energy changes topic 5

how much energy can chemical store take in, if products of reaction store more energy then, if they store less, why doesnt the overall amount of energy change

how much energy can chemical store take in, if products of reaction store more energy then, if they store less, why doesnt the overall amount of energy change

• chemicals store certain amount energy & diff chemicals store diff amounts

• if products of reaction store more energy than og reactants, then must have taken in diff in energy between products & reactants from surroundings during reaction

• if store less, then excess energy was transferred to surroundings during reaction

• overall amount energy doesn’t change because energy is conserved in reactions → cant be created, destroyed only moved around = energy in universe stays same

whats exothermic reactions & hows it shown, 2 examples, how is oxidation reactions sometimes exothermic, 2 everyday uses of it

• exothermic reaction → transfers energy to surroundings usually by heating, shown by rise in temp

• example → burning fuels or sometimes called combustion = gives out loads energy, very exothermic

• neutralisation reactions (acid + alkali) r also exothermic

• many oxidation reactions r exothermic → adding sodium to water releases energy = exothermic. The reaction releases energy & sodium moves about on surface of water as its oxidised

• exothermic reactions have lots everyday uses

  • some hand warmers use exothermic oxidation of iron in air (w salt solution catalyst) to release energy

  • self heating cans of hot chocolate & coffee also rely on exothermic reactions between chemicals in their bases

whats endothermic reaction & hows it shown, they r…, 2 examples, everyday use

• endothermic reaction → takes in energy from surroundings, shown by fall in temp

• much less common than exothermic reactions but they include:

  • reaction between citric acid & sodium hydrogencarbonate

  • thermal decomposition - heating calcium carbonate causes it to decompose into calcium oxide (quicklime) & carbon dioxide

  • CaCo_3 (+heat) → CO_2 + CaO

• endothermic reaction everyday uses:

  • used in some sports injury packets - chemical reactions allows pack to become instantly cooler without having to put it into freezer

how can u measure amount of energy released by chemical reactions, biggest prob w energy measurements…, how to reduce this, what else can we measure w this method

1. can measure amount of energy released by chemical reaction (in solution) by taking temp of reagents, mixing them in polystyrene cup & measuring temp of solution at end of reaction

2. biggest problem w energy measurements is amount energy lost to surroundings

3. can reduce it a bit by putting polystyrene cup into beaker of cotton wool to give more insulation & putting lid on cup to reduce energy lost by evaporation

4. can also use method to investigate what effect diff variables have on amount of energy transferred (mass/ concentration of reactants used)

how could u test effect of acid concentration on energy released in neutralisation reaction between hydrochloric acid (HCl) & sodium hydroxide (NaOH) (5 steps)

1. put 25cm³ of 0.25mol/dm³ of hydrochloric acid & sodium hydroxide in separate beakers

2. place beakers in water bath set to 25°C until both r at same temp

3. add HCl followed by NaOH to polystyrene cup w lid

4. take temp of mixture every 30 seconds & record highest temp

5. repeat steps 1-4 using 0.5mol/dm³ & then 1mol/dm³ of hydrochloric acid

whats reaction profiles, exo: y is it an exothermic reaction profile, diff in height, intial rise in energy reps, whats activation energy, greater activation energy…, endo: whys it endothermic reaction profile, diff in height reps

• reaction profiles → diagrams that show relative energies of reactants & products in reaction & how energy changes over course reaction

• exothermic reactions:

  • products at lower energy than reactants

  • diff in height reps overall energy change in reaction (given out) per mole

  • initial rise in energy reps energy needed to start reaction → activation energy (E_a)

  • activation energy is minimum amount of energy reactants need to collide w each other & react

  • greater activation energy = more energy needed to start reaction (has to be supplied by heating reaction mixture)

• endothermic reactions:

  • products at higher energy then reactants

  • diff in height reps overall energy change during reaction (energy taken in) per mole

during chemical reactions…, how is bond breaking an endothermic process, how is bon formation a exothermic process, which releases more energy in endothermic & exothermic reactions

• during chemical reactions old bonds r broken & new bonds r formed

• energy must be supplied to break existing bonds - so bond breaking is an endothermic process

• energy is released when new bonds r formed - bond formation is an exothermic process

• in exothermic reactions energy releases by forming bonds is greater than energy used to break them. In endothermic reactions energy used to break bonds is greater than energy released by forming them

how to calculate energy changes for reactions using bond energies & equation, what do u need to compare them

1. find energy required to break og bonds

2. find energy released by forming new bonds

3. find overall energy change for reaction using the equation:

   overall energy change = energy required to break bonds - energy released by forming bonds

• cant compare overall energy changes unless you know numerical diffs in bond energies

whats an electrochemical cell, 2 electrodes must & r usually.., whats electrolyte, what do chemical reactions set up, what happens if electrodes r then connected by wire

• electrochemical cell is a basic system made up of 2 diff electrodes in contact w an electrolyte:

1. 2 electrodes must be able to conduct electricity & so r usually metals

2. electrolyte is a liquid that contains ions which react w electrodes

3. chemical reactions between electrodes & electrolyte set up charge difference between electrodes

4. if electrodes r then connected by wire, charge is able to flow & electricity is produced. A voltmeter can be connected to circuit to measure voltage of cell

what do diff metals reacting differently w electrolyte cause, bigger diff in reactivity of electrodes…, how does electrolyte affect size of voltage, how is a battery formed

• diff metals react differently w same electrolyte → this causes charge difference/ voltage of cell, type of electrodes used affect voltage of cell

• bigger the diff in reactivity of electrodes = bigger the voltage of cell

• (can predict voltage of cell might be from info about voltages of other cells)

• electrolyte used in cell affects size of voltage since diff ions in solution react differently w metal electrodes used

• battery is formed by connecting 2/ more cells tgther in series. voltage of cells in battery r combined so theres bigger voltage overall

what happens in some cells, what happens over time to reacting particles, why is no electricity produced, what r non-rechargeable batteries (definition), what can happen in rechargeable cell

• in some cells chemical reactions happen at electrodes r irreversible

• over time reacting particles - ions in electrolyte & metal ions on electrode - get used up & turned into products of reaction

• once any one of reactions is used up the reaction cant happen & so no electricity is produced

• non-rechargeable batteries → alkaline batteries, contain cells which use irreversible reactions, once one of reactants is used up they dont produce any more charge & have to replace them

• in rechargeable cell, reaction can be reversed by connecting it to an external electric current

whats fuel cell, what happens when fuel enters cell, what can diff fuel cells use, important example & what does it produce

• fuel cell → electrical cell thats supplied w fuel & oxygen (/ air) & uses energy from reaction between them to produce electrical energy efficiently

• when fuel enters cell it becomes oxidised & sets up pd within cell

• few diff types of fuel cells, using diff fuels & diff electrolytes,

• important example → hydrogen-oxygen fuel cell, combined hydrogen & oxygen to produce nice clean water & release energy

How does a hydrogen-oxygen fuel cell work (reduction, oxidisation, electrolysis) - involves redox reaction

• electrolyte is often an acid (phosphoric acid). Electrodes r often porous carbon w a catalyst

• hydrogen goes into anode compartment & oxygen goes into cathode compartment (electrolysis)

• at -ve electrode (anode), hydrogen loses electrons to produce H^+ ions → oxidisation

• H^+ ions in electrolyte move tk cathode (+ve):

  H_2 → 2H^+ + 2e^-

• at +be (cathode) oxygen gains electrons from cathode & reacts w H^+ ions (from acidic electrolyte) to make water → reduction

  O_2 + 4H^+ + 4e^- → 2H_2O

• electrons flow through external circuit from anode to cathode → electric current

• overall reaction is hydrogen + oxygen which gives water:

  2H_2 + O_2 → 2H_2O

3 pros of hydrogen-oxygen fuel cells used in vehicles

• Fuel cell vehicles dont produce as many pollutants as other fuels - no greenhouse gases, nitrogen oxides, sulfur dioxide or carbon monoxide. Only by-products r water & heat. Electrical vehicles dont produce as many pollutants either - but their batteries r more polluting to dispose of than fuel cells because made from highly toxic metal compounds

• batteries in electric vehicles r rechargeable but theres a limit ti hiw many times can be reCharged before need replacing, & batteries r more expensive to make fuel cells

• batteries also store less energy then fuel cells & so would need to be recharged more often - can take long time

3 cons of using hydrogen-oxygen fuel cells in vehicles

• hydrogen is gas so it takes up loads more space to store than rechargeable battery

• hydrogen is explosive when mixed w air so its hard to store safely

• hydrogen fuel is often made either from hydrocarbons (from fossil fuels) or by electrolysis of water which uses electricity (& electricity has got to be generated somehow - usually fossil fuels)