Studied by 0 people
Looks like no one added any tags here yet for you.
where is energy conserved?
energy is conserved in chemical reactions, so the total amount of energy remaisn the same as it was before.
Where is energy stored
Chemical bonds
What does each chemical bond have
Different amounts of energy in their chemical energy stores
where is energy transferred when a chemical reaction happens
to or from the surrounding
endothermic- energy is transferred from the surroundings
exothermic- energy is transferred to the surroundings
what is an exothermic reaction?
when thermal energy is released to the surroundings.
it causes the temperature of the surroundings to increase
What are examples of exothermic reactions
combustion reactions
oxidation reactions
most neutralisation reactions
what are examples of everyday uses for exothermic reactions
self heating cans
hand warmers
what is an endothermic reaction
when thermal energy is taken in from the surroundings
the temperature of the surroundings decrease
if happened inside a closed system energy would decrease
what are examples of endothermic reactions
thermal decomposition
the reaction of citric acid and sodium hydrogencarbonate
what are everyday uses of endothermic reactions?
instant ice packs which can be used in sport injuries
What is activation energy
The minimum amount of energy that particles need in order to collide with each other and react
temperature changes in reacting solutions (R.P), what is the aim and apparatus
aim= investigate variables that affect temperature change in reacting solutions
apparatus= polystyrene cup, thermometer, 250cm3 glass beaker, measuring cylinder, top pan balance.
temperature changes in reading solutions (R.P) context
you could investigate one or more chemical reactions, for example:
acids reacting with metals
acids reacting with metal carbonates
acids reacting with alkalis
displacement reactions of metals
temperature changes in reading solutions (R.P) method for reacting 2 solutions
e.g acid and alkali
place the polystyrene cup (insulator) inside the glass beaker to make it more stable.
measure an appropriate volume of each liquid, e.g 25cm3
place one of the liquids in a polystyrene cup
record the temperature of the solution
add the second solution and record the highest or lowest temperature obtained
change your independent variable. Your independent variable could be the concentration of one if the reactants, or the type of acid/ alkali being used, or the type of metal/ metal carbonate used.
temperature changes in reading solutions (R.P) method for reacting a solid with a solution
e.g metal and acid
place the polystyrene cup (insulator) inside the glass beaker to make it more stable.
measure an appropriate volume of solution e.g 25cm3
measure appropriate mass of solid, or select a suitable sized piece of metal
place the solution in a polystyrene cup
record the temperature of the solution
add the solid and record the highest or lowest temperature obtained
change your independent variable, and repeat the experiment. This could be the surface area of the solid or the type of acid being used or the type of metal being used
temperature changes in reading solutions (R.P) analysis
the bigger the temperature change, the more energy is absorbed or released. Remember that endothermic reactions absorb energy from surrounding and exothermic reactions transfer heat into the surroundings
what si the biggest error in temperature changes in reading solutions (R.P)
biggest source of error is unwanted heat transfer, using a lid can prevent this
what are hazards, risks and precautions of temperature changes in reading solutions (R.P)
hazard= dilute acid and alkalis
this can cause possible harm as it may irritate they eyes or skin so possible precautions should be to avoid any contact with skin so rinse off skin if necessary and wear eye protection
hazard= solutions of metal salts
this can cause possible harm as its dangerous to enviroment. The possible precaution is to disspose of metal salt solutions as directed by teacher as some cannot be poured down normal drain
what happens, in terms of bonds, during a chemical reaction
bonds are broken in the reactant
new bonds made in the product
the difference btween energy needed to break bonds and energy release new bonds is determained by what?
exothermic = more heat energy is released in making bonds in products than taken in when breaking bonds in reactants
endothermic= if less heat energy is released in making bonds in the products than taking in when breaking bonds in reactants
what is a bond energy
amount of energy needed to break one mole of a particular covalent bond
different bonds, have different what?
bond energies
Does breaking bonds require or release engery
requires energy
how do you calculate energy change for a reaction?
add together the bond energies for all the bonds in reactants (energy in)
add together the bond energies for all the bonds in products (energy out)
energy change = energy in- energy out
what are chemical cells
chemical cells use a chemical reactions to transfer energy by electricity.
what does the voltage of the cell depend on?
material of electrodes → the greater the difference in reactivity the greater the voltage will be
substance used and concentration for electrolytes
Conditions such as temperature
How do you make a cell
Connect 2 different electrodes with a wire and place them in contact with the electrolyte
Often a voltmeter to measure voltage of the cell
how can a simple cell be made
made by connecting two different metals in contact with an electrolyte.
what is a battery?
two or more cells connected together in series to provide a greater voltage
what happens in non rechargeable cells?
a voltage is produced until one of the reactants are used up, causing the battery to go flat
an example is alkaline batteries
what happens in rechargeable cells and batteries?
the chemical reactions can be reversed when an external current is suplied
What type of cell is a fuel cell
Electrochemical cell
→ converts energy between electrical energy and chemical energy
Chemical energy, fuel and oxygen, it turned into electrical energy used to power things
what do fuel cells produce
a continuous voltage, as long as its supplied with:
a constant supply of suitable fuel
oxygen, eg from air
what happens to the fuel
the fuel is oxidised electrochemically, rather than being burned, so the reaction takes place at a lower temperature than if if was burned.
in fuel cells, what is energy released as?
electrical energy , NOT thermal energy
what is an alternative to rechargeable fuel cells
hydrogen- oxygen fuel cells
hydrogen and oxygen are used to produce a voltage. Water is the only product.
What’s the structure of a hydrogen fuel cell
centre has the electrolyte → a solution that ions can move through. Most common type if potassium hydroxide
Electrodes on either side → anode on left (negative) and cathode on right (positive)
Electrodes connected by a wire → wire allows electrons to flow from the anode around to the cathode
On the outside of the electrodes you have the anode compartment and the cathode compartment
Summary of how fuel cells work
as fuel enters the cell, hydrogen enters (through the left inlet) the anode compartment and oxidised by the anode, SETTING UP A POTENTIAL DIFFERENCE ACROSS THE CELL
Each hydrogen atom loses an electron to become an ion
Due to potential difference there is a difference in charge tween the two electrodes, driving the electrodes around the circuit to generate electricity
Electrons pass around the wire to the cathode
The hydrogen ions move through the electrolyte to the cathode
On the right oxygen enters the inlet on the cathode compartment
The electrons and hydrogen ions react with the oxygen to form 2 water which leaves via heat through the outlet
What do the half equations show
Loss or gain of electrons
In fuel cells, what are the electrolytes alwats made of
Porous carbon
→ have lots of tiny holes
→ contains. Catalyst to speed up the reaction
What is the overall reaction in hydrogen- oxygen fuel cells
hydrogen + oxygen → water
2H2 + O2 → 2H2 O
what is the half equation at the negative electrode
H2 → 2H+ + 2e-
oxidation of hydrogen
Loss of electrons
what is the half equation at the positive electrode
O2 + 4H+ + 4e- → 2H20
pros and cons of alkaline cell
pros: cheaper to manufacture
cons: may end up in landfill sites once fully discharged, recyclable though is expensive
What’s 5he overall equation for a hydrogen fuel cell
O2 + 2H2 → 2H2O
pros and cons of rechargeable cell
pros: can be recharged many times before being recycled, reducing the use of resources
cons: costs more to manufacture
In fuel cells, where does this electrical energy come from
The movement of electrons through the wire, powered by the chemical reaction
pros and cons for hydrogen fuel cell
pros: easy to maintain as there are no moving parts; small size; water is only chemical product, no pollutants so less waste, last longer then batteries
cons: very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas, takes up large spaces to store, explosive when mixed with air, requires energy to make which often comes from fossil fuels
where are hydrogen- oxygen fuel cells used
in space craft as the water they produce water which useful as drinking water for astronauts
Note 
Flashcard (25)