C7 - Energy Changes Flashcards
Describe what an exothermic reaction is
- They transfer energy to their surroundings as heat energy which increases the temperature of the surroundings
- The products have less energy than the reactants
- Energy changes are shown as negative
Examples of exothermic reactions
- Combustion
- Certian oxidation reaction
- Some neutralisation reactions
Pracitcal uses of exothermic reactions
- Hand warmers
- Self heating food cans
Describe what an endothermic reaction is
- They take in energy from their surroundings which decreases the temperature of the surroundings
- The products have more energy than the reactants
- Energy changes are shown as positive
Examples of endothermic reactions
- Thermal decomposition
- reaction of citric acid with sodium hydrogen carbonate
What is the activation energy
- When particles collide, they must have sufficient energy to react
- Th activation energy is the minimum amount of energy required that particles must have to react
What do bond energies state
- Breaking chemical bonds requires energy so is endothermic
- Making chemical bonds releases energy so is exothermic
Variables for the neutralisation exothermic reaction of HCl and NaOH
- Independent - volume of sodium hydroxide
- Dependent - max temp reached
- Control - voulme of HCl and concentration of both HCl and NaOH
Method for experiment for exothermic reaction of HCl and NaOH
- Measure 30cm3 of dilute HCl using measuirng cylinder and pour in a polystyrene cup
- Stand the cup inside a beaker to stop it falling over
- Use a thermometer to measure the initial temp of the HCl
- Pour 5cm3 NaOH into the cup and put a lid with a thermometer and stir
- The temperature will increase and record the temp change
- Rinse out cup and repeat with increasing volumes of NaOH
- Repeat whole thing to find a mean
Explain the results of the exothermic experiment with HCl and NaOH
- As the volume of NaOH increases, the max temp increases as more particles react with the HCl
- After a certain point, the temperature decreases due to there being too much NaOH(in excess) and some that doesn’t react
- This means the energy released is spread out into a larger volume and the cold NaOH absorbs the heat energy lowering the temp
Describe exothermic and endothermic reactions in terms of bonds energies
- Exothermic reaction - the energy released from forming new bonds is greater than the energy needed to break existing bonds.
- Endothermic reaction - the energy needed to break existing bonds is greater than the energy released from forming new bonds
How is a simple cell set up and what happens
- Put two different metals in an electrolyte ( conductive solution)
- There will be a voltage between them and a current will flow through the wire
- This is due to the chemical reaction occuring on the electrodes
- Electrons pass from the more reactive metal to the less reactive metal generating a flow of electrons
Key ideas about cells
What affects voltage, are cells indefinite
- Cells can only produce electricity for a set amount of time as the chemicals run out
- Electricity can only be produced if metals with different reactivities are used
- The greater the different in reactivity, the greater the voltage
- Th electrolyte also affects the voltage
What is a battery
Two or more cells connected in series and this produces a greater voltage than a single cell
How can rechargeable batteries be recharged
- The chemical reactions that occur in them are reversible
Why can some batteries not be recharged
- in alkaline batteries, the reactants run out producing no more electricity
- This is due to the reactions being non-reversible
What is a fuel cell
- A reaction vessel where a fuel e.g. hydrogen is supplied and reacted with oxygen in the air
- These chemical reactions produce a potential difference
Full, balanced equation for the reaction inside a hydrogen fuel cell
- 2H2 + O2 —> 2H2O
- The hydrogen is being oxidised
What happens at the negative electrode in a hydrogen fuel cell
- Hydrogen molecules split into hydrogen ions and electrons
- Half equation: 2H2 —-> 4H+ + 4e-
- Electrons pass through wire giving current
What happens at the positive electrode in a hydrogen fuel cell
- Oxygen atoms combine with hydorgen ions and electrons
- Half equation: O2 + 4H+ + 4e- —> 2H2O
Advantages of hydrogen fuel cells
- Produce electricity as long as hydrogen is supplied and don’t need to be recharged
- Don’t get less efficient the longer they run
- Can be a source of drinking water
- Only waste product is water - not pollutant
- More reliable as they don’t have moving parts
- Don’t have a finite life like lithium ion batteries
Disadvantages of hydrogen fuel cells
- Hydrogen has to be kept under high pressure which is difficult to store
- Hydrogen can be explosive
- Lots are needed together to produce a large enough potential difference
- Very expensive
What do catalysts do
- Increase the rate of reaction
- By providing an alternative reaction pathway witha lower activation energy
- This increases the likelihood of successful collisions
Difference in processes between electrolysis and in chemical cells
- Electrolysis used electricity to produce chemical reactions
- Chemical cells use chemical reactions to produce electricity
