Energy changes - chemsistry Flashcards
What does it mean if the products of a reaction store more energy than the original reactants
They must have taken in the difference in energy between the products and reactants from the surroundings during this reaction
What does it mean if the products of a reaction store less energy than the original reactants
The excess energy was transferred from the surroundings during the reaction
Why does overall amount of energy in a reaction not change
Because energy is conserved in reactions - it can’t be created or destroyed, only moved around.
Exothermic reaction
A reaction which transfers energy to the surroundings, usually by heating. This is shown by a rise in temperature
Example of exothermic reaction
- Combustion
- Neutralisation reactions
- Oxidation reactions
Every day uses of exothermic reactions
- Hand warmers use the exothermic oxidation of iron in air to release energy
- Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases
Endothermic reaction
A reaction which takes in energy from the surroundings. This is shown by a fall in temperature
Example of endothermic reactions
- Citric acid and sodium hydrogen carbonate
- Thermal decoposition
Every day uses of endothermic reactions
Used in some sport injury packs
Practical to test the effect of acid concentration on the energy released in a neutralisation reaction
Put 25cm cubed of 0.25 mol/dm cubed of hydrochloric acid and sodium hydroxide in separate beakers
Place the beakers in a water bath set to 25 degrees celcius until they are both at the same temperature (25 degrees)
Add the HCl followed by the NaOH to a polystyrene cup with a lid
Take the temp of the mixture every 30 seconds, and record the highest temperature
Repeat steps using a higher concentration of HCl
Problem with energy measurements and how to reduce it
You cannot control the amount of energy lost to the surroundings so to reduce this problem you can put a polystyrene cup into a beaker of cotton wool to give it more insulation and putting a lid on the cup to reduce energy lost by evaporation
What must be supplied to break bonds
Energy
What process is the breaking of bonds
Endothermic process
What process is the formation
Exothermic process
In exothermic reactions the energy released by forming bonds is
Greater than the energy used to break them
In endothermic reactions the energy used to break bonds
Is greater than the energy released by forming them
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 and so are usually metals
Electrolyte in electrochemical cells
A liquid that contains ions which react with the electrodes . The chemical reaction between the electrodes and electrolyte set up a charge difference between the electrodes.
Why are electrodes connected by a wire in electrochemical cells
The charge set up by the chemical reaction between the electrolyte and the electrode is able to flow and electricity is produced. A voltmeter can also be connected to the circuit to measure the voltage of the cell.
What factors does the voltage of a cell depend on
- Different metals will react different.y with the same electrolyte so the type of electrodes used will affect the voltage of a cell
- The bugger the difference in reactivity of the electrodes, the bigger the voltage of the cell
- The electrolyte used in a cell since different ions i solution will react differently with the metal electrode
- More cells the bigger the overall voltage
How do reactants in non-rechargeable batteries get used up
The chemical reaction is irreversible sp over time the reacting particles (the ions in the electrolyte and the metal ions in the electrode) get used up and turn into the products of the reaction. Once any one of the reactants is used up the reactuon can’t happen and so no electricty ios produced
Why are rechargeable cells rechargeable
The reaction can be reversed by connecting it into an external electric current
Fuel cell
A fuel cell is an electrical cell that’s supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
How do fuel cells work
When the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell.
