Chemistry - Energy Changes Flashcards
exothermic reactions
Energy from the reacting chemicals is transferred to the surroundings, which often increases in temperature as a result
examples of exothermic reactions
combustion
neutralisation
oxidation
endothermic reaction
energy from the surroundings is transferred to he reacting chemicals, causing the temperature of the surroundings to decrease
examples of endothermic reactions
thermal decomposition
photosynthesis
how do reusable hand warmers work
use the energy released by iron oxidation. they source their energy from the crystallisation of salt solutions. Boiling the pack re-dissolves the salt solutions so it is ready to be activated once more
how do sports injury packs work
when squeezed tightly ammonium nitrate and water mix in the pack causing instant cooling.
requirements for a reaction to happen
collisions-particles of the reactants have to collide
activation energy-the collisions that happen must take place with enough energy, the threshold amount of energy is called the activation energy
activation energy
the minimum amount of energy that reacting particles must have in order for a reaction to occur
if the total energy released by forming bonds is less than total energy required to break bonds then the reaction is
endothermic
bond making
releases energy - exothermic
bond breaking
needs energy-endothermic
how can the voltage output of a cell be changed
changing the type of electrode, the type of electrolyte or both
what does changing the electrolyte do
changes the reaction that happens at the electrodes resulting in the voltage output of the cells changing
what does changing the electrodes do
a greater difference in reactivity between the two electrodes will result in the cell having a greater voltage
what is a battery
a collection of more than one cell joined in a series.
how do you calculate the voltage of the battery
the sum of voltages of the voltages of its individual cell components
why is a battery disposable
when the chemicals of a battery have run out the battery can not be used anymore
what happens in non-rechargeable cells and batteries
irreversible reactions take place at the electrodes this means that electricity can no longer be produced as soon as one of the reacting metals has run out
what happens in rechargeable cells and batteries
connections to an electric current reverses the reaction that happens at an electrode
what are fuel cells
Fuel cells produce electrical energy using a reaction between an external fuel source (often hydrogen) and oxygen, the fuel is added to a cell and then there is a constant supply of oxygen. The fuel is oxidised and that creates a voltage
advantages of fuel cells
- compact and lightweight
- reliable
- less pollution than traditional methods
- high efficiency
in fuel cells what is the only waste product
water
anode half equation
hydrogen is pumped in at the anode. hydrogen ions are formed as hydrogen molecules are oxidised and lose electrons.
2H2 → 4H+ + 4e-
cathode half equation
Oxygen is pumped in at the cathode. water is formed as the oxygen reacts with the H+ ions that have moved through the electrolyte from the anode.
O2 + 4H+ + 4e- → 2H2O
