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Bonding, Structure and Properties of Matter > Energy Changes > Flashcards

Energy Changes Flashcards

1
Q

what is an exothermic reaction?

A
  • reaction which transfers heat to the surroundings
  • shown by rise in temp
  • by heating
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2
Q

give an example of an exothermic reaction

A
  • combustion (burning of fossil fuels)
  • nuetralisation reactions
  • many oxidation reactions e.g sodium added to water releases energy
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3
Q

give examples of exothermic reaction in every day uses

A
  • hand warmers –> oxidation of iron in air with salt solution catalyst to release energy
  • self heating cans –> exothermic reactions between chemicals in their bases
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4
Q

what is an endothermic reaction?

A
  • reaction which takes in energy from surroundings
  • shown by fall in temp
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5
Q

give examples of endothermic reactions

A
  • photosynthesis –> chlorophyll absorbs energy from sun
  • thermal decomposition –> heating calcium carbonate decomposes into calcium oxide and carbion dioxide
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6
Q

give the symbol equation of thermal decomposition of calcium carbonate

A

CaCO2 (+HEAT) –> CO2 + CaO

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7
Q

give examples of endothermic reactions in every day uses

A

-sports injury packs –> instantly cool without freezer

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8
Q

how do you measure the amount of chemicals released by a chemical reaction in a solution?

A
  • place polystyrene cup in glass beaker for stability
  • measure temp of reagents
  • mix in polystyrene cup
  • measure temp of solution at end of reaction
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9
Q

how do you reduce energy loss to surroundings in a practical?

A
  • polystyrene cup in beaker of cotton wool
  • more insulation
  • lid reduces energy lost thru evaporation
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10
Q

what else can you investigate using the polystyrene cup method?

A
  • effect different variables have on amount of energy transfered
  • e.g mass or concentration of reactants used
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11
Q

how to test the effect of acid concentration on energy released in a neutralisation reaction between HCl and NaOH?

A
  1. 25cm3 of 0.25mol/dm3 of HCl and NaOH in seperate beakers
  2. place beakers in water bath set to 25°C until both same temp
  3. -add HCl followed by NaOH to polystyrene cup with lid
  4. take temp of mixture every 30 seconds and record highest temp
  5. -repeat steps 1-4 using 0.5mol/dm3 and then 1mol/dm3 of HCl
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12
Q

what do reaction profiles show?

A
  • relative energies of reactants and products in reaction
  • activation energy
  • overall energy change
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13
Q

Describe the exothermic reaction profile:

A
  1. exothermic reaction - products at lower energy level than reactants
  2. difference in height represents overall energy change (energy given out) per mole
  3. initial rise in energy represents energy needed to start reaction (activation energy)
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14
Q

what is activation energy?

A

-minimum amount energy needs to collide with eachother and react

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15
Q

Describe the endothermic reaction profile: ​

A
  1. endothermic reaction - products at higher energy than reactants
  2. difference in height represents overall energy change during reaction (energy taken in) per mole
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16
Q

how does a catalyst increase the rate of reaction?

A
  • provides alternative reaction route
  • that has lower activation energy
  • by weakening bonds
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17
Q

how does a covalent bond hold atoms together?

A
  • electrostatic forces of attraction
  • between shared pair of negatively charged electrons
  • and both positively charged nuclei
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18
Q

what happens to the bonds in a chemical reaction?

A
  • old bonds are broken
  • new bonds are formed
19
Q

what process is bond breaking?

A
  • 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
20
Q

what process is bond formation?

A
  • 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
21
Q

energy change in an endothermic reaction is…

A

positive

22
Q

energy change in an exothermic reaction is…

A

negative

23
Q

bond energies are measured in…

A

kJ/mol

24
Q

what is an electrochemical cell?

A
  • 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
25
Q

what affects the voltage of the cell?

A
  • type of electrodes
  • bigger difference in reactivity, bigger voltage of cell
26
Q

what causes charge difference?

A
  • different metals reacting differently
  • with same electrolyte
27
Q

what does the electrolyte used in cells affect?

A
  • size of voltage
  • different ions in solution will react differently with electrodes used
28
Q

how is a battery formed?

A
  • two or more cells connected in a series
  • voltage of cells combined = bigger voltage overall
29
Q

why are there irreversible chemical reactions at the electrodes in some cells?

A
  • 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
30
Q

what is a non-rechargeable battery?

A
  • 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
31
Q

what is a rechargeable cell?

A
  • reaction is reversible
  • by connecting to external electric current
32
Q

what is a fuel cell?

A
  • 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
33
Q

give an example of a fuel cell

A
  • hydrogen-oxygen fuel cell
  • combines H and O2 to produce clean water and release energy
34
Q

when dealing iwth fuel cells what charge is the electrodes?

A

positive = cathode

negative = anode

35
Q

in hydrogen-oxygen fuel cells. what often is the electrolyte?

A

potassium hydroxide

36
Q

in hydrogen-oxygen fuel cells, what often is the electrodes made out of?

A

porous carbon with catalyst

37
Q

what happens in a fuel cell at the negative anode?

A
  • hydrogen goes into anode compartment
  • loses electrons to produce H+ ions; oxidation
  • H+ ions in electrolyte moves to positive cathode
38
Q

what happens in a fuel cell at the positive cathode?

A
  • oxygen goes into cathode compartment
  • gains electrons from cathode
  • reacts with H+ ions to form water; reduction
39
Q

how does the electric current flow in a fuel cell?

A
  • electrons flow through external current
  • from anode to cathode
40
Q

what is the oxidation reaction involved in a hydrogen-oxygen fue cell?

A

H2 –> 2H+ + 2e-

41
Q

what is the reduction reaction involved in a hydrogen-oxygen fuel cell?

A

02 + 4H+ + 4e- –> 2H2O

42
Q

conventional fuel for vehicles…

A
  • has finite supply
  • very polluting
  • vehicles using electrical energy becoming more popular
43
Q

what are the advantages to fuel cells in comparison to batteries?

A
  • 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
44
Q

what are the disadvantages to hydrogen fuel cells?

A
  • 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

Bonding, Structure and Properties of Matter (11 decks)

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