Energy profiles and bond energy

Question 1

Exothermic reactions

Answer 1

Where energy is transferred from the molecules ——> the surroundings

Question 2

Temperature in exothermic reactions

Answer 2

The surrounding temperature increases
Molecule temperature decreases

Question 3

Examples of exothermic reaction?

Answer 3

Combustion
Certain oxidation
Neutralisation

Question 4

Energy profile diagram

Answer 4

Shows the increase and decrease of energy in a reaction

Question 5

Exothermic energy profile

Answer 5

The reactants are placed higher than the products

Question 6

Why are the reactants higher than the products in an exothermic energy profile?

Answer 6

Because energy has been given from the reactant molecules to the surroundings
Therefore the product molecules has less energy

Question 7

Difference between reactant line and product line in exothermic reaction

Answer 7

The amount of energy given out into the surroundings

Question 8

Uses of exothermic reactions

Answer 8

Hand warmers
Self heating cans eg food or drink

Question 9

Endothermic reactions

Answer 9

Take in energy from the surroundings

Question 10

Temperature change in endothermic reactions

Answer 10

Surroundings cool down because energy was taken in
Therefore the molecules temp increases because it takes it in

Question 11

Examples of endothermic reactions

Answer 11

Thermal decomposition
Photosynthesis

Question 12

Endothermic reaction profile

Answer 12

The reactants are lower than the products

Question 13

Why are reactants lower than products in an endothermic reaction?

Answer 13

Because the reactants took in energy from the surroundings
Therefore the products have taken in more energy than the reactants had before

Question 14

Difference between reactants and products in an endothermic energy profile diagram

Answer 14

The amount of energy the molecules took in from the surroundings

Question 15

Where is the activation energy

Answer 15

From the peak down to the REACTANTS
Even if it’s endothermic or exothermic, always from the peak to the reactants

Question 16

What is the activation energy?

Answer 16

For reactants to react, they must collide with each other
They need sufficient energy to do this
This minimum amount of energy needed = activation energy

Question 17

If the energy decreases from the reactants to products (exothermic) what changes the answer?

Answer 17

It is a negative number
As you are decreasing a higher amount of energy (reactants) from the lower parts (products)

Question 18

What would the energy change in an exothermic reaction be like?

Answer 18

Negative because the reactants have less energy than the products
Ie a loss in energy

Question 19

What would the change in energy in an endothermic reaction be like?

Answer 19

Positive because the reactants have less energy than the products
Ie a gain in energy from the surroundings

Question 20

Breaking a chemical bond

Answer 20

Needs energy (endothermic)
Because you gain energy from the surroundings

Question 21

Making a chemical bond

Answer 21

Releases energy
(Exothermic)
Because temperature is released into the surroundings

Question 22

Energy value

Answer 22

Something a chemical bond has
Tells us energy required to break the bond

Question 23

How to find energy changes in a reaction?

Answer 23

Note down structures/ bonds of each molecules involved
See which bonds are broken, usually LHS
Add up energy values of LHS
Do the same for RHS
Number is positive if bonds are broken but negative if being made
Carry out calculation

Question 24

What to remember when calculating the energy values of molecules?

Answer 24

Include the big number
Ignore the small number as that is just included in the structure

Question 25

How to remember endothermic and exothermic energy transfers?

Answer 25

ENdothermic - ENter (energy is transferred from surroundings into to reacting substances) EXothermic - EXit (energy is released by reacting substances out into the surroundings)

Question 26

How to remember bonds broken and made in terms of exothermic and endothermic?

Answer 26

Bendo (B for bonds being Broken, endo for endothermic) Mexo (M for bonds being Made, exo for exothermic)

Question 27

In an exothermic reaction, which is greater: the energy released from forming new bonds, or the energy needed to break existing bonds?

Answer 27

More energy needed making bonds MEXO M= making bonds Exo = energy released into surrounds Thus energy released making new bonds

Question 28

Electrolyte

Answer 28

Solution that can conduct electricity For example an ionic compound solution in electrolysis

Question 29

How can we make electricity

Answer 29

Placing 2 different metals in an electrolyte

Question 30

Components of a cell

Answer 30

2 different strips of metals in an electrolyte solution Connected together with a voltmeter Therefore a current will flow between them

Question 31

How do cells conduct electricity?

Answer 31

A chemical reaction is happening on the surface of the metals, producing electricity

Question 32

Properties of a cell

Answer 32

Can only produce electricity for a period of time because the reactants run out / reaction stop Only produces electricity if we use metals with different reactivity

Question 33

What determines the voltage produced by the cell?

Answer 33

The difference in reactivity between the metals Greater difference eg Mg and Cu = high V Lower difference eg Zn and Fe = low V The electrolyte

Question 34

Components of a battery

Answer 34

2 or more cells connected in series in order to make a greater voltage

Question 35

What are non rechargeable batteries?

Answer 35

Alkaline batteries The reactions in the electrolyte + metal in the cells aren’t reversible so therefore not rechargeable

Question 36

What are rechargeable batteries?

Answer 36

Batteries that have reversible reactions between electrolyte and the metal that can be reversed when a current is applied Therefore reactants aren’t used up

Question 37

Fuel cells

Answer 37

A type of cell Hydrogen reacts with oxygen or pure air to produce an electric current Water is the product and leaves the cell

Question 38

What happens in a fuel cell in terms of electrons?

Answer 38

Hydrogen atoms lose electrons to form positive ions These electrons join with the oxygen molecules and hydrogen ions to form water molecules

Question 39

Half equation at the negative electrode (fuel cell)

Answer 39

2H₂——> 4H⁺ + 4e⁻

Question 40

Half equation at the positive electrode (fuel cell)

Answer 40

O₂ + 4H⁺ + 4e⁻ ——> 2H₂ O

Question 41

Overall equation of the fuel cell

Answer 41

2H₂ + O₂ ——> 2H₂O

Question 42

What is happening to the hydrogen in a fuel cell?

Answer 42

It’s gaining oxygen Therefore it’s oxidised

Question 43

Advantages of a fuel cell

Answer 43

Produces elec as long as you provide hydrogen Do not get less efficient the longer they run Potential store of drinking water (waste product) eg in a space craft

Question 44

Disadvantages of a hydrogen fuel cell

Answer 44

They can run out of hydrogen Hydrogen is an explosive gas so difficult to store safely Produce a relatively low voltage so need more than 1

Question 45

Advantages of a rechargeable battery

Answer 45

No dangerous fuels needed Can produce a larger voltage

Question 46

Disadvantages of a rechargeable battery

Answer 46

Runs out and need to be recharged More charging cycles = stores less electricity so eventually needs replacing If not manufactured properly can catch fire

Question 47

Hydrogen fuel cells vs rechargeable batteries

Answer 47

HFCs produces electricity so long as you provide hydrogen, RBs needs recharging HFCs don’t get less efficient as they go on, RBs do RBs produce more voltage than HFCs RBs contain no explosive chemicals like hydrogen, HFCs do HFCs are much more expensive and difficult to store