Energy Changes Flashcards
what is an exothermic reaction?
- reaction which transfers heat to the surroundings
- shown by rise in temp
- by heating
give an example of an exothermic reaction
- combustion (burning of fossil fuels)
- nuetralisation reactions
- many oxidation reactions e.g sodium added to water releases energy
give examples of exothermic reaction in every day uses
- hand warmers –> oxidation of iron in air with salt solution catalyst to release energy
- self heating cans –> exothermic reactions between chemicals in their bases
what is an endothermic reaction?
- reaction which takes in energy from surroundings
- shown by fall in temp
give examples of endothermic reactions
- photosynthesis –> chlorophyll absorbs energy from sun
- thermal decomposition –> heating calcium carbonate decomposes into calcium oxide and carbion dioxide
give the symbol equation of thermal decomposition of calcium carbonate
CaCO2 (+HEAT) –> CO2 + CaO
give examples of endothermic reactions in every day uses
-sports injury packs –> instantly cool without freezer
how do you measure the amount of chemicals released by a chemical reaction in a solution?
- place polystyrene cup in glass beaker for stability
- measure temp of reagents
- mix in polystyrene cup
- measure temp of solution at end of reaction
how do you reduce energy loss to surroundings in a practical?
- polystyrene cup in beaker of cotton wool
- more insulation
- lid reduces energy lost thru evaporation
what else can you investigate using the polystyrene cup method?
- effect different variables have on amount of energy transfered
- e.g mass or concentration of reactants used
how to test the effect of acid concentration on energy released in a neutralisation reaction between HCl and NaOH?
- 25cm3 of 0.25mol/dm3 of HCl and NaOH in seperate beakers
- place beakers in water bath set to 25°C until both same temp
- -add HCl followed by NaOH to polystyrene cup with lid
- take temp of mixture every 30 seconds and record highest temp
- -repeat steps 1-4 using 0.5mol/dm3 and then 1mol/dm3 of HCl
what do reaction profiles show?
- relative energies of reactants and products in reaction
- activation energy
- overall energy change
Describe the exothermic reaction profile:
- exothermic reaction - products at lower energy level than reactants
- difference in height represents overall energy change (energy given out) per mole
- initial rise in energy represents energy needed to start reaction (activation energy)
what is activation energy?
-minimum amount energy needs to collide with eachother and react
Describe the endothermic reaction profile:
- endothermic reaction - products at higher energy than reactants
- difference in height represents overall energy change during reaction (energy taken in) per mole
how does a catalyst increase the rate of reaction?
- provides alternative reaction route
- that has lower activation energy
- by weakening bonds
how does a covalent bond hold atoms together?
- electrostatic forces of attraction
- between shared pair of negatively charged electrons
- and both positively charged nuclei
what happens to the bonds in a chemical reaction?
- old bonds are broken
- new bonds are formed
what process is bond breaking?
- endothermic
- energy must be supplied to break existing bonds
- energy used to break bonds is greater than energy released by forming them
- because overall energy has been taken in
what process is bond formation?
- exothermic
- energy is released when new bonds are formed
- which is greater than the energy used to break them
- because overall energy has been released
energy change in an endothermic reaction is…
positive
energy change in an exothermic reaction is…
negative
bond energies are measured in…
kJ/mol
what is an electrochemical cell?
- two electrodes (conductors)
- electrolyte (contains ions to react with electrodes)
- chemical reaction between them sets charge difference between electrodes
- if connected by wire, charge flows electricity produced
what affects the voltage of the cell?
- type of electrodes
- bigger difference in reactivity, bigger voltage of cell
what causes charge difference?
- different metals reacting differently
- with same electrolyte
what does the electrolyte used in cells affect?
- size of voltage
- different ions in solution will react differently with electrodes used
how is a battery formed?
- two or more cells connected in a series
- voltage of cells combined = bigger voltage overall
why are there irreversible chemical reactions at the electrodes in some cells?
- over time reacting particles (ions in electrolyte and metal ions on electrode) get used up and turn into the products of reaction
- once a reactant is used up, reaction can’t happen, no electricity produced
- product can’t be turned back into reactants, cell can’t be recharged
what is a non-rechargeable battery?
- e.g alkaline batteries
- contain cell which use irreversible reactions
- once one reactant used up, don’t produce any more charge
- products can’t turn back into reactants
what is a rechargeable cell?
- reaction is reversible
- by connecting to external electric current
what is a fuel cell?
- electrical cell supplied with fuel and oxygen
- uses energy from reaction to produce electrical energy efficiently
- when fuel enters cell, becomes oxidised, sets up p.d within cell
give an example of a fuel cell
- hydrogen-oxygen fuel cell
- combines H and O2 to produce clean water and release energy
when dealing iwth fuel cells what charge is the electrodes?
positive = cathode
negative = anode
in hydrogen-oxygen fuel cells. what often is the electrolyte?
potassium hydroxide
in hydrogen-oxygen fuel cells, what often is the electrodes made out of?
porous carbon with catalyst
what happens in a fuel cell at the negative anode?
- hydrogen goes into anode compartment
- loses electrons to produce H+ ions; oxidation
- H+ ions in electrolyte moves to positive cathode
what happens in a fuel cell at the positive cathode?
- oxygen goes into cathode compartment
- gains electrons from cathode
- reacts with H+ ions to form water; reduction
how does the electric current flow in a fuel cell?
- electrons flow through external current
- from anode to cathode
what is the oxidation reaction involved in a hydrogen-oxygen fue cell?
H2 –> 2H+ + 2e-
what is the reduction reaction involved in a hydrogen-oxygen fuel cell?
02 + 4H+ + 4e- –> 2H2O
conventional fuel for vehicles…
- has finite supply
- very polluting
- vehicles using electrical energy becoming more popular
what are the advantages to fuel cells in comparison to batteries?
- fc produces heat and water, no CO2
- renewable source: plentiful supply of water
- batteries more polluting to dispose of, made from highly toxic metal compounds
- batteries have limit to be recharged
- batteries more expensive to make
- batteries store less energy, recharge more often, takes long time
what are the disadvantages to hydrogen fuel cells?
- gas, takes up more space to store than rechargeable battery
- dangerous reactivity: explosive when mixed with air
- often made from fossil fuels or electrolysis of water using electricity which is expensive
