Exothermic and endothermic reactions (5.1) (M)

Question 1

Energy is (…) in chemical reactions

Answer 1

conserved

Question 2

The amount of energy in the universe at the end of a chemical reaction is the (…) as before the reaction takes place

Answer 2

same

Question 3

Why would energy be transferred to the surroundings after a reaction?

Answer 3

the product molecules must have had less energy than the reactants, by the amount transferred

Question 4

What is an exothermic reaction?

Answer 4

one that transfers energy to the surroundings so the temperature of the surroundings increases

Question 5

What happens to the temperature of the surroundings after an exothermic reaction?

Answer 5

It increases

Question 6

3 needed

What are examples of exothermic reactions?

Answer 6

• Combustion
• Many oxidation reactions
• Neutralisation

Question 7

2 needed

What are some everyday uses of exothermic reactions?

Answer 7

self-heating cans and hand warmers

Question 8

What is an endothermic reaction?

Answer 8

one that takes in energy from the surroundings

Question 9

What happens to the temperature of the surroundings after an endothermic reaction?

Answer 9

It decreases

Question 10

2 needed

What are examples of endothermic reactions?

Answer 10

• Thermal decompositions
• The reaction of citric acid and sodium hydrogencarbonate

Question 11

What is an everyday use of endothermic reactions?

Answer 11

Some sports injury packs

Question 12

The temperature of the surroundings has decreased, the reaction therefore must be…

Answer 12

endothermic

Question 13

Chemical reactions can occur only when what happens?

Answer 13

reacting particles collide with each other and with sufficient energy

Question 14

What is activation energy?

Answer 14

The minimum amount of energy that particles must have to react

Question 15

3 needed

What do reaction profiles show?

Answer 15

• the relative energies of reactants and products
• the activation energy
• the overall energy change of a reaction

Question 16

Draw the reaction profile for an exothermic reaction

Answer 16

Question 17

Draw the reaction profile for an endothermic reaction

Answer 17

Question 18

2 things

What must happen to bonds in a chemical reaction?

Answer 18

• energy must be supplied to break bonds in the reactants (endothermic)
• energy is released when bonds in the products are formed (exothermic)

Question 19

What is bond energy?

Answer 19

the energy needed to break bonds

or

the energy released when bonds are formed

Question 20

How do you calculate overall energy change of a reaction?

Answer 20

the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

Question 21

For an exothermic reaction, the overall energy change will always be (…), whereas in an endothermic reaction, the overall energy change will always be (…)

Answer 21

negative

positive

Question 22

In an exothermic reaction, the energy released from forming new bonds is (…) than the energy needed to break existing bonds

Answer 22

greater

Question 23

In an endothermic reaction, the energy needed to break existing bonds is (…) than the energy released from forming new bonds

Answer 23

greater

Question 24

How can you tell from a reaction profile that a reaction is exothermic?

Answer 24

the products are at a lower energy level than the reactants

Question 25

What might a bond in the product being weaker mean for the energy released?

Answer 25

Less energy is released when forming bonds (more positive overall energy change - so more in endothermic direction) as less energy is required to form the bonds

Question 26

6 marker

Use the above statement to predict and explain how the overall energy change for the reaction of ethene with chlorine will differ from the overall energy change for the reaction of ethene with bromine.

Answer 26

Size and strength

• chlorine atoms have fewer electron energy levels
• chlorine atoms form stronger bonds
• Cl-Cl bond stronger than Br-Br
• C-Cl bond stronger than C-Br

Energies required

• more energy required to break bonds with chlorine
• more energy given out when making bonds with chlorine
• Overall energy change depends on size of energy changes

Conclusions

• if Cl-Cl bond changes more the more/less exothermic
• cannot tell how overall energy change will differ as do not know what changes more

Question 27

2 points

The bond energy of the reactants is calculated to be 6000kJ/mol and the bond energy of the products is 7148kJ/mol.

Explain why the reaction is exothermic (use values)

Answer 27

More energy is released (7148kJ/mol) when bonds in the products are formed than the energy used (6000kJ/mol) to break bonds in the reactants.

So energy is released to the surroundings