C5 - Energy Changes Flashcards
Define an exothermic reaction
a reaction where heat energy is transferred to the surroundings (make bonds)
Define an endothermic reaction
a reaction where heat energy is absorbed from the surroundings (breaks bonds)
What do you use to remember which is exo or endothermic (about bonds) ?
BENDOMEXO
endo = breaks bonds
exo = makes bonds
If the reaction gets hotter, is it exo or endo?
exothermic
If the reaction gets colder, is it exo or endo?
endothermic
How could you measure whether a reaction is exothermic or endothermic?
- use thermomter
- touch it
- observe steam
Describe an exothermic graph
The energy level of reactants is larger than products so the overall energy change is negative
Describe endothermic graph
The energy level of reactants is smaller than products so the overall energy change is positive
Describe the RPA for measuring temperature change
- 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
Why use a polystyrene cup for the temp change experiment?
Polystyrene is a better insulator than glass - reduces amount of energy lost to surroundings
Why use a lid for the temp change experiment?
It reduces the energy lost by evaporation - more accurate
Why stir the mixture in the temp change experiment?
so temperature is more even throughout
How do you do bond energy calculations?
- 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)
Describe the set-up of a chemical cell
two electrodes made of different metals connected by a wire with a voltmeter and in electrolyte
Describe what happens in a chemical cell
the more reactive metal will donate electrons to the less reactive metal creating a current
What are some factors that will affect the voltage of a chemical cell?
- difference in reactivity of the metals (bigger difference = bigger voltage)
- temperature
- type of electrolyte solution
Why does a battery run out?
the reactants ran out
Why can some batteries be recharged?
the reaction can be reversed in certain cells by connecting it to an external electric current
Define activation energy
the energy required to break bonds at the start of the chemical reaction
Explain the difference between electrolysis and chemical cells
electrolysis uses electricity to make a chemical reaction but in chemical cells, a chemical reaction is used to make electricity
Define a redox reaction
A chemical reaction involving both reduction (gaining electrons) and oxidation (loss of electrons)
State the 2 half equations for the electrodes in a hydrogen fuel cell
2H₂ –> 4H+ + 4e-
4H+ + 4e- + O₂ –> 2H₂O
State the overall symbol and word equation for a hydrogen fuel cell
2H₂ + O₂ –> 2H₂O
hydrogen + oxygen –> water
Describe how a hydrogen fuel cell works
- 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
Name some advantages of hydrogen fuel cells:
- water is the only byproduct
- ## store more energy than batteries = less recharging required
Name some disadvantages of hydrogen fuel cells:
- hydrogen is hard to store and transport safely - explosive
- often non-renewable because hydrogen often taken from methane
