3.3 Energy calculations Flashcards
What is the equation we can use to work out the amount of energy released? (include units)
energy released = mass (of substance e.g.water) x specific heat capacity (of substance e.g.water) x rise in temperature
Joules = grams x degrees C x degrees C
( Q=mc^t)
What happens when fuels and food react with oxygen?
energy is released in an exothermic reaction
What can a simple calorimeter be used for?
can be used to compare the energy released by different fuels or different foods in a school chemistry lab
What examples are there for reactions that can be used with the temperature change energy transfer with a calorimeter technique?
neutralisation and displacement reactions
What is the difference between the energy levels of the reactants and the products?
the energy change during a reaction
What is the energy change during a reaction measured in?
kJ/mol
How can we show the relative difference in energy of the reactants and products?
energy level diagrams
Describe the energy level diagram in an exothermic reaction:
the energy level of the reactants is higher than that of the products and energy is released in the reaction to the surroundings which get hotter
Describe the energy level diagram in an endothermic reaction:
the products are at a higher energy level than the reactants so energy is absorbed from the surroundings in the reaction so they get colder
What is the activation energy?
the minimum amount of energy needed to start a reaction (before colliding particles can react)
How can catalysts increase the rate of reaction?
provide an alternative pathway to the products which has a lower activation energy
this means that the higher proportion of reactant particles now have the energy to react
What happens when new bonds are formed, what process is it?
energy is released
exothermic process
What happens when bonds are broken, what process is it?
energy has to be supplied to break chemical bonds and energy is taken in from the surroundings
endothermic process
What is the name of the energy needed to break the bond between two atoms called?
the BOND ENERGY for that bond
What are bond energies measured in?
kJ/mol
How can we use bond energies?
to work out the energy change for many chemical reactions
What do we need to work out to calculate the energy change for a chemical reaction?
- how much energy is needed to break the chemical bonds in the reactants
- then how much energy is released when the new bonds are formed in the products
What decides whether a reaction is endothermic or exothermic (considering bonds)?
Endothermic: the energy released when new bonds are formed is less than the energy absorbed when bonds are broken
Exothermic: the energy released when new bonds are formed is greater than the energy absorbed when bonds are broken
What are the problems with using fossil fuels?
- they are running out
- releases carbon dioxide which contributes to greenhouse effect (global warming)
- releases sulfur dioxide and nitrogen oxide which creates acid rain
- sourcing is not good for environment etc
What is much of the world’s pollution caused by?
the increasing numbers of vehicles on our roads
What is being considered as an alternative fuel and what is its reaction?
hydrogen:
hydrogen + oxygen –> water
2H2 + O2 –> 2H2O
What are the advantages and disadvantages of hydrogen fuel?
Advantages:
- does not produce CO2 which could help tackle global warming
Disadvantages:
- safety and storage is an issue
- supplying hydrogen to burn in cars is also an issue… if we use electrolysis then electricity generated from non-renewable fossil fuels doesn’t help the environment
What is a more efficient way to use energy from oxidising hydrogen and how does it work?
fuel cell:
- cells fed with hydrogen and oxygen which produce water
- most of energy released in reaction is transferred to electrical energy (can be used to run vehicle)
- constant supply required
What is the challenge of hydrogen-powered engines?
to match the performance, price and convenience petrol or diesel cars whilst being cleaner with great benefits
