Energy changes Flashcards
What are the key features of an exothermic reaction?
- Chemical energy is transferred to surroundings
- Usually manifests in a temperature increase
- Bonds are made
- Negative sign in overall energy equation result
What are the key features of an endothermic reaction?
- Chemical energy is taken in from the surroundings
- Usually manifests in a temperature decrease
- Bonds are broken
- Positive sign in overall energy equation result
How do chemical reactions occur?
When reacting particles collide
What is activation energy?
The minimum energy that particles must have to react `
What can be used to show the relative difference in the energy of reactants and products ?
Reaction profiles
How does a catalyst work?
Lowers the activation energy by providing an alternative pathway
What are the examples of exothermic reactions?
- Combustion
- Respiration
- Displacment
- Neutralisation
- Hand warmers
- Self heating cans
- Oxidation reactions (normally)
What are examples of endothermic reactions?
- Thermal decomposition
- Sports injury packs
- Photosynthesis
- Reaction between citric acid and sodium hydrocarbonate
What are the advantages of self-heating drinks cans?
- People can have hot drinks without requirement for an external heat source
- Shops and manufacturers can profit from sale of the product
What are the disadvantages of self-heating drinks cans?
- Lots of the volume of the can is taken up by the heating chamber
- In cold weather, the drink can’t be heated to a high enough temperature
- Can can only be used once
- Not easily recycled
In an exothermic reaction profile, is the energy of the products higher or lower than the reactants?
Lower
In an endothermic reaction profile, is the energy of the products higher or lower than the reactants?
Higher
What is the relationship between the energy for bond breaking and making in an exothermic reaction?
The energy released from making bonds is greater than the energy used to break them
What is the relationship between the energy for bond breaking and making in an endothermic reaction?
The energy used to break bonds is greater than the energy released by making them
How do you calculate the overall energy change of a reaction?
Sum of the energies needed to break bonds in the reactants - the energy released when new bonds are formed in the products
What does the term bond energy mean?
The energy needed to break the bonds between two atoms to form gaseous atoms
What does an electrochemical cell consist of?
Two different metals in an electrolyte connected by a wire
How does an electrochemical cell produce electricity?
The chemical reactions between the metals and electrolyte set up a charge/potential difference between the electrodes. This charge is then able to flow through the wire and electricity is produced.
How is a battery formed?
By connecting multiple cells in series
How is the voltage of a battery calculated?
By adding up the individual voltages of each cell in the battery
What does a cell convert?
Converts chemical energy into electrical energy
Which direction do the electrons flow in the cell? Why?
They flow from the more reactive metal to the less reactive metal due to the more reactive metal being able to give away electrons easier
WHat causes the charge/potential difference between the electrodes?
Different metals react differently with the same electrolyte
Why do non-rechargeable batteries run out?
Because one of the reacting particles (ions in electrolyte or metal ions on electrode) gets used up and turned into the products of the reaction. It is an irreversible reaction.
What is a common example of a non-rechargeable battery?
Alkaline battery
How are cells made rechargeable?
By reversing the reaction by connecting the cell to an external electric circuit
What is a fuel cell?
An electrical cell that is constantly supplied with fuel and oxygen (or air) ans uses energy from the reaction between them to produce electrical energy efficiently
What two factors affect the voltage produced by the cell?
- Types of metals used
- Electrolyte
How does the types of metals the terminals are made from affect the voltage produced by the cell?
-The further apart the two metals are in the reactivity series, the higher the potential difference of the cell
How does the electrolyte affect the voltage produced by the cell?
changing the electrolyte and/or the concentration of the electrolyte can affect the potential difference as the metals react differently
What could the electrolyte be in the cell?
- Salt solution
- Acid
- Alkali
What type of reaction is electrolysis?
Endothermic
What type of reaction occurs in a cell?
Exothermic
What is the negative terminal in a cell?
The more reactive metal
What is the positive terminal in a cell?
The less reactive metal
What is the negative terminal also referred to as? Why?
Reductant or reducing agent as it causes an oxidation reaction to occur
What is the positive terminal also referred to as? Why?
Oxidant or oxidising agent as it causes reduction to occur
What happens to the ngative terminal over time in a non-rechargeable battery?
It decreases in size and eventually disintigrates due to losing its ions and electrons
Does oxidation or reduction occur at the negative terminal?
Oxidation
How does the fuel in a fuel cell set up a potential difference?
Because as it enters the cell, it becomes oxidised
Describe the process by which a hydrogen-fuel cell produces electricity:
1) Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
2) At the anode, hydrogen loses electrons to produce hydrogen ions
3) Hydrogen ions move to cathode in the electrolyte
4) At the cathode, oxygen gains electrons from the cathode and reacts with the hydrogen ions to make water
5) The elctrons flow through an external circuit from the anode to the cathode, this is the electric current
What is the overall reaction in a hydrogen-oxygen fuel cell?
Hydrogen + oxygen = water
What reaction occurs at the anode to the fuel in a fuel cell?
oxidation
What reaction occurs at the cathode to the oxygen in a fuel cell?
reduction
What are advantages of cells and batteries?
- Provide a very convenient portable source of electricity which can be used in many devices
- Relatively cheap and readily available
- Some are rechargeable and so can be used many times over
What are the disadvantages of cells and batteries?
- Contain toxic or harmful chemicals
- Can harm environment unless these chemicals are recycled.
- need replacing
What are the advantages of hydrogen fuel cells?
Less polluting to dispose off than batteries
Produces water – no pollution
Light weight and highly energy efficient
Cheaper to make than batteries
What are the disadvantages of hydrogen fuel cells?
Hydrogen is a gas so it needs a lot of space to store it.
Hydrogen is explosive – difficult and dangerous to store.
Hydrogen fuel is made from hydrocarbons from fossil fuels of methane
CH4 + H2O (g) CO + 3H2(g)
Steam requires energy to make it.
Or from the
Electrolysis of water. (also required electicity – expensive.
What are the advantages of a hydrogen fuelled car over a battery car?
- Batteries are more polluting to dispose of than fuel cells because they are made from highly toxic metal compounds
- Bateries are more expensive to make than fuel cells
- Rechargeable batteries have a limit as to how many times they can be recharged
- Batteries store less energy (charging more often and less distance travelled)
- Both are unpolluting
What are the disadvantages of a hydrogen fuelled car over a battery car?
- Hydrogen is a gas so takes up more space to store than a battery
- Hydrogen is explosive with air so hard to safely store
- Hydrogen fuel is often made from hydrocarbons or electrolysis of water which uses electricity
- Widespread charging points but hydrogen fuelled cars aren’t widely available or used
Why does hydrogen not combust with oxygen without a flame?
- Because the hydrogen and oxygen particles do not meet the activation energy requirement when they collide in order for them to react
Describe the experiment investigating the variables which affect temperature change in chemical reactions:
- measure set volume of acid into polystyrene cup e.g. 25 cm cubed
- stand the cup inside a beaker
- measure and record temperature of the acid
- measure 5cm cubed of alkali and add to polystyrene cup
- put a lid on the cup and gently stir the solution with the thermometer through the hole in the lid
- when the reading on the thermometer stops changing, record the temperature
- repeat previous three steps to add further 5cm cubed increments of alkali to the cup
- repeat experiment
- calculate the mean maximum temperature reached for each volume of alkali
How can ethanol be evaporated safely as it is flammable?
Using a water bath instead of a Bunsen burner
