Energy Changes Flashcards
When chemical reaction occurs, what happens to energy?
It’s transferred to or from the surroundings
What is an exothermic reaction?
One that transfers energy to the surroundings, usually by heating
How is an exothermic reaction shown?
By a rise in temperature
What is an endothermic reaction?
One which takes in energy from the surroundings
How is an endothermic reaction shown?
By a fall in temperature
Give three examples of an exothermic reaction
- Combustion 2. Neutralisation reactions 3. Oxidation reactions
Give 2 everyday uses of exothermic reactions
- Hand warmers 2. Self-heating cans
Explain how hand warmers use exothermic reactions
Use exothermic oxidation of iron in air (with a salt solution catalyst) to release energy
What do self-heating cans rely on?
Exothermic reactions between chemicals in their bases
Give an example of an endothermic reaction
Thermal decomposition
Give a everyday use of an endothermic reaction
In some sports injury packs
Explain how sports injury packs work
Chemical (endothermic) reaction allows pack to become instantly cooler without having to put it in the freezer
Give an example of a reversible reaction (that has the forward endothermic reaction and backwards exothermic reaction)
Hydrated copper sulfate (blue) ⇌ Anhydrous copper sulfate (white) + water
How can you measure the energy released when a substance is burnt?
Via simple calorimetry
Why is calorimetry used?
Used to compare amounts of energy released
What is the bad thing about calorimetry and why?
Not accurate because of energy losses
What equation can you use to calculate and compare amount of energy released by different fuels?
Q = mc ΔT
How is energy measured?
In J or kJ for a given mass/mole e.g. J/mol
How can the amount of energy produced by a chemical reaction in solution be calculated + when can this method be used?
Can be calculated from measured temperature change of the solution when the reagents are mixed in an insulated container Method can be used for reactions of solids with water or for neutralisation reactions
When doing bond energy calculations, what are the 3 things you need to remember?
- Reactants − Products 2. − = exothermic 3. + = endothermic
In endothermic reactions why must energy must be supplied?
To break existing bonds
In exothermic reactions when is energy released?
When new bonds are formed
In endothermic reactions: (comment on energy + bonds = comparison)
Energy used to break bonds > energy released by forming them
In exothermic reactions: (comment on energy + bonds = comparison)
Energy released by forming bonds > energy used to break them
In a energy level diagram, what does the difference in height of (the reactants and products) represent?
Overall energy change (energy given out/taken in) in reaction per mole
In a energy level diagram, what does the initial rise represent?
Activation energy (energy needed to break old bonds and start reaction)
What is activation energy?
Minimum amount of energy needed for reactants to collide with each other and react
What does it mean if a substance has a large activation energy?
Large amount of energy needed to start the reaction (needed to be supplied e.g. via heating)
In a chemical reaction, what must happen?
Old bonds are broken & new bonds are formed
What does a chemical cell do?
Produces a potential difference until the reactants are used up
What does a fuel cell do?
Supplied with fuel and oxygen (or air) & uses energy from reaction between them = to produce electrical energy efficiently
What happens to fuel when it enters a fuel cell?
Becomes oxidised & sets up a potential difference within cell
What is an example of a fuel cell?
Hydrogen-oxygen fuel cell
What does a hydrogen-oxygen fuel cell do?
Combines hydrogen and oxygen to produce clean water + energy
What can a hydrogen-oxygen fuel cell be used for?
Can be used in vehicles
Name 3 pros of hydrogen-oxygen fuel cells
- Don’t need to be electrically recharged
- Don’t produce as many pollutants as other fuels (by-products are water & heat)
- Can be designed in a range of sizes for different uses
Name 4 cons of hydrogen-oxygen fuel cells
- Hydrogen is a gas = takes up more space store than liquid fuels
- Hard to store safely
- Not actually that environmentally friendly
- Constant supply of hydrogen is needed to run fuel cell
Why are hydrogen-oxygen fuel cell hard to store safely?
Hydrogen is explosive when mixed with air
Why aren’t hydrogen-oxygen fuel cell 100% environmentally friendly?
Hydrogen fuel is made from hydrocarbons (from fossil fuels) or by electrolysis of water which uses electricity (generated by fossil fuels)
How can we reduce the amount of space hydrogen takes up?
Can use high-pressure cylinders but have safety problems in crashes
What is the downside of using high-pressure cylinders?
Have safety problems in crashes
How do you work out enthalpy change?
ΔH = mcΔT
n
What does the letters stand for in this formula?
ΔH = mcΔT
n
ΔH - enthalpy change (aka energy change per mole)
m - mass of water used
c - specific heat capacity of water
ΔT - change in temperature
n - moles of fuel used
Draw a energy level diagram for endothermic reaction
Draw a energy level diagram for exothermic reaction
Illustrate the effect of a catalyst on a energy level diagram for exothermic reaction
