C3.3 Energy Change Flashcards
What is an exothermic reaction?
More energy is being released from bond making than is being absorbed from bond breaking - this increases the temperature of the surrounding
What is an endothermic reaction?
More energy is being absorbed from bond breaking than is being transferred from bond making - this decreases the temperature of the surrounding
Give examples of endo- and exo- thermic reactions
Endothermic: when a substance thermally decomposes
Exothermic: when a fuel burns
What happens when bonds are being broken?
Energy is supplied to break bonds, making bond breaking an endothermic process
What happens when bonds are being made?
Energy is released when bonds are made, making bond making an exothermic process
What is bond energy?
How much energy is required to break one bond of that particular substance
How can you use bond energies to calculate the overall energy change for a reaction?
- Draw out the display formula of all the molecules present in the reaction e.g. H-H + Cl-Cl => H-Cl, H-Cl
- Add up all the bond energies from the reactants
- Add up all the bond energies from the products
- Use this formula: Energy change =
Energy of bond breaking - Energy of bond making or in other words: Reactants - Products
From bond energy calculations, if the final value is negative what does this mean?
The reaction is exothermic - more energy was released from bond making than bond breaking
From bond energy calculations, if the final value is positive what does this mean?
The reaction is endothermic - more energy was absorbed from bond breaking than bond making
What is an energy level diagram?
A graph that shows how the energy in a reaction changes as the reaction progresses
How is an energy level diagram drawn?
The graph starts at the energy level of the reactants and finishes at the energy level of the products - these 2 points are joined by a smooth curve
How do you find the overall energy change of a reaction from an ELD?
The difference in height between the energy level of the products and the energy level of the reactants
How can you tell if a reaction is endothermic from an ELD?
If the reaction is endothermic, the reactants in the ELD will be lower than the products (as they have less energy)
How can you tell if a reaction is exothermic from an ELD?
If the reaction is exothermic, the reactants in the ELD will be higher than the products (as they have more energy)
What is the minimum amount of energy needed by reacting particles to break their bonds called?
The activation energy
How can you tell what the activation energy is of an ELD?
It is the difference between where the curve starts and its highest point
How will an EDL change when a catalyst is added to the same reaction?
It creates a different pathway to the original - it has a lower activation energy, but the energy level of the reactants and products are the same
What is energy transfer measured in?
Kilojoules per gram (kJ/g) or kilojoules per mole of reactant (kJ/mol)
How can you measure the energy transfer of a reaction in a solution?
By taking the original temperature, and then adding the reactant(s) and then taking the temperature at the end of the solution - this method works for reactions where you mix two solutions like neutralisation reactions
How will you know if a reaction is endo/exothermic after taking its original and final temp?
Exo - temperature of solution will increase, endo - temperature of solution will decrease
What method can you use to measure how much energy is released from a fuel?
Calorimetry
How can you work out the energy transferred of a reaction?
Q = m x c x ΔT ==> energy transferred (J) = mass of water (g) x specific heat capacity of water (4.2) x temperature change (°C)
What problems are there with burning fuels? (2)
Releases greenhouse gases causing global warming and other forms of climate change, and crude oil is running out
Why can hydrogen be used as a fuel?
Because the reaction is exothermic when it reacts with oxygen
How can you harness the energy released from burning hydrogen?
By reacting hydrogen and oxygen in a fuel cell
Why is hydrogen very clean as a fuel?
Because it only produces water when burnt
What is the word and symbol equation for the combustion of hydrogen?
Hydrogen + oxygen ==> water
2H2* + O2* ==> 2H2*O
What are the disadvantages of using hydrogen as a fuel? (3)
The fuel cell which hydrogen needs to be able to be used as a fuel is very expensive, the obtaining of hydrogen still requires another source of energy and it’s hard to store as it is very explosive
What is a fuel cell?
An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to generate electricity
What benefits do hydrogen-oxygen fuel cells have over a battery?
A fuel cell doesn’t run down or need recharging from the mains
What energy does a fuel cell produce? (2)
It’ll produce energy in the form of electricity and heat as long as fuel is supplied
What is the main benefit of a fuel cell?
It doesn’t produce any conventional pollutants
Why doesn’t hydrogen replace crude oil as the main source of fuel? (3)
Because it is a gas, it takes up much more space than liquid fuels like petrol, hydrogen is extremely explosive and is hard to store safely, and the hydrogen is usually obtained from either hydrocarbons, or the electrolysis of water - which both need energy which usually comes from a fossil fuel
