rate & extent of chemical change topic 6

whats rate of chemical reaction, slow reactions example, another slow example & explain, moderate speed example, 2 fast examples

whats rate of chemical reaction, slow reactions example, another slow example & explain, moderate speed example, 2 fast examples

• rate of chemical reaction → how fast reactants r changed into products

• one of slowest → rusting of iron

• other slow reactions → chemical weathering (acid rain damage to limestone buildings)

• moderate speed → metal magnesium reacting w acid to produce gentle stream bubbles

• fast → burning, explosions r even faster & release lots gas all over in fraction of second

what does collision theory, what 2 things rate of chemical reaction depends on, whats activation energy

• reaction rate is explained by collision theory

• rate of chemical reaction depends on:

  • collision frequency of reacting particles (how often collide), more collisions = faster reaction

  • energy transferred during collision, particles have to collide w enough energy for collision to be successful

    minimum amount of energy particles need to react → activation energy (need this much energy to break bonds in reactants & start reaction

4 things rate of reaction depends on

1. temperature

2. concentration of solution/ pressure of gas

3. surface area - changes depending on size of lumps of solid

4. presence of catalyst

how does temp, concentration/ pressure, SA, catalyst increase the rate

• increasing temp increases rate:

  1. when temp increases = particles move faster

  2. moving faster = collide more frequently

  3. also move faster = more energy they have = more collision will have enough energy make reaction happen

• increasing concentration/ pressure increases rate:

  1. solution made more concentrated = more particles knocking about in same volume of water/ other solvent

  2. pressure of gas increases = same no. particles occupy smaller space

  3. this makes collisions between reactant particles more frequent

• increasing SA increases rate:

  1. one of reactants is solid, breaking it up into smaller pieces will increase its SA:V ratio

  2. means for same volume of solid particles around it have more area to work on → Collisions happen more frequently

• using catalyst increases rate:

  1. catalyst → substance that speeds up reaction, w/out being used up itself (not apart of overall reaction equation)

  2. diff catalysts r needed for diff reactions but all work by decreasing activation energy needed for reaction to occur → provide alternative reaction pathway w lower activation energy

  3. enzymes = biological catalysts (catalyse reactions of living things)

equation for rate of reaction, what r the units for gas, solid & time

• rate of reaction = amount of reactants or amount of product formed / time

• gas = cm³, moles

• solid = grams, moles

• time = s

how can we measure rate of reaction using precipitation & colour change, change in mass, volume of gas given off

• precipitation & colour change:

  1. can record visual changes in reaction if initial solution is transparent & product is a precipitate which clouds solution (becomes opaque)

  2. can observe mark through solution & measure how long it takes for it to disappear

  3. if reactants r coloured & products r colourless (vice versa), can time how long it takes for solution to lose/ gain its colour

  4. cant plot rate of reaction graph from results

• change in mass (usually gas given off):

  1. measuring speed of reaction that produces gas can be carried out using mass balance

  2. gas released = mass disappearing is measured on balance

  3. take measurements at regular intervals, can plot rate of reaction graph & find rate quite easily

  4. most accurate of 3 methods because mass balance is very accurate but has disadvantages of releasing gas straight into room

• volume of gas given off:

  1. involves using gas syringe to measure volume of gas given off

  2. more gas given off during given time interval = faster reaction

  3. gas syringes usually give volume accurate to nearest cm³ = quite accurate, take measurements at regular intervals & plot rate of reaction graph but have to be careful → if reaction is too vigorous can easily blow plunger out of end of syringe

how to measure rate of reaction using magnesium & HCl which react to produce H_2 gas (6 steps)

1. add set volume of dilute hydrochloric acid to a conical flask

2. add some magnesium ribbon to acid & quickly attach empty gas syringe to flask

3. start stopwatch. Take readings of volume of gas in gas syringe at regular intervals, recording results in table

4. plot graph w time on x-axis & volume of gas on y-axis

5. repeat w more concentrated acid solutions. variables such as amount of magnesium ribbon & volume of acid used should be kept same each time - only change acids concentration to make experiment fair test

6. 3 graphs show higher concentration of acid gives faster rate of reaction

how to measure rate of reaction of sodium thiosulfate & HCl which produce a cloudy precipitate (8 steps)

1. these 2 chemicals r both clear solutions. React tgther to form yellow precipitate of sulfur

2. add set volume of dilute sodium thiosulfate to conical flask

3. place flask on piece of paper w black cross drawn on it. Add some HCl to flask & start stopwatch

4. watch black cross disappear through cloudy sulfur & time how long it takes

5. reactants should be replaced w solutions of either reactant at diff concentrations (only change 1 at a time). Depth of liquid must be kept same each time

6. results show effect of increasing concentration of HCl on rate of reaction, when added to excess of sodium thiosulfate

7. higher concentration = quicker reaction & therefore less time takes for mark to disappear

8. doesnt give set of graphs

hows a rate of reaction graph shown, how to find mean rate for whole reaction, find mean rate of reaction between 2 points

• rate of reaction graph show amount of product formed or amount of reactant used uo on y-axis & time on x-axis

• find mean rate for whole reaction, just work out overall change in y-value & divide this by total time taken for reaction

• can also use graph to find mean rate of reaction between any 2 points in time:

  mean rate of reaction = change in y/ change in x

how to find reaction time at particular point

…

whats reversible reaction, as reactants react…, hows a system at equilibrium, at equilibrium…, equilibrium can only be reached…

• reversible reaction → products can react to form reactants again:

  • as reactants react their concentrations fall so forward reaction will slow down but as more products r made & their concentrations rise the backward reaction will speed up

  • after while forward reaction will be going at exactly same rate as backward one → system is at equilibrium

  • at equilibrium → both reactions r still happening but theres no overall effect (dynamic equilibrium) = concentrations of reactants & products have reached balance & wont change

  • equilibrium only reached if reaction takes place inside closed system (non of reactants or products can escape & nothing can get in)

when a reactants at equilibrium…, equilibrium lies to right…, to left…, 3 things position of equilibrium depends on

• when a reactants at equilibrium it doesnt mean amounts of reactants & products r equal

• equilibrium lies to right = concentration of products is greater than that of reactants

• equilibrium lies to left = concentration of products is greater

• position of equilibrium depend on:

  • temp → heating reaction moves equilibrium to right & cooling to left

  • pressure (only affects equilibrium involving gases)

  • concentration of reactants & products

in reversible reactions…, energy transferred from surroundings by endothermic = …, give example using blue hydrated copper (II) sulfate crystals

• in reversible reactions if reaction is endothermic then it will be exothermic in other direction

• energy transferred from surroundings by endothermic reaction = energy transferred to surroundings during exothermic reaction

• example: heat blue hydrated copper (II) sulfate crystals, it drives water off & leaves white anhydrous copper (II) sulfate powder → endothermic

• add couple drops of water to white powder get blue crystals back → exothermic

whats Le Chatelier’s principle, what can it be used for

• Le chatelier’s principle → idea if change conditions of reversible reaction at equilibrium, the system will try to counteract that change

• can be used to predict effect of any changes make to reaction system

what are all reactions, decrease temp.., raise temp.., (involves exothermic & endothermic & equilibrium)

• all reactions r exothermic & endothermic in each direction

• decrease temp = equilibrium moves in exothermic direction to produce heat → get more products for exothermic reaction & fewer for endothermic reaction

• raise temp = equilibrium moves in endothermic direction to try & decrease it → get more products for endothermic reaction & fewer products for exothermic reaction

changing pressure affects…, increasing pressure.., decreasing pressure…, what can use a balanced equation for

• changing pressure only affects equilibrium involving gases

• increase pressure → equilibrium tries to reduce it = moves in direction where there r fewer molecules of gas

• decrease pressure → equilibrium tries increase it = moves in direction where there r more molecules of gas

• can use balanced symbol equation for reaction to see which side has more molecules of gas

changing concentration…, system responds…, increasing concentration of reactants…, decreasing concentration of products..

• change concentration of either reactants/ products the system will no longer be at equilibrium

• system responds to bring itself back to equilibrium again

• increase concentration of reactants→ system tries to decrease it by making more products

• decrease concentration of products → system tries to increase it again by reducing amount of reactants