C5 - Energy Changes

Question 1

Define an exothermic reaction

Answer 1

a reaction where heat energy is transferred to the surroundings (make bonds)

Question 2

Define an endothermic reaction

Answer 2

a reaction where heat energy is absorbed from the surroundings (breaks bonds)

Question 3

What do you use to remember which is exo or endothermic (about bonds) ?

Answer 3

BENDOMEXO
endo = breaks bonds
exo = makes bonds

Question 4

If the reaction gets hotter, is it exo or endo?

Answer 4

exothermic

Question 5

If the reaction gets colder, is it exo or endo?

Answer 5

endothermic

Question 6

How could you measure whether a reaction is exothermic or endothermic?

Answer 6

• use thermomter
• touch it
• observe steam

Question 7

Describe an exothermic graph

Answer 7

The energy level of reactants is larger than products so the overall energy change is negative

Question 8

Describe endothermic graph

Answer 8

The energy level of reactants is smaller than products so the overall energy change is positive

Question 9

Describe the RPA for measuring temperature change

Answer 9

• put 25 cm³ of (named) acid to a polystyrene cup
• use a thermometer to measure starting temp
• add 5g of (metal (oxide)) and stir and add a lid
• measure temp every 30 secs and record the max temp
• repeat
• if the temp increases, it is exo but decreases = endo

Question 10

Why use a polystyrene cup for the temp change experiment?

Answer 10

Polystyrene is a better insulator than glass - reduces amount of energy lost to surroundings

Question 11

Why use a lid for the temp change experiment?

Answer 11

It reduces the energy lost by evaporation - more accurate

Question 12

Why stir the mixture in the temp change experiment?

Answer 12

so temperature is more even throughout

Question 13

How do you do bond energy calculations?

Answer 13

• use a table provided to work out the energy needed to break the bonds in the reactants - do number of bonds x energy from table
• do the same for the products
• do first number - second number to calculate overall energy change (probably negative)

Question 14

Describe the set-up of a chemical cell

Answer 14

two electrodes made of different metals connected by a wire with a voltmeter and in electrolyte

Question 15

Describe what happens in a chemical cell

Answer 15

the more reactive metal will donate electrons to the less reactive metal creating a current

Question 16

What are some factors that will affect the voltage of a chemical cell?

Answer 16

• difference in reactivity of the metals (bigger difference = bigger voltage)
• temperature
• type of electrolyte solution

Question 17

Why does a battery run out?

Answer 17

the reactants ran out

Question 18

Why can some batteries be recharged?

Answer 18

the reaction can be reversed in certain cells by connecting it to an external electric current

Question 19

Define activation energy

Answer 19

the energy required to break bonds at the start of the chemical reaction

Question 20

Explain the difference between electrolysis and chemical cells

Answer 20

electrolysis uses electricity to make a chemical reaction but in chemical cells, a chemical reaction is used to make electricity

Question 21

Define a redox reaction

Answer 21

A chemical reaction involving both reduction (gaining electrons) and oxidation (loss of electrons)

Question 22

State the 2 half equations for the electrodes in a hydrogen fuel cell

Answer 22

2H₂ –> 4H+ + 4e-

4H+ + 4e- + O₂ –> 2H₂O

Question 23

State the overall symbol and word equation for a hydrogen fuel cell

Answer 23

2H₂ + O₂ –> 2H₂O
hydrogen + oxygen –> water

Question 24

Describe how a hydrogen fuel cell works

Answer 24

• hydrogen gas enters anode and loses electrons (oxidation)
• electrons flow through wire = current
• hydrogen ions from oxidation reaction diffuse through electrolyte solution
• the hydrogen ions combine with electrons and oxygen gas to produce water at the cathode

Question 25

Name some advantages of hydrogen fuel cells:

Answer 25

• water is the only byproduct
• ## store more energy than batteries = less recharging required

Question 26

Name some disadvantages of hydrogen fuel cells:

Answer 26

• hydrogen is hard to store and transport safely - explosive
• often non-renewable because hydrogen often taken from methane