode (the negative electrode)
- oxygen is reduced at the cathode (the positive electrode)
