Energy Changes Flashcards
what happens in an exothermic reaction?
there is a transfer of energy to surroundings which registers as a temperature increase
what happens in an endothermic reaction?
there is a transfer of energy from the surroundings which registers as a temperature decrease
what is the overall energy change for a reaction also known as?
delta H or enthalpy change
when is a reaction exothermic?
energy required to break bonds in reactants is lower than energy given out when new bonds are formed
when is a reaction endothermic?
energy required to break bonds in reactants is higher than energy given out when new bonds are formed
is breaking bonds exothermic or endothermic?
endothermic
is forming bonds exothermic or endothermic?
exothermic
how do you calculate delta H?
bonds to break - bonds formed
what does a negative value for delta H mean?
the reaction is exothermic
how do chemical cells work?
contain chemicals which react to produce electricity
what factors affect voltage in chemical cells?
- identity of metals; bigger difference in reactivity = bigger voltage
- electrolyte; identity and concentration
what is a battery?
2 or more cells connected
how do you make a 6V battery?
4 1.5V cells in series
which electrode is more reactive in chemical cells?
the anode
what are the 3 types of cell?
normal (chemical) cell
hydrogen fuel cell
rechargeable cell
how do normal cells work?
go flat as chemicals used up
how do hydrogen fuel cells work?
chemicals pumped in to replace those used up
how do rechargeable cells work?
chemical reactions reverse to reproduce chemicals used up
what is the chemical reaction in a hydrogen fuel cell?
hydrogen (fuel) reacts with pure oxygen or air
what is the product of the reaction in a fuel cell?
water
what are 2 positives of hydrogen fuel cells?
- produce electricity as long as hydrogen is provided
- do not get less efficient
what are 2 negatives of hydrogen fuel cells?
- hydrogen gas is explosive and difficult to store safely
- produce low voltages so many are required
what are 2 negatives of rechargeable battery cells?
- will run out and need recharging
- store less electricity over time
what are 2 positives of rechargeable battery cells?
- no dangerous fuel required
- produce greater voltages compared to hydrogen fuel cells
Explain the difference between the processes in electrolysis and in a chemical cell.
electrolysis uses electricity to produce a chemical reaction BUT cells use a chemical reaction to produce electricity
