C7 (Exothermic And Endothermic Reactions) Flashcards
Exothermic reaction definition
Energy transfers from the reactants to the surroundings, and the energy of the products is less than the reactants
- decrease temperature
Endothermic definition
Energy transfers from the surroundings to the reactions and products have more energy than the reactants
- increase temperature
3 examples of Exothermic reactions
- combustion (burning fuels)
- neutralisation (acid + alkali)
- oxidation (eg. sodium + water)
2 everyday uses of exothermic reactions
- hand warmers uses oxidation of iron in air
- self heating cans rely on the chemicals in their bases
2 examples of endothermic reactions (less common)
- reaction of citric acid and sodium hydrogen carbonate
- thermal decomposition (calcium carbonate to calcium oxide)
1 example of the everyday use of endothermic reactions
Sports injury pack, chemical reactions make it cooler
How do you measure the energy transfer?
- Take the temperature of the solution before, putting in polystyrene cup with cotton wool and taking the temperature after
What are the 3 types of reactions the polystyrene cup test works for?
- neutralisation
- reaction between metal and acids
- reaction between carbonates and acids
What is a reaction profile?
Diagrams to show relative energies of reactants and products
What is the reaction profile for an exothermic reaction?
- increase for activation energy
- products less energy than reactant
- height is the energy released per mole
What is the reaction profile for an endothermic reaction?
- increases due to activation energy
- slight drop but products energy more than reactants
- height is the energy absorbed per mole
What happens to the bonds in an endothermic reaction?
- energy supplies that breaks strong bond
- energy input greater than output
What happens to the bonds in an exothermic reaction?
- energy released which forms the formation of bonds
- output energy greater than input
Overall energy change =
Energy required to break bonds - energy released from bonds
What is an electrochemical cell?
- basic system with two metal electrodes that conduct in contact with an electrolyte
- flow of charge created, use voltmeter to measure
3 factors the voltage of cell depends on
1) metals react differently with the electrolyte
2) bigger difference in reactivity of the electrodes, bigger the voltage
3) electrolyte used, as different ions react differently with metals
What is a battery?
- This is two or more cells together in a series
- cells combined for more voltage
What is a non-rechargeable battery?
+ example
- battery contains irreversible reactions, once one of the reactants is used up
- alkaline batteries
What is a fuel cell?
An electrical cell which uses fuel and oxygen, and uses the energy from the reaction to produce electricity efficiently
What happens when the fuel enters the cells?
- oxidised and sets up potential difference
What is the difference between fuel cells and electrolysis?
+ what does it mean for OILRIG
- In fuel cells the anode is negative and the cathode is positive
- Oxidation is at the positive and reduction at the negative
What is a hydrogen-oxygen fuel cell?
- This is where the electrolyte is potassium hydroxide
- Hydrogen is at the anode and the oxygen is at the cathode
What is the overall reaction of a fuel cell (equation)
2H2 + O2 -> 2H20
Advantages of fuel cells
Adv:
- no pollutants
- rechargeable
- cheaper than batteries
- store more energy than batteries
Disadvantages of Fuel Cells
- hydrogen takes up more space than rechargeable battery
- hydrogen explosive when reacts with air
- uses fossil fuels to create the hydrogen fuel
