Chemistry - Energy Changes

Question 1

exothermic reactions

Answer 1

Energy from the reacting chemicals is transferred to the surroundings, which often increases in temperature as a result

Question 2

examples of exothermic reactions

Answer 2

combustion
neutralisation
oxidation

Question 3

endothermic reaction

Answer 3

energy from the surroundings is transferred to he reacting chemicals, causing the temperature of the surroundings to decrease

Question 4

examples of endothermic reactions

Answer 4

thermal decomposition
photosynthesis

Question 5

how do reusable hand warmers work

Answer 5

use the energy released by iron oxidation. they source their energy from the crystallisation of salt solutions. Boiling the pack re-dissolves the salt solutions so it is ready to be activated once more

Question 6

how do sports injury packs work

Answer 6

when squeezed tightly ammonium nitrate and water mix in the pack causing instant cooling.

Question 7

requirements for a reaction to happen

Answer 7

collisions-particles of the reactants have to collide
activation energy-the collisions that happen must take place with enough energy, the threshold amount of energy is called the activation energy

Question 8

activation energy

Answer 8

the minimum amount of energy that reacting particles must have in order for a reaction to occur

Question 9

if the total energy released by forming bonds is less than total energy required to break bonds then the reaction is

Answer 9

endothermic

Question 10

bond making

Answer 10

releases energy - exothermic

Question 11

bond breaking

Answer 11

needs energy-endothermic

Question 12

how can the voltage output of a cell be changed

Answer 12

changing the type of electrode, the type of electrolyte or both

Question 13

what does changing the electrolyte do

Answer 13

changes the reaction that happens at the electrodes resulting in the voltage output of the cells changing

Question 14

what does changing the electrodes do

Answer 14

a greater difference in reactivity between the two electrodes will result in the cell having a greater voltage

Question 15

what is a battery

Answer 15

a collection of more than one cell joined in a series.

Question 16

how do you calculate the voltage of the battery

Answer 16

the sum of voltages of the voltages of its individual cell components

Question 17

why is a battery disposable

Answer 17

when the chemicals of a battery have run out the battery can not be used anymore

Question 18

what happens in non-rechargeable cells and batteries

Answer 18

irreversible reactions take place at the electrodes this means that electricity can no longer be produced as soon as one of the reacting metals has run out

Question 19

what happens in rechargeable cells and batteries

Answer 19

connections to an electric current reverses the reaction that happens at an electrode

Question 20

what are fuel cells

Answer 20

Fuel cells produce electrical energy using a reaction between an external fuel source (often hydrogen) and oxygen, the fuel is added to a cell and then there is a constant supply of oxygen. The fuel is oxidised and that creates a voltage

Question 21

advantages of fuel cells

Answer 21

• compact and lightweight
• reliable
• less pollution than traditional methods
• high efficiency

Question 22

in fuel cells what is the only waste product

Answer 22

water

Question 23

anode half equation

Answer 23

hydrogen is pumped in at the anode. hydrogen ions are formed as hydrogen molecules are oxidised and lose electrons.
2H2 → 4H+ + 4e-

Question 24

cathode half equation

Answer 24

Oxygen is pumped in at the cathode. water is formed as the oxygen reacts with the H+ ions that have moved through the electrolyte from the anode.
O2 + 4H+ + 4e- → 2H2O

Question 25

overall equation of hydrogen fuel cells

Answer 25

2H2 + O2 → 2H2O

Question 26

what are the risks of hydrogen fuel cells

Answer 26

hydrogen has to be stored at a high pressure and hydrogen is explosive