Topic 5 - Energy changes

Question 1

What happens to energy during chemical reactions

Answer 1

It is moved around, some energies either taking in from the atmospheres or giving energy out, normally in the form of thermal energy
Obviously, no energy is created or destroyed

Question 2

What is an exothermic reaction

Answer 2

This is a reaction where energies transferred to the surroundings in the form of heat energy. This can be shown by a rise in temperatures

Question 3

What are some examples of exothermic reactions in real life

Answer 3

Many hand warmers use oxidation

Self heating cans of hot chocolate and coffee

Question 4

What is an Endothermic reaction

Answer 4

This is a reaction where energy is taken in from the surroundings, which can be proved by a fall in temperatures
They are less common the exothermic reactions

Question 5

What are some examples of reactions which are exothermic

Answer 5

combustion - burning fuels
Neutralisation reactions
Many oxidation reactions

Question 6

What are some examples of reactions which are endothermic

Answer 6

Endothermic reactions are less common then exo
thermal decomposition
The reaction between citric acid and sodium hydrogencarbonate

Question 7

What are some examples of endothermic reactions in real life

Answer 7

Sports injury packs: The chemical reaction allows the pack to become instantly cooler without a freezer

Question 8

Describe a practical where energy transfer is measured testing the effects of acid concentrations

Answer 8

1) Put 25cm3 of 0.25mol/dm3 hydrochloric acid and sodium hydroxide in separate beakers
2) Place the beakers in25 degrees water until they are at 25 degrees
3) Mix the two reactants in a polystyrene cup with a lid
4) Measure the temperature change every 30s
5) Repeat with different acid concentrations

Question 9

What are reaction profiles

Answer 9

These are diagrams showing the energy transfers during a reactions, by showing the reactants and the products energy

Question 10

What is the activation energy?

Answer 10

This is the energy needed to start a reaction. Overall, on a reaction profile, it is the initial rise of energy before it drops

Question 11

How can you tell if a reaction is endo or exo from just a reaction profile

Answer 11

On the profile, if the energy amount of the products are lower then that of the reactants, the reaction is exothermic as energy has been released
If the energy of the products is higher then the energy of the reactants, then energy has been taken in and the reaction is endothermic

Question 12

Is energy lost or gained if a bond is BROKEN?

Is it endothermic or exothermic

Answer 12

If a bond is broken, then energy is taken in from the surroundings (So the process is endothermic)

Question 13

Is energy lost or gained if a bond is FORMED?

Is it endothermic or exothermic

Answer 13

If a bond is formed, energy is released, so the process is exothermic

Question 14

In terms of breaking and forming bonds, how can you tell if the overall reaction is exothermic or endothermic?

Answer 14

If the energy taken in to break bonds is greater then the energy released, then the reaction is endothermic
If more energy is released from forming bonds, then the reaction is exothermic

Question 15

How would you calculate energy change for a reaction, and work out if it is exo or endo

Answer 15

1) Add up the energy needed to break all the bonds
2) Add up all the energy released from forming bonds
3) Energy used to break bonds - Energy released to form bonds
4) If the final answer is NEGATIVE then the reaction is exothermic. If the final answer is POSITIVE then the reaction is ENDOTHERMIC

Question 16

Calculate the energy change for the reaction: H2 + Cl2 = 2HCL
( H – H = 436 )
( Cl – Cl = 242)
( H – Cl = 431)

Answer 16

(1 x 436) + (1 x 242) = 678
2 x 431 = 862

678 - 862 = -184kJ/mol
So the reaction is exothermic