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C7 Energy Changes (Exothermic and Endothermic Reactions (Required…
C7 Energy Changes
Exothermic and Endothermic Reactions
Exothermic Reactions
reactions that transfer energy from the reacting chemicals to its surroundings
release energy that heats up the surroundings
a rise in temperature
Examples
Combustion
burning fuels
Oxidation reactions
respiration
keeps temperature of warm-blooded animals steady
Neutralisation reactions
acids and bases
the products have less energy than the reactants
Uses
Hand Warmers
Single Use
oxidation of iron
iron into hydrated iron(III) oxide
similar to rusting
catalysed by sodium chloride(common salt)
disposable
lasts for hours
Reusable
reversible reaction of salt crystalisation
sodium ethanoate
supersaturated salt solution
dissolving as much salt into hot water then allow to cool
pressing a small metal disc starts reaction
small metal particles scrape off and start crystalisation
reuse by re-dissolving the crystals in boiling water
ready to activate again once cooled
only lasts 30 mins
Self-heating Cans
for foods or drink
no external heating device needed
calcium oxide + water → calcium hydroxide
breaking a seal allows two substances to mix and starts reaction
hard and expensive to develop
lots of space taken by reactants for reaction
temperature rise not high enough when cold
Endothermic Reactions
less common than exothermic reactions
reactions that transfer energy from the surroundings to the reacting substances
take in energy to cool down surroundings
a fall in temperature
Examples
Thermal Decomposition
require a supply of energy to keep the reaction going
breaks down compounds by absorbing energy
calcium carbonate into calcium oxide and carbon dioxide
citric acid and sodium hydrogen carbonate
products have more energy than reactants
Uses
Chemical Cold Packs
ammonium nitrate and water
when they mix, ammonium nitrate dissolves in water
kept separate in the pack
reduces swelling and numbs pain for injuries
included in first aid kit
packs are single use
reaction is reversible but not in the pack
lasts for 20 mins
Chill Drink Cans
ammonium nitrate and water
Law of Conservation of Energy
energy is conserved in chemical reactions
total energy before and after reaction remains the same
energy cannot be created or destroyed
Required Practical
Investigating Temperature Changes
measure 30cm³ of dilute acid using a measuring cylinder
transfer the acid into a polystyrene cup (calorimeter) which insulates and reduces heat loss
stand the calorimeter (heat measuring device) inside a beaker to stop the cup falling over
measure and record the initial temperature of the acid using a thermometer
measure 5cm³ of dilute alkali solution using a measuring cylinder and add to the polystyrene cup
add a plastic lid (to reduce heat loss to air) to the cup and place a thermometer through the lid's hole
make sure thermometer's bulb is in the solution and use thermometer to gently stir the solution
record the highest temperature reached (or lowest if endothermic reaction) and calculate the temperature rise
repeat method for different volumes of alkali solution to see effects on temperature change
repeat entire experiment again to obtain 2 sets of results and calculate mean temperature rise for each volume
as volume of alkali increases, more alkali reacts with acid to release energy, so max. temperature reached increases
at a certain volume of alkali solution, the maximum temperature reached starts to decrease
energy released has reached maximum because all the acid has reacted
as greater volumes of alkali added, energy released is
spread out
in a greater volume, so max. temperature falls
Reaction Profiles
Endothermic :blue_heart:
difference in energy of reactants and products show the amount of energy
taken in
products shown at a higher energy level on the diagram
Exothermic
:<3:
difference in energy of reactants and products show the amount of energy
released
products are at a lower energy level on the diagram
show the relative amounts of
energy
contained in reactions and products
measured in kilojoules per mole (kJ/mol)
curved line shows progress of reaction
up shows energy going in
down shows energy given out
difference in energy level of the reactant and product is the
overall energy change
of reaction
Activation Energy
the minimum energy required to start a reaction
shown as the difference in energy between the reactants and the peak of the curve
Bonds
Breaking and Making
then new chemical bonds are formed to make products
when new bonds form, energy is released to the surroundings
exothermic
process
in chemical reactions, bonds in the reactants are broken
breaking bonds requires energy from the surroundings
endothermic
process
in reality bond breaking and making happen at the same time
not all bonds are broken before all new bonds form
Bond Energy Calculations
comparing energy needed to break bonds and the energy released from making bonds gives us the
overall energy change
we can decide if the reaction is endothermic or exothermic
Exothermic :<3: : energy released from making bonds is greater than energy need to break bonds
Endothermic :blue_heart: energy needed to break bonds is greater than energy released from making bonds
the same amount of energy is involved in breaking and making a particular bond
the energy needed to break the bond between two atoms is called the
bond energy
for that bond
bond energy is measured in kJ/mol
≡ triple and = double bonds are stronger than – single bonds
using the balanced equation, write/draw out how many of each bond is present in the reactants and products (use the BIG multipliers)
using the table of bond energies provided, calculate the total amount of energy needed to break all of the bonds in the reactants
then calculate the total amount of energy released in making all of the bonds in the products
the difference between the two totals is the
overall energy change
of the reaction
if the value is -ve, more energy is released than taken in so the reaction is
exothermic
negative so 'lose' (release) energy
if the value is +ve, more energy is taken in than released so the reaction is
endothermic
positive so 'gain' (take in) energy
Overall energy change
=
Energy taken in
-
Energy released
total
reactant's
bond energy
minus
total
product's
bond energy
Chemical Batteries
electrical cells
can be made using the difference in reactivity of two metals
a battery is two or more cells joined together
to increase the voltage :zap:
join two metals together by a wire and dip them into an electrolyte (e.g. salt solution or dilute acid)
electrons will flow through the wire from the more reactive metal to the less reactive metal
the more reactive metal donates electrons via the wire to the less reactive metal
the more reactive metal is the negative terminal
electrons flow from negative to positive
the more reactive metal has a greater tendency to give away electrons and form a positive ion
the
reducing agent
loses electrons and are oxidised
the less reactive metal will accept the donated electron and turn into atoms
the
oxidizing agent
gains electrons and are reduced
the flow of electrons is an
electric current
the current will flow until one of the metal reactants is used up
the greater the
difference in reactivity
of the two metals used, the higher the voltage produced
the greater the voltage, the bigger the difference in the metal's power to give away electrons
the voltage produced in a cell also depends on the
electrolyte
Non-rechargeable Batteries
Primary Cells
zinc-carbon dry cell
1.5V
prone to leakage if left in device too long
alkaline cells
produce a larger voltage
once one reactant runs out, cell stops working
disposed of at recycling centre
Rechargeable Batteries
can be recharged and used again
cell is connected to a power supply to reverse the reactions at each electrode when cell discharges
original reactants regenerated
Secondary Cells
Fuel Cells
using hydrogen as a fuel
Advantages
produces no pollutants
only waste product is water
a source of drinkable water
reduce global warming
alternative to fossil fuels
hydrogen burns well
in combustion engines
used in fuel cells
can be a range of sizes for different uses
do not need to be electrically recharged
alternative to rechargeable cells
they run with a constant hydrogen supply
do not get less efficient the longer they run
do not need to be replaced like rechargeable batteries
Disadvantages
producing hydrogen using electrolysis requires electricity
may come from non-renewable resources
hydrogen is highly flammable
hydrogen is difficult to store
fuel cells produce a relatively low voltage (than batteries)
several are needed together
Alkaline Fuel Cells
an efficient use of the energy from
oxidising hydrogen
(fuel)
most of the energy released is transferred into electricity
fed with hydrogen and oxygen and produce water
hydrogen gas is supplied to the negative electrode
diffuses through graphite electrode and reacts with hydroxide ions
forms water
provides a source of electrons to an external circuit
2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻
oxygen gas is supplied to the positive electrode
diffuses through graphite electrode and reacts with water
form hydroxide ions
accepts electrons from the external circuit
O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
overall reaction when two half equations combine and cancel out
2H₂(g) + O₂(g) → 2H₂O(l)
has an alkaline electrolyte
potassium hydroxide solution
