Energy Changes Flashcards
What do exothermic reactions do?
They transfer energy from the reacting molecules to the surroundings - surroundings get hotter
Examples of exothermic reactions
- Neutralisation
- Combustion
- Oxidation
- Freezing
What do energy profile diagrams show?
Scientists represent the energy changes that take place in reactions by drawing these
What are endothermic reactions?
They take in energy from their surroundings - surroundings get colder
Example of endothermic reactions
- Thermal decomposition
- Reaction between citric acid and sodium hydrogencarbonate
- Melting
What is the activation energy?
Minimum amount of energy the reactants need to collide with each other and react
What does a greater activation energy mean?
The greater the ae. the more energy needed to start the reaction - this has to be supplied
What is the method to measure energy transfer?
- Use the 50cm3 measuring cyclinder to put 30cm3 dilute hydrochloric acid into the polystyrene cup
- Stand the cup in the beaker. This will make it more stable
- Use the thermometer to measure the temperature of the acid. Record it
- Pour5cm3 sodium hydroxide in the measuring cyclinder
- Pour sodium hydroxide into the cup - gently stir with thermometer in the hole of lid
- Repeat - pour more 5cm3 amounts to the cup - total of 40cm3 needs to be added
- Repeat previous steps for second trial
- Calculate mean of max temp
Exothermic reactions in everday life examples
- Hand warmers - uses exothermic oxidation of iron in air to release energy
- Self heating cans
Endothermic reactions in everday life examples
- Sports injury packs
How can you reduce amount of energy lost to the surroundings?
- Putting polystrene cup into a beaker of cotton wool to give more insulation
- Putting a lid on the cup to reduce the energy lost by evaporation
What does height represent in reaction profiles?
Overall energy change in the reaction per mole
Is bonding breaking endothermic or exothermic, why?
Endo because energy must be supplied to break existing bonds
Is bonding forming endothermic or exothermic, why?
Exo because energy is released when new bonds are formed
How does the amount of energy related to bonds differ in exothermic reactions?
Energy released by forming bonds is greater than the energy used to break them
How does the amount of energy related to bonds differ in endothermic reactions?
Energy used to break bonds is greater than energy released by forming them
How do you work out the overall energy change?
Sum of energies needed to break bonds in the** reactants** - energy released when the new bonds are formed in the products
What is an electromechanical cell?
Basic system made up of 2 diff electrodes in contact with ann electrolyte
What causes the charge difference/ voltage of the cell?
Diff metals will react diff with the same electrolyte
What is produced when there is a greater difference in reactivity of the metals?
- greater pd produced by the cell
- electrolyte also affects pd
What else would affect the size of voltage in fuel cells?
Electrolyte used in a cell - diff ions in solution will react diff with the metal electrodes used
How is a battery formed?
2 or more cells together in series = greater voltage
Example of non-rechargable batteries
Alkaline batteries contain cells which use irreversible reactions - once 1 reactant is used up, they produce no more charge and replacement is needed
How do rechargable cells work?
- Can reverse the chemical reactions when we apply an electrical current
Whats a fuel cell?
- supply fuel = hydrogen
- react with pure o2 or air
- chemical reaction = electric current
- waste product = water
What happens to the fuel when it enters the cell?
Becomes oxidised and sets up a pd within the cell
What does a hydrogen-oxygen fuel cell do, type of reaction?
Combines O and H to produce water and release energy through a redox reaction
What does a H-O fuel cell consist of?
- Electrolyte - usually an acid (phosphuric aicd(
- Electrodes - porous carbon with a catalyst
- Negative anode and positive cathode
Process of H-O fuel cell
- H goes into anode compartment
- O goes into cathode
- At anode, H loses electrons to produce H+ ions - oxidation
- Electrons move through wire to the cathode
- H+ ions in the electrolyte move to cathode
- At cathode, O gains electrons and reacts with H+ ions from acidic eletrolyte to make water - reduction
- Electrons flow through an external circuit from the anode to cathode - electric current
Pros on fuel cells
- Not as many pollutants compared to other fuels - only by products are water and heat
- do not get less efficient the longer they run
- source of drinkable water
- will produce electricity for as long as you provide hydrogen
Cons on fuel cells
- H is a gas, takes up more space to store than a rechargable battery
- H is explosive when mixed with air - hard to store safely
- H fuel made from hydrocarbons (from fossil fuels) or electrolysis of water (electricity from ff)
- produce low pd so several are needed together
How do you know whether a reaction is mainly endo or exo thermic by using bond energies?
If the answer is negative, its exothermic as it has given that amount of energy to the surroundings
If its positive, its endothermic, has gained that muh energy from surroundings
What does bond energy mean?
Amount of energy required to break one mole of a particular covalent bond.
Fuel cell equation at negative electrode
2H₂ –> 4H⁺ + 4e⁻
Fuel cell equation at positive electrode
O₂+ 4H⁺ + 4e⁻ –> 2H₂0
Overall equation for fuel cells
2H₂ + O₂ –>2H₂0
hydorgen is oxidised
Practical Method: Temp changes
- MC for 30cm3 dilute HCL
- Transfer into polyesterne cup
- put into beaker - stops it falling over
- measure tem p of acid
- MC for 5cm3 sodium hydroxide and transfer into cup
- put a lid + thermometer into lid
- stir solution
- record temp rise + record highest temp reached
- Repeat, rinse and use 10cm3 sodium hydroxide solution
- increase vol of sodium hydroxide by 5cm3 until 40cm3
- repeat + mean value for max temp reached for each vol.
Describe what happens as you increase the volume of sodium hydroxide solution in HCL
- As you increase vol, max temp increases
- because they react with HCL - more energy released as its exo
- at a certain vol, max temp starts to decrease becaue there isnt enough HCl to react
Why do we use a polysterene cup and a lid?
reduce heat losses
poly = good thermal insulator
lid = reduces heat loss to air
How is electricity produced using metals?
2 diff metals, place them into an electrolyte
How long can a cell produce electricity for?
a certain period of time, chemicals in the cell will run out and reaction stops
When will cells only produce electricity?
if metals with diff reactivities are used
Cons of rechargable batteries
- run out and need to be recharged
- store less electricity the more charging cycles they go - need to be replaced
Pros of rechargeable batteries
- no dangerous fuels are required - some can catch fire if not manufactured correctly
- produce greater pd
In a fuel cell, what charges is the anode and cathode?
anode = negative
cathode = positive
What is used to make the electrodes and why?
Graphite (carbon) has delocalised elctrons - carry charge
Structure of a hydorgen fuel cell
- Negative anode on left
- Positive cathode on right
- Electroylte - ions move
- Top - both electrodes connected by a wire
Exam answer shorter of how fuel cell works
- As hydrogen enters the cell it becomes oxidised (lose e)
- This sets up a pd across the cell (diff in charge between electrodes - drives electrons around circuit)
Reasons why hydrogen fuel cells are better than rechargable cells for cars and other things
- no toxic chemicals to dispose at the end of the cell life
- travel further before refuelling
- faster to refuel
- hydrogen can be renewable
- hydrogen produces a constant voltage
- no loss of efficiency (ver time)
limitations of using a gas particle model
- particles shown as spheres
- particles shown as solids
- doesnt show movement or speed
- its 2d
