Chemistry - Energy Changes

What is the mass-mr-moles equation

Mass = mr x moles

How to find a concentration in mol/dm3

Concentration = moles/volume

From cm3 to dm3

Divide by 1000

How to go from mol/dm3 to g/dm3

Conc(g) = conc(mol) x mr

The bonds between reactants must be made/broken in a reaction

Broken

Breaking bonds is endo/exothermic

Endothermic

The bonds of a product must be made/broken in a reaction

made

Making bonds is endo/exothermic

exothermic

How to calculate energy change in a reaction

Draw out the equation with lines (eg. O=O)
Look up each type of bond in the data sheet
Multiply energy by the number of that bond
Do this for every bond
Add reactants. Add products
Reactant energy - product energy

If the energy given out when forming/breaking bonds is is greater than the energy needed to form/break bonds, then the sign of the answer will be postive/negative, and the reaction will be exothermic

Forming, break, negative, exothermic

If the energy given out when forming/breaking bonds is is less than the energy needed to form/break bonds, then the sign of the answer will be postive/negative, and the reaction will be endothermic

Forming, break, positive, endothermic

What is the first straight bit of the energy profile diagram

The energy held in the bonds of a reactant

What is the steep bit going up on an energy profile diagram

The energy given to a reaction e.g. by a flam

What is the steep bit going down on an energy profile diagram

The reaction is occuring

What is the last straight bit on the energy profile diagram

The energy held between the bonds of products

Energy profile diagram: if the products bit is higher than the reactants bit, then…

The reaction is endothermic

Energy profile diagram: if the products bit is lower than the reactants bit, then…

The reaction is exothermic

What is a limiting agent in a reaction

The reaction that there is the least of. This is because, when this is used up, the reaction ends, even if there is still some of the other reactant(s)

How to calculate the energy change in a reaction

Q=mcΔt
Energy = mass x sp.heatcap x temp change
*if there is no mass given, use volume in ml, assuming density is 1
Then
Change in energy = energy/moles of limiting agent

How to make a cell

Connecting two different electrodes with a wire and placing them in an electrolyte (often a voltmeter as well to measure potential difference)

What is an electrolyte

A liquid through which charged particles can flow

Why are the electrodes made of metal

So they can conduct electricity

What is a battery

A number of cells connected in series to create a greater voltage

Factors affecting the voltage of a cell

Difference in reactivity between the two metals (greater difference = greater reactivity)
Type and concentration of the electrolyte
Conditions like temperature

How do rechargeable batteries work? Where might you find them?

The chemical reactions in them can be reversed when they are supplied with an external electrical current. They are found in laptops and phones

How do non-rechargeable batteries work? Where might you find them?

The reaction stops when one of the reactants has been used up, which means the battery runs out. You may find them in smoke alarms and tv remotes (alkaline batteries)

In a cell the BLANK reacts with the BLANK

Electrodes, electrolyte

State the reactivity series

Potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, nickel, tin, lead, hydrogen, copper, mercury, silver, gold, platinum

Police Sergeant Little Charlie MAZINTL (he) Caught Me Stealing Gold Plate
When you’re stuck on carbon don’t forget, it's between the start and the end of the alphabet
Hydrogen is between lead and copper

What gives a bigger voltage: Magnesium + copper cell or a magnesium + zinc cell and why?

Magnesium and copper as there is a greater difference in reactivity

The BLANK the reaction between the electrolyte and the electrodes, the greater the voltage

Faster

If 3 cells connected in series have a voltage of 1V each, how big is the voltage of the battery

3V

What does a fuel cell need to produce electricity

Oxygen and fuel

What does a hydrogen oxygen fuel cell use? What does it produce

Needs: hydrogen and oxygen
Produces: water

Hydrogen
Electron flow
Oxygen
Water
Cathode
Electrolyte
anode

Is the anode positive or negative

Negative (andy is a really pessimistic guy)

Is the cathode positive or negative

Positive (cathy is very optimistic)

What are the electrodes made of

Porous carbon and a catalyst

How does a hydrogen fuel cell work with equations (full description0

Hydrogen enters from the left side and is oxidised by the anode (this means it loses its electrons
Hydrogen is now H+, a positive ion, (H2 = 2H+ + 2e-)
The electrons pass through the wire to the cathode
Hydrogen ions move through the electrolyte to the cathode
The oxygen gains 2 electrons and reacts with hydrogen (O2 + 4H+ + 4e- → 2H2O)
Water leaves the fuel cell via the outlet

What are the two equations in a hydrogen fuel cell called (hint: this name describes both of them)

Half equations

In a fuel cell, hydrogen goes through BLANK

Oxidation

In a fuel cell, oxygen goes through BLANK

Reduction

How is electrical energy created in a fuel cell

Through the electrons moving through the wire at the top

Describe how a fuel cell works (exam words)

The fuel is oxidised, which sets up a potential difference across the cell, where the anode and the cathode have different charges. This drives the electrons to move through the wire, generating electrical energy

Where would hydrogen fuel cells mostly be used

Car batteries

Pros of hydrogen fuel cells

Only uses hydrogen and oxygen, which are abundant
Only produces water, (no toxic waste products like how normal car batteries produce CO2)
Simple device which is not very polluting to dispose of and lasts longer than normal batteries

Cons of hydrogen fuel cells

Hydrogen is a gas, which means that is takes up a lot more space to store than fossil fuels
It is explosive when it reacts with air, so is dangerous to store
Making hydrogen fuel requires a lot of energy in itself, and this is normally obtained from fossil fuels

Pros of alkaline cells

Cheap to make and easy to use

Cons of alkaline cells

Often end up in landfill after only one use

Pros of rechargeable cells

Can be used over and over again

Cons of rechargeable cells

Expensive to make and can take a long time to recharge