C3 - Energy Flashcards
What is an endothermic reaction?
One that takes in energy (usually heat) and is shown by a fall in temperature
What is an exothermic reaction?
A reaction that gives out energy to its surroundings usually in the form of heat and is shown as a rise in temperature
What causes a reaction to be endothermic and why?
A reaction is endothermic if more bonds are broken than formed because energy has to be supplied to break bonds
What causes a reaction to be exothermic and why?
A reaction is exothermic if more bonds are formed than broken because energy is released when bonds are formed
Describe how energy transfer of a reaction can be measured and explain why everything is done
For the equipment:
You put a polystyrene cup inside a beaker and fill all of the gaps between the 2 with cotton wool (to insulate the cup and reduce heat loss)
You also put a lid on the cup to also reduce heat kiss
For the method:
You take the temperature (thermometer) of the 2 reactants (liquid or solution) and if they are the same, you record the temperature and put the right amounts of the reactants for a complete reaction in the cup and close the lid
You open the lid when the reaction is complete and take the temperature to see how much energy has been transferred or absorbed
What are the 3 issues with burning fuels for energy?
Has various environmental effects like global warming
It will be expensive to change to other energy sources that are better
Crude oil is a finite resources and will run out eventually
What is calorimetry?
The method used to calculate how much energy is stored in a given mass of a fuel
Describe how the equipment is set up for calorimetry
A copper can is suspended above a spirit burner
This equipment is within a draught excluder
Describe how the practical part of calorimetry is done
Put 50g of water inside the copper can and record its temperature using a thermometer
Weigh the spirit burner with the fuel inside it and its lid on top of it then put the spirit burner underneath the copper can
Turn on the spirit burner and take off its lid
Put the lid over the spirit burner when the temperature of the water reaches 50C and turn it off
Weigh the spirit burner with the lid on top of it again
Describe how the calculation of calorimetry is done
The mass of the fuel burned = the mass of spirit burner before - mass of burner after
Temperature change of water = temperature after - temperature before
The mass of water is 50g
It takes 4.2 joules to heat up 1g of water by 1C (specific heat capacity)
The energy transferred by the amount of fuel burned = mass of water (50g) * specific heat capacity of water (4.2) * temperature change of water
What is an energy level diagram?
A graph that shows if a reaction is exothermic or endothermic and the activation energy
What is activation energy?
The amount of energy needed to break the bonds of reacting molecules (to trigger a reaction)
What would the energy level diagram look like for an exothermic reaction?
The curve would go up first to represent the activation energy and then go down further than the start height to represent the products having less energy than the reactants
What would an energy level diagram look like for an endothermic reaction?
The curve will go upwards and then down slightly meaning that the activation energy is shown and the energy of the products is higher than the energy of the reactants
What effect would a catalyst have on an energy level diagram?
The activation energy would be lower
How do catalysts work?
They lower the activation energy by providing an alternative pathway for the reaction
What is bond energy?
The amount of energy needed to break the bonds of a specific molecule which is the same amount of energy released when that bond is formed
Calculate the overall energy change for the reaction H^2 + Cl^2 –> 2HCl
Assume that:
H-H = 1kJ/mol
Cl-Cl = 2kJ/mol
H-Cl = 2kJ/mol
H^2 + Cl^2 –> 2HCl
H-H + Cl-Cl –> 2H-Cl
1kJ/mol + 2kJ/mol - 2(2kJ/mol) = -1kJ/mol
This means that -1kJ/mol is taken up by the reaction
This would be stated as 1kJ/mol has been released by the reaction
How does hydrogen gas fuel work?
If hydrogen gas is burned, that provides the activation energy for hydrogen to react with oxygen in the air to form water
This is an exothermic reaction meaning energy is given out so hydrogen gas can be used as a fuel
What is the advantage of using hydrogen gas fuel?
The only product is water so it is very environmentally friendly
What are the 3 disadvantages of hydrogen gas fuel?
You need a special, expensive engine
You need to use energy from another source to make hydrogen gas from water
It’s very explosive so it is hard to store
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 is the advantage of fuel cells?
It doesn’t need recharging because only fuel and oxygen needs to be supplied
What are the 2 advantages of using fuel cells in cars and replacing engines?
Could help countries to become less dependent on crude oil
No environmentally harmful waste products
