topic 5 - energy changes Flashcards
what is the conservation of energy principle?
- energy is conserved in chemical reactions
- the amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place
what is an exothermic reaction?
- a reaction where energy is transferred to the surroundings so that the surroundings temperature increases
what is an endothermic reaction?
- a reaction where energy is taken in from the surroundings so the surroundings temperature decreases
what is activation energy?
- minimum amount of energy that particles need to react
what is a reaction profile?
- a graph which shows the relative energies of reactants and products, as well as the activation energy of the reaction
what are examples of exothermic reactions?
- combustion
- certain oxidation reactions
- neutralisation
what are examples of endothermic reactions?
- thermal decomposition
- reaction of citric acid
- sodium hydrogen carbonate
describe exothermic and endothermic reactions in terms of bond energies
- when we break a chemical bond, this requires energy (endothermic)
- making bonds releases energy (exothermic)
- this tells us the energy required to break that bond
what are the variables in the temperature changes required practical?
- (independent variable is the volume of sodium hydroxide solution)
- (dependent variable is the maximum temperature reached)
- (control variables are the volume of hydrochloric acid, and the concentrations of both the hydrochloric acid and the sodium hydroxide solution)
what are bases?
- bases are chemicals which can neutralise acids and produce salt and water
- bases which are soluble in water are also called alkalis
what do the most common acids produce?
hydrochloric acid -> … chlorides
sulfuric acid -> … sulfates
nitric acid -> … nitrates
describe how to investigate the variables that affect temperature changes in chemical reactions
- use a measuring cylinder to measure 30cm of dilute hydrochloric acid
- transfer the acid to a polystyrene cup
- now we stand the polystyrene cup inside a beaker
- this stops the cup from falling over
- use a thermometer to measure the temperature of the acid
- record this in a table
- use a measuring cylinder to measure 5cm3 of sodium hydroxide solution and transfer this to the polystyrene cup
- fit a plastic lid to the cup and place the thermometer through the hole in the lid
- the bulb of the thermometer must be in the solution
- use the thermometer to gently stir the solution
- because this reaction is exothermic, it releases energy
- this means that the temperature of the solution will increase
- when the reading on the thermometer stops changing, record the highest temperature reached
- rinse out and dry the polystyrene cup
- repeat the whole experiment using 10cm3 of sodium hydroxide solution
- Carry out the experiment several more times + each time we increase the volume of sodium hydroxide by 5cm3 until we reach a maximum of 40cm3 of sodium hydroxide solution
- repeat the whole experiment one more time so that we have two sets of results
- use these to calculate a mean value for the maximum temperature reached for each volume of sodium hydroxide solution
- at the end plot a graph of the results
what is a cell?
- a cell is composed of two electrodes dipped in an electrolyte solution
- contain chemicals which react to produce electricity
what is an electrolyte?
- a solution that can conduct electricity
what is a battery?
- a battery contains two or more cells connected in series to produce a greater voltage
how long can a cell produce electricity?
- a cell can only produce electricity for a certain period of time
- eventually, the chemicals in the cell run out and the reaction stops
what are the conditions needed for the cell to produce electricity?
- cells can only produce electricity if we use metals with different reactivities
what determines the amount of p.d produced by the cell?
- the greater the difference between the reactivity of the metals, the greater the potential difference produced by the cell
- the electrolyte also affects the potential difference
what are the advantages and disadvantages of rechargeable batteries?
- expensive
- used many times
- cheap in the long run
- output stays constant until flat
- creates less chemical pollution
what are the advantages and disadvantages of non-rechargeable batteries?
- cheap
- only can be used once
- expensive in the long run
- output falls gradually
-creates a lot of chemical pollution
what is an electrode?
- a conductor that is used to gain electrical contact with a circuit
how can you create a simple cell using electrodes and an electrolyte?
connect two different metals and dip them into an electrolyte
how do cells generate electricity?
- the more reactive metal gets oxidised
- the electrons flow through the wire from the more reactive metal to the less reactive metal
- the rate of flow if electrons is an electric current
what are the 4 factors that affect the voltage of a cell?
- the type of electrode used
- the electrolyte used
- the concentration of the electrolyte
- the temperature
why can a rechargeable battery be recharged?
- the chemical reactions are reversed using an external electric current
what are fuel cells?
- cells that are supplied by an external source of fuel
what is the overall reaction in a hydrogen fuel cell?
hydrogen + oxygen -> water
in a hydrogen fuel cell, where does the oxidation of hydrogen take place?
at the anode
in a hydrogen fuel cell, what happens at the cathode?
the oxygen is reduced
give 3 advantages of using hydrogen fuel cells over rechargeable batteries
- do not need to be electrically recharged
- no pollutants are produced
- can be a range of sizes for different uses
give 3 disadvantages of using hydrogen fuel cells over rechargeable cells and batteries
- hydrogen is highly flammable
- hydrogen is sometimes produced for the cell by non-renewable sources
- hydrogen is difficult to store
