TOPIC 5 - ENERGY CHANGES Flashcards
Explain exothermic reactions.
A reaction that transfers energy to the surroundings, usually by heating. This is how by a rise in temperature.
Eg combustion, neutralisation reactions, oxidation reactions
Can be used for hand warmers or in self heating cans.
What are endothermic reactions?
Takes in energy from the surroundings. This is shown by a fall in temperature.
Eg the reaction between citric acid and sodium hydrogencarbonate and thermal decomposition.
Can be used in some sport injury packs.
What practical can be used to measure energy transfer?
Taking the temperature of the reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction. The problem with this is that energy will be lost to the surroundings. This can be reduced by putting the polystyrene cup in a beaker of cotton wool to give more insulation, and put a lid on the top to prevent energy lost by evaporation. This method works for neutralisation or to investigate the effect of different variables have on the amount of energy transferred. Eg mass or concentration used.
To test the effect of acid concentration on the energy released in a neutralisation reaction between hydrochloric acid and sodium hydroxide.
- put 25ml of 0.25 mol/dm*3 of HCl and NaOH in separate beakers.
- add the beakers in a water bath and set to 25 degrees celcius until they both reach 25 degrees celcius.
- add the HCl followed by the NaOH to a polystyrene cup with a lid.
- take the temperature of the mixture every 30 seconds, and record the highest temperature.
- repeat these steps using 0.5 then 1 mol/dm*3 of hydrochloric acid.
Explain the exothermic reaction profile.
The products are at a lower energy than the reactants. The difference in height represents the overall energy change in the reaction per mole. (Energy given out).
The initial rise in energy represents the activation energy, this is the energy needed to start the reaction.
The activation energy is the minimum amount of energy the reactants need to collide with each other and react. The greater the activation energy, the more energy needed to start the reaction.
Explain the endothermic reaction profile.
The products are at higher energy than the reactants.
The difference in height represents the overall energy change during the reaction (the energy taken in) per mole.
What process is bond breaking?
Endothermic because energy needs to be supplied.
In endothermic reactions, the energy used to break bonds is greater than the energy released by forming the.
What process is bond making?
Exothermic because energy is relaxed.
In exothermic reactions, the energy released by forming bonds is greater than the energy used to break them.
What is an electrochemical cell?
A basic system made up of two different electrodes in contact with an electrolyte.
The two electrodes must be able to conduct electricity so are usually metals. The electrolyte is a liquid that contains ions which react with the electrodes. The chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes. If the electrodes are then connected by a wire, the charge is able to flow and electricity produced. A voltmeter can also be connected to the circuit to measure the voltage of the cell.
What does the voltage of a cell depend on?
Different metals will react differently with the same electrolyte. So, the type of electrodes used will affect the voltage of the cell.
The bigger the difference in reactivity of the electrodes, the bigger the voltage of the cell.
The electrolyte will affect the size of the voltage because the different ions in the solution will react differently with the metal electrodes used.
What are non-rechargeable batteries?
The reactants get used up. In some cells, the chemical reactions that happen at the electrodes are irreversible.
Over time the reacting particles, the ions in the electrolyte and the metal jo s on the electrode, get used up and tuned into the products of the reaction. Once any one of the reactants are used up, the reaction can’t happen anymore so no electricity is produced. The products can’t be tuned back into the reactants, so the cell can’t be recharged.
Eg alkaline batteries.
What are fuel cells?
They use fuel and oxygen to produce electrical energy. When fuel enters the cell it becomes oxidised and sets up potential difference within the cell.
Eg hydrogen-oxygen fuel cell - this fuel cell combines hydrogen and oxygen to produce clean water and release energy.
How do hydrogen oxygen fuel cells involve a redox reaction?
The electrolyte is often a solution of potassium hydroxide. The electrodes are often porous carbon with a catalyst. Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment. (Anode and cathode are the other way round when dealing with fuel cells in terms of positive and negative)
At the anode (-ve) hydrogen loses electrons to produces H+ ions. This is oxidation.
H+ ions in the electrolyte move to the cathode (+ve)
At the cathode oxygen gains electrons (reduction) from the cathode and reacts with H+ ions from the electrolyte.
The electrons flow through an external circuit from the anode to the cathode - this is the electric current.
The overall reaction is hydrogen plus oxygen which gives water.
What are some positives of hydrogen-oxygen fuel cells?
Don’t produce as many pollutants as other fuels. Better than electric vehicles as well because their batteries are more polluting to dispose of than fuel cells because they are made from highly toxic metal compounds.
There is a limit to how many times electric vehicles can be recharged, and they are more expensive than fuel cells.
Batteries would need to be recharged more often than fuel cells because they store less energy.
Wha are some disadvantages of hydrogen fuel cells?
Hydrogen is a gas, so it takes up more space to store than a rechargeable battery.
Hydrogen is explosive when it is mixed with air so it is hard to store safely.
Hydrogen fuel is often made from hydrocarbons (from fossil fuels), or by electrolysis of water, which uses electricity (usually created from fossil fuels).
