Why is there a transfer of energy when chemical reactions occur?
Because chemicals store energy in different amounts
What happens to the total amount of nerdy before and after a chemical reaction?
Stays the same because energy cant be created or destroyed
Examples of exothermic reaction?
- combustion
- neutralisation
- many oxidation reactions
Practical uses of endothermic reactions?
- hand warmers (exothermic oxidation of iron)
- self heating cans
Examples of exothermic reactions and practical uses?
- reacting citric acid with sodium hydrogencarbonate/ thermal decomposition
- sports injury packs: absorb heat and cool without freezing
How can we measure energy transfers in chemical reactions?
Monitoring temprature changes.
Why do we use polystyrene cup in experiment?
Insulating material reduces ENERGY LOSS to surroundings
Method: investigating energy changes for neutralisation reaction?
1) prepare separate solutions of HCl and NaOH with known concentration
2) measure initial temperature of the solutions using thermometer
3) mix the reactants in insulated cups and immediately cover with lid to prevent heat loss; stir the solution
4) measure and record the change in temperature at regular intervals (e.g. every 30s)
5) when the highest temprature is reached, subtract that from the initial temperature
6) repeat process for different concentrations of acid
Method: investigating energy changes when metal reacts with acid?
1) add a solution of HCl to insulated cup, measure initial temperature
2) add known mass of magnesium, immediately cover insulated cup with lid to prevent heat loss and stir
3) measure the temprature at regular intervals (e.g. 30s)
4) find temp changes after highest temprature reached
5) repeat with different masses of magnesium
What happens when bonds break in reactions?
- energy taken in to break the bonds of reactants
- so it’s exothermic
What happens when bonds are made?
- atoms rearrange themselves to create products by bonding
- releases energy: exothermic
What determines whether reaction is endothermic or exothermic?
- exothermic: more energy released making bonds than breaking
- endothermic: more energy take in breaking bonds than released making
Units of bond energies?
KJ/ mol
What are chemicals cells? Set up of simple chemical cells?
- cells that generate electricity through chemical reactions
- Similar to electrolysis: electrolyte with TWO DIFFERENT METAL electrodes submerged in it that are connected by wires
How can we produce a larger voltage of electricity in simple chemical cells?
Use two electrodes with bigger difference in reactivity
What else affects the size of the voltage in simple chemical cells?
The ions in the electrolyte - different metal ions react with the electrodes differently
Why do non rechargeable batteries stop working? An example of one?
-Because reactants get used up so chemical reaction stops
- alkaline batteries
Why can we recharge some batteries?
Reaction can be reversed with an external electrical current
What are batteries?
Multiple cells connected in series to produce greater voltage.
What are fuel cells?
Electrochemical cells that convert chemical energy of a fuel and oxygen into electrical energy
Why are hydrogen fuel cells a better alteration to rechargeable cells and batteries?
- don’t release toxic chemicals when disposed of
- in cars they allow it to travel further before refuelling
- quicker to refuel
downsides of hydrogen fuel cells?
- hydrogen comes from fossil fuels that are non renewable
- hydrogen difficult to store
- hydrogen is explosive
How do fuel cells produce electricity?
- they are supplied by an external supply of fuel such as hydrogen, and also supplied oxygen/ air
- the fuel gets oxidised at the anode
- its oxidised electrochemically to produce a potential difference (voltage)
What happens at each component of a hydrogen fuel cell?
- anode: hydrogen is oxidised. There will be H+ ions
- cathode: oxygen is reduced and reacts with H+ ions to form water
