Energy in bonds, fuels and graphs

Question 1

Changes in energy during reactions can be explained by the …..?

Answer 1

making or breaking of bonds

Question 2

What is an exothermic reaction?

e.g

Answer 2

One that gives out energy to the surroundings usually in the form of heat and is usually shown as a rise in temperature.
combustion of fuels

Question 3

What is an endothermic reaction?

e.g

Answer 3

One that takes in energy from the surroundings usually in the form of heat and is usually shown as a fall in temperature.
photosynthesis

Question 4

Energy must always be supplied to …

It must always be released for …

Answer 4

break bonds

bonds to form

Question 5

What happens during a chemical reaction to bonds?

Answer 5

old bonds are broken and new bonds are formed

Question 6

Why is bond breaking an endothermic process?

Answer 6

As energy must be supplied to break the existing bonds, energy taken from the outside

Question 7

Why is bond formation an exothermic process?

Answer 7

As energy is released when new bonds are formed

Question 8

In an endothermic reaction the … required to …. old bonds is … than the energy …. when new … are formed.

Answer 8

energy, break, more, released, bonds

Question 9

In an exothermic reaction the … released in bond … is … than the energy used in … old bonds.

Answer 9

energy, formation, more, breaking

Question 10

How can you measure energy released by a chemical reaction?
What is the problem with this?
How can you make this better?
What does this method work for? Reactions of (2):

Answer 10

By taking the temp of the reactants (both must be the same) mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.
Energy is lost to surroundings which is not shown in the results.
Reduce the radiation by putting the polystyrene cup unto a beaker of cotton wool to give more insulation and putting a lid on it to prevent evaporation
solids with water, neutralization reactions

Question 11

What is an example of a reaction where energy released is measured? (4)
What else could this method be used for?

Answer 11

Place dilute HCl in a polystyrene cup and record the temp
Put dilute sodium hydroxide in a measuring cylinder and record temp’
As long as they’re at the same temp add the alkali and stir into the acid
Take the temp every 30 secs and record the highest temp it reaches
When energy is absorbed, there will be a fall in temp

Question 12

Fuels release…

burning fuels affects ….. . For example burning fossil fuels…. causing…..

Answer 12

energy
the environment
releases the greenhouse gas CO2, global warming and other types of climate change

Question 13

Why is it expensive to use fossil fuels?

Answer 13

It’s expensive to slow down the negative effects and and to put things right, as well as developing alternative solutions

Question 14

What’s running out that our fuels are made from?
So as they … …. they will get …. …..
So everything …. by … , …. , or ….gets more … too. So the price of … … has a large … effect.

Answer 14

Fossil fuels
run out, more expensive
transported, lorry, train, plane, expensive. crude oil, economic

Question 15

What is calorimetry?

Answer 15

When you try to find out how much energy released when a fuel is burnt by burning the fuel to heat water and measuring the temperature change.

Question 16

What is the calorimetry method?(5)

Answer 16

1. Put a measured amount of water in the copper can and record its temperature.
2. Weigh the spirit burner and lid.
3. Put the burner under the can and light the wick. Stir the water and until the temp reaches 50 degrees.
4. put out the flame using the burner lid and measure the final temp of water.
5. Weigh the spirit burner.

Question 17

What is the formula to find out the energy transferred in the calorimetry test?

Answer 17

Energy transferred (Joules) = mass of water (in g) x specific heat capacity (=4.2) x temperature change (Δ)(in degrees)

Question 18

What do energy level diagrams show?

Answer 18

If an experiment is endo or exothermic

Question 19

In exothermic reactions, what type of energy change is there?

Answer 19

ΔH (energy change) is negative

the energy in the reactants goes down, it’s released.

Question 20

The energy level diagram is for an exothermic reaction.
Describe its shape… 1. axes 2. starting height, middle, finishing height, 3. ΔH is what? 4.What are the horiz labeled as?

Answer 20

X is progress of reaction, y energy
horizontal flat line going right, a bump of energy needed to break the old bonds, the activation energy, then it falls down lower than the original line and flattens out.
-ve
reactants, products

Question 21

In an exothermic reaction diagram the lines are at different heights, which are where?
What does the difference in height represent?
What are the energy transfers in the two types of diagram (positive or negative?)

Answer 21

The products are lower than the reactants
The energy given out in the reaction per mole.
Exo = ΔH -ve, Endo = ΔH+ve

Question 22

The energy level diagram is for an endothermic reaction.
Describe its shape… 1. axes 2. starting height, middle, finishing height, 3. ΔH is what? 4.What are the horiz labeled as?

Answer 22

X is progress of reaction, y energy
horizontal flat line going right low on the diagram, the line rises high, then fall a bit, then flattens out onto the horizontal.
3. +ve,
4. reactants, products

Question 23

The energy level diagram is for an endothermic reaction.

5. What does the difference in height represent? 6. What is the activation energy? 7. What is the energy absorbed?

Answer 23

5. The energy taken in during the reaction. 6. From the first horizontal to the top of the peak. The distance between the first horizontal and the second.

Question 24

What does the activation reaction represent?
What does a catalyst do?
What is this represented by on the energy level diagrams?
But what stays the same?

Answer 24

The minimum energy needed by reacting particles to break their bonds.
Provides a different pathway for a reaction that has a lower activation energy so it happens more quickly and easily.
A lower curve on the diagram showing a lower AEnergy
The overall energy change in the reaction ΔH

Question 25

What does every chemical bond have?
What changes depending on what?
What can you do with these known … ….?

Answer 25

Bond energy associated with it
This energy changes depending on what compound the bond occurs in
bond energies, calculate the overall energy change in the reaction

Question 26

How do you calculate bond energy? (4)

Answer 26

Write the balanced symbol equation
Work out which bonds you have there and note how many of each
1. Using the info provided work out how much energy is needed to break all the bonds on one side of the equation
2. Work out how much energy is released when the new bonds are made, if it is 2HCl for example you need to times the energy it says by two as it is two moles worth
So overall the energy released is: the energy released by part 2 when the bonds are made - (take away) the energy needed to break them in 1. .
If this energy is +ve then it is released, so the energy of the molecules has gone down so it’s then written:
ΔH= -184 KJ/mol = exothermic, gives it out
So if the energy is -ve then it’s the other way around
ΔH= +184 KJ/mol = endothermic, the energy has been absorbed

Question 27

What are the two ways hydrogen can be used as a fuel?

Answer 27

By burning it or using it in a fuel cell

Question 28

What does hydrogen and oxygen produce? which is…
The reaction is … thermic. So it … energy.
So what’s good about hydrogen for a fuel?

Answer 28

H2O - not a pollutant
Exo, releases
It burns cleanly and releases energy

Question 29

Hydrogen gas can be burnt for what? by doing what?

Answer 29

Powering vehicles, as a fuel in combustion engines

Question 30

What are the pros for burning hydrogen in vehicles?

What are the cons for burning hydrogen in vehicles? (3)

Answer 30

It combines with O2 to form water so it’s clean
You need a special expensive engine
H can be sourced form water which is fine but it requires energy to extract from another source (fossil fuels?)
It’s hard to store safely as it’s very explosive

Question 31

What is a fuel cell?

Answer 31

An electrical cell that’s been supplied with fuel and oxygen and uses energy from the reaction between them to generate electricity.

Question 32

What can hydrogen be used in?

Answer 32

A hydrogen-oxygen fuel cell

Question 33

When were fuel cells developed as part of what? Why?
Why instead of solar cells?
Why instead of nuclear power?
Why is it better/different to a battery?

Answer 33

The 1960s as part of the space program, to provide electrical power on spacecraft
More practical
Safer
It doesn’t run down or need charging from the mains. it produces energy in the form of elecy and heat as long as fuel is supplied

Question 34

Why is the car industry developing fuel cells?

Answer 34

To replace conventional fossil fuel engines

Question 35

Why are fuel cell cars good?
Where would this be useful?
Why would this be good for countries?

Answer 35

They don’t release fossil fuels
The byproducts are water and heat so it is very clean for a city where air pollution is a big problem
Help them stop being so dependent on fossil fuels

Question 36

Why won’t fuel cells be the end for power stations and fossil fuels? (3)

Answer 36

H is a gas and takes up a lot more space to store than liquid fuels like petrol
It’s explosive and difficult to store safely
The hydrogen fuel is often made form hydrocarbons from fossil fuels or by electrolysis of water which uses electricity which is usually generated by fossil fuels.

Question 37

In terms of bond energy explain why hydrogen can be used as a fuel: (2 marks) - real exam question

Answer 37

It reacts with oxygen to release more energy in bond making than bond breaking
So the reaction is exothermic