3- Energy Calculations Flashcards
What can you use to measure the amount of energy released when a substance is burnt?
A calorimeter- a simple one is water in a glass beaker or metal can
The burning substance heats the water, the temperature rise depends on the amount of energy released
What equation is used to calculate the amount of energy transferred to the water from the substance/solution?
Q=m x c x T
Q=energy transferred (J) m=mass of water (g)
c=specific heat capacity (4.2J/g’C) T=temperature change (degrees C)
What is the advantages and disadvantages to a simple calorimeter?
Ad: cheep and easy to make, good for comparing
Dis: do not give accurate results because much of the energy released heats the surroundings and not the water
How can the energy change in kg/mol be found?
Energy change in kg/g x relative formula mass of the substance
What happens when fuels and food react with oxygen?
The reaction is exothermic
Different amounts of energy are released by different fuels and foods (usually measured in Joules, but sometimes in calories)
1cal=4.2J
How can you make energy transfer and released experiments more accurate?
Do the reactions in an insulated container to reduce energy transfer to the surroundings
What do we assume when calculating energy transferred?
Solutions behave like water:
1cm3 of solution =1kg
The specific heat capacity is always is 4.2J/g’C
Give two examples of reactions that can use the calorimeter technique?
Neutralisation and displacement reactions
He do you find the energy changed (kJ/mol) from the energy transferred (J)?
Divide the energy transferred (J) by 1000 (kJ)
Then divide the energy transferred (kJ) by the mass (g) to get kJ/g
Then multiply that by the substances relative formula mass
to get (kJ/mol)
What is the energy change and how is it shown?
It is the difference between the energy levels of the reactant and the product.
It is shown on an energy level diagram
What must happen for a reaction to happen?
Bond must be broken (endothermic- taking in)
What is the activation energy?
The minimum energy needed for the reaction to happen
After the activation energy, why does the line on the graph start going downwards?
Energy is being released as products form (exothermic)
How can you tell on a energy level diagram if the reaction is
1) endothermic 2) exothermic?
1) the products have more energy than the reactants
2) the reactants have more energy than the products
How can a catalyst effect an energy level diagram?
It can lower the activation energy
What is bond energy?
The energy needed (activation energy) to break the bond between two atoms - an equal amount of energy is needed when bonds are formed
What needs to be worked out to find the energy change for a reaction?
A balanced equation
Total Energy needed to break all the bonds in the reactants
Total Energy needed to make all the bonds in the products
The difference between the two totals
What are the advantages of using hydrogen and a fuel?
It burns easily
Releases a large amount of energy per gram
Produces no carbon dioxide, only water
Can be produced from renewable sources
What are the disadvantages of using hydrogen as a fuel?
It is in short supply
Safety problems
Can reduce a vehicles performance
What’s the problem with fossil fuels?
They are non renewable and cause pollution when extracting
