Energy profiles and bond energy Flashcards
Exothermic reactions
Where energy is transferred from the molecules ——> the surroundings
Temperature in exothermic reactions
The surrounding temperature increases
Molecule temperature decreases
Examples of exothermic reaction?
Combustion
Certain oxidation
Neutralisation
Energy profile diagram
Shows the increase and decrease of energy in a reaction
Exothermic energy profile
The reactants are placed higher than the products
Why are the reactants higher than the products in an exothermic energy profile?
Because energy has been given from the reactant molecules to the surroundings
Therefore the product molecules has less energy
Difference between reactant line and product line in exothermic reaction
The amount of energy given out into the surroundings
Uses of exothermic reactions
Hand warmers
Self heating cans eg food or drink
Endothermic reactions
Take in energy from the surroundings
Temperature change in endothermic reactions
Surroundings cool down because energy was taken in
Therefore the molecules temp increases because it takes it in
Examples of endothermic reactions
Thermal decomposition
Photosynthesis
Endothermic reaction profile
The reactants are lower than the products
Why are reactants lower than products in an endothermic reaction?
Because the reactants took in energy from the surroundings
Therefore the products have taken in more energy than the reactants had before
Difference between reactants and products in an endothermic energy profile diagram
The amount of energy the molecules took in from the surroundings
Where is the activation energy
From the peak down to the REACTANTS
Even if it’s endothermic or exothermic, always from the peak to the reactants
What is the activation energy?
For reactants to react, they must collide with each other
They need sufficient energy to do this
This minimum amount of energy needed = activation energy
If the energy decreases from the reactants to products (exothermic) what changes the answer?
It is a negative number
As you are decreasing a higher amount of energy (reactants) from the lower parts (products)
What would the energy change in an exothermic reaction be like?
Negative because the reactants have less energy than the products
Ie a loss in energy
What would the change in energy in an endothermic reaction be like?
Positive because the reactants have less energy than the products
Ie a gain in energy from the surroundings
Breaking a chemical bond
Needs energy (endothermic)
Because you gain energy from the surroundings
Making a chemical bond
Releases energy
(Exothermic)
Because temperature is released into the surroundings
Energy value
Something a chemical bond has
Tells us energy required to break the bond
How to find energy changes in a reaction?
Note down structures/ bonds of each molecules involved
See which bonds are broken, usually LHS
Add up energy values of LHS
Do the same for RHS
Number is positive if bonds are broken but negative if being made
Carry out calculation
What to remember when calculating the energy values of molecules?
Include the big number
Ignore the small number as that is just included in the structure
How to remember endothermic and exothermic energy transfers?
ENdothermic - ENter (energy is transferred from surroundings into to reacting substances)
EXothermic - EXit (energy is released by reacting substances out into the surroundings)
How to remember bonds broken and made in terms of exothermic and endothermic?
Bendo (B for bonds being Broken, endo for endothermic)
Mexo (M for bonds being Made, exo for exothermic)
In an exothermic reaction, which is greater: the energy released from forming new bonds, or the energy needed to break existing bonds?
More energy needed making bonds
MEXO
M= making bonds
Exo = energy released into surrounds
Thus energy released making new bonds
Electrolyte
Solution that can conduct electricity
For example an ionic compound solution in electrolysis
How can we make electricity
Placing 2 different metals in an electrolyte
Components of a cell
2 different strips of metals in an electrolyte solution
Connected together with a voltmeter
Therefore a current will flow between them
How do cells conduct electricity?
A chemical reaction is happening on the surface of the metals, producing electricity
Properties of a cell
Can only produce electricity for a period of time because the reactants run out / reaction stop
Only produces electricity if we use metals with different reactivity
What determines the voltage produced by the cell?
The difference in reactivity between the metals
Greater difference eg Mg and Cu = high V
Lower difference eg Zn and Fe = low V
The electrolyte
Components of a battery
2 or more cells connected in series in order to make a greater voltage
What are non rechargeable batteries?
Alkaline batteries
The reactions in the electrolyte + metal in the cells aren’t reversible so therefore not rechargeable
What are rechargeable batteries?
Batteries that have reversible reactions between electrolyte and the metal that can be reversed when a current is applied
Therefore reactants aren’t used up
Fuel cells
A type of cell
Hydrogen reacts with oxygen or pure air to produce an electric current
Water is the product and leaves the cell
What happens in a fuel cell in terms of electrons?
Hydrogen atoms lose electrons to form positive ions
These electrons join with the oxygen molecules and hydrogen ions to form water molecules
Half equation at the negative electrode (fuel cell)
2H₂——> 4H⁺ + 4e⁻
Half equation at the positive electrode (fuel cell)
O₂ + 4H⁺ + 4e⁻ ——> 2H₂ O
Overall equation of the fuel cell
2H₂ + O₂ ——> 2H₂O
What is happening to the hydrogen in a fuel cell?
It’s gaining oxygen
Therefore it’s oxidised
Advantages of a fuel cell
Produces elec as long as you provide hydrogen
Do not get less efficient the longer they run
Potential store of drinking water (waste product) eg in a space craft
Disadvantages of a hydrogen fuel cell
They can run out of hydrogen
Hydrogen is an explosive gas so difficult to store safely
Produce a relatively low voltage so need more than 1
Advantages of a rechargeable battery
No dangerous fuels needed
Can produce a larger voltage
Disadvantages of a rechargeable battery
Runs out and need to be recharged
More charging cycles = stores less electricity so eventually needs replacing
If not manufactured properly can catch fire
Hydrogen fuel cells vs rechargeable batteries
HFCs produces electricity so long as you
provide hydrogen, RBs needs recharging
HFCs don’t get less efficient as they go on, RBs do
RBs produce more voltage than HFCs
RBs contain no explosive chemicals like hydrogen, HFCs do
HFCs are much more expensive and difficult to store
