Energetics and cells Flashcards
What is the principle of energy conservation in chemical reactions?
Energy is conserved in chemical reactions; products must have less energy than reactants if energy is transferred to surroundings.
What is chemical energy?
Energy that’s stored in chemical bonds.
What defines exothermic reactions?
Reactions that transfer energy to the surroundings.
Give examples of exothermic reactions.
- Combustion (burning)
- Many oxidation reactions
- Neutralisation
What happens to the temperature in exothermic reactions?
The reaction mixture and surroundings become hotter.
How is the energy change represented in exothermic reactions?
The energy change is negative.
What defines endothermic reactions?
Reactions that take in energy from the surroundings.
Give examples of endothermic reactions.
- Electrolysis
- Thermal decompositions
What happens to the temperature in endothermic reactions?
The reaction mixture and surroundings become colder.
How is the energy change represented in endothermic reactions?
The energy change is positive.
What is a common use for endothermic reactions?
Used in some sports injury packs.
What is activation energy?
The minimum amount of energy that particles must have to react.
What should you do when answering exam questions about reaction profiles?
Replace the reactants and products with the actual reactants and products.
In a reaction profile, what does the increase in energy to the top of the hill represent?
Energy being put in to break the bonds.
Fill in the blank: In exothermic reactions, the overall energy change is ______.
negative
Fill in the blank: In endothermic reactions, the overall energy change is ______.
positive
What is the energy change during bond breaking?
Energy is required to break the bonds in the reactants, therefore bond breaking is endothermic.
What happens during bond making?
New bonds are formed which gives out energy, making bond making exothermic.
How do you calculate overall energy change in a reaction?
Energy Change = Energy of bonds broken - Energy of bonds made (BREAK - MAKE)
When is a reaction considered endothermic?
The reaction is overall endothermic if the energy needed to break the bonds in the reactants is more than the energy given out when new bonds form.
When is a reaction considered exothermic?
The reaction is overall exothermic if the energy given out when new bonds form is more than the energy needed to break the bonds in the reactants.
What are the steps for bond energy calculations?
- Calculate bonds broken.
- Calculate bonds made.
- Cross off the bonds as you add them up.
- Do break - make.
- Explain if it’s exothermic or endothermic.
What indicates an exothermic reaction in terms of bond energies?
More energy is released making bonds than needed to break bonds.
What indicates an endothermic reaction in terms of bond energies?
Less energy is released making bonds than needed to break bonds.
What is the simplest design of a cell?
Two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by a wire.
What role does the more reactive metal play in a cell?
The more reactive metal forms ions more easily with the electrolyte, releases ions, which gives it a negative charge and sets up a charge difference.
What occurs as electrons flow in a cell?
Electrons flow around the circuit to the now more positive electrode.
Fill in the blank: A cell contains ______ that react to produce electricity.
[chemicals]
What is the purpose of using a thermometer in a practical energy change experiment?
To measure the temperature change during the reaction.
What do you plot to analyze temperature changes during a reaction?
Plot results and two lines of best fit - one when it’s increasing and one when the temperature is decreasing.
Where does the maximum temperature occur in a temperature change graph?
Where the lines intersect.
What happens during the energy release after the top is reached in a reaction?
Energy is being released whilst making the bonds.
What is the practical method to measure temperature change in a reaction?
Place a beaker in a polystyrene cup and use a thermometer.
What should you do after adding sodium hydroxide into hydrochloric acid?
Measure the temperature change.
How should you plot the results of a temperature change experiment?
Plot two lines of best fit - one when it’s increasing and one when the temperature is decreasing.
What does the intersection of the two lines in a temperature change graph represent?
The maximum temperature.
What type of process is bond breaking?
Endothermic.
What type of process is bond making?
Exothermic.
What is the formula for calculating energy change in a reaction?
Energy Change = Energy of bonds broken - Energy of bonds made.
Under what condition is a reaction overall endothermic?
If the energy needed to break the bonds in the reactants is more than the energy given out when new bonds form.
Under what condition is a reaction overall exothermic?
If the energy given out when new bonds form is more than the energy needed to break the bonds in the reactants.
What is the first step in bond energy calculations?
Do two separate calculations for bonds broken and bonds made.
What should you do after adding the bonds in energy calculations?
Cross off the bonds as you add them up.
What does it mean if a reaction is exothermic?
More energy is released making bonds than needed to break bonds.
What does it mean if a reaction is endothermic?
Less energy is released making bonds than needed to break bonds.
What is contained in a cell that produces electricity?
Chemicals that react.
What is the simplest design of a cell?
Two electrodes made from metals of different reactivity immersed in an electrolyte.
What role does the more reactive metal play in a cell?
It forms ions more easily with the electrolyte and releases ions, giving it a negative charge.
What happens to electrons in a cell?
They flow around the circuit to the now more positive electrode.
What causes a voltage to be produced in an electrochemical cell?
The difference in the ability of the electrodes to release electrons
This difference in reactivity between the electrodes is crucial for voltage generation.
What is the role of the negative electrode in an electrochemical cell?
It is the more reactive metal, loses electrons, and is oxidised
The negative electrode is often referred to as the anode.
What is the role of the positive electrode in an electrochemical cell?
It is the less reactive metal, gains electrons, and undergoes reduction
The positive electrode is commonly known as the cathode.
What factors affect the voltage produced in an electrochemical cell?
Electrodes and electrolyte
Specific factors include the difference in reactivity, surface area of the electrode, distance between them, type, concentration, and temperature of the electrolyte.
How does the difference in reactivity relate to voltage?
The greater the difference in reactivity, the greater the voltage
This principle is fundamental to the design of batteries and electrochemical cells.
What is a battery?
Two or more cells connected in series
Batteries can be classified into rechargeable and non-rechargeable types.
What happens to non-rechargeable batteries over time?
The electrodes degrade and the reactions are non-reversible
This degradation leads to the eventual death of the battery once reactants are consumed.
What are the advantages of non-rechargeable batteries?
They are cheap
However, they pose disposal challenges due to toxic chemicals.
What are the disadvantages of non-rechargeable batteries?
Disposal issues and non-reversible reactions
The toxic chemicals in used cells require careful handling.
What defines rechargeable batteries?
The reactions are reversible and can be recharged
This feature allows them to be used multiple times.
What are the advantages of rechargeable batteries?
Can be used many times and are cheaper than fuel cells
Their longevity makes them a preferred choice for many applications.
What are the disadvantages of rechargeable batteries?
Takes time to recharge and are more expensive than non-rechargeable cells
This can affect their usability in certain situations.
What is a fuel cell?
A cell supplied by an external source of fuel and oxygen or air
Fuel cells convert chemical energy directly into electrical energy.
How does a fuel cell operate?
Fuel donates electrons at one electrode while oxygen gains electrons at the other
This electrochemical process generates electricity.
What occurs as the fuel enters the fuel cell?
It becomes oxidised, setting up a voltage within the cell
Oxidation of the fuel is essential for energy release.
What is produced in a hydrogen-oxygen fuel cell?
Energy and water
This reaction highlights the efficiency and environmental benefits of fuel cells.
What is the negative electrode in hydrogen-oxygen fuel cells?
H2 → 2e + 2H*
The negative electrode is also known as the anode.
What is the positive electrode reaction in hydrogen-oxygen fuel cells?
O2 + 4H* + 4e → 2H2O
The positive electrode is also known as the cathode.
What is the overall reaction in hydrogen-oxygen fuel cells?
O2 + 2H2 → 2H2O
List three advantages of hydrogen-oxygen fuel cells.
- They do not produce any pollution.
- They produce more energy per kilogram than either petrol or diesel.
- No power is lost in transmission as there are no moving parts.
What is a disadvantage of hydrogen-oxygen fuel cells related to materials?
Materials used in producing fuel cells are expensive.
True or False: Hydrogen-oxygen fuel cells are affected by low temperatures.
True
Fill in the blank: Hydrogen is _______ to produce and store.
expensive
What is the environmental benefit of hydrogen-oxygen fuel cells regarding waste?
No batteries to dispose of which is better for environment + no toxic chemicals.
What type of process do hydrogen-oxygen fuel cells use for energy production?
Continuous process
What are high-pressure tanks needed for in hydrogen-oxygen fuel cells?
To store oxygen and hydrogen
List two reasons why hydrogen-oxygen fuel cells can be considered dangerous.
- High pressure tanks needed to store oxygen and hydrogen
- Hydrogen is expensive to produce and store
