The Rate and Extent of Chemical Change

Rates of a chemical reaction

The rate of a reaction is 'the time taken for a product to form, or reactants to be used up'. 

 

mean rate=average rate

specific rate= specific to moment in time

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Factors + Collision Theory

4 main factors that affect how quicky a reaction goes:

  •  Temp - the higher the temp = faster the reaction
  • Concentration/pressure for gases - the more concentrated reactants/higher the pressure = faster reaction
  • Surface Area - larger surface area = faster reaction
  • Catalysts - reactions with catalyst = faster than without

Collision Theory - reaction rates explained by collision theory. Collision theory depends on 2 things: how often they collide = more there are in certain amount time=faster reaction and energy transferred during collision=particles have to collide with enough energy to be successful. Changing either of these=change rate of reaction. 1 way is to increase freq. of collisions so probability of successful collision increases. Other way is to increase energy of collisions so more collisions=successful

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Increasing freq. of collisions

Increasing the frequency of Collisions

Temperature - when temp increased all particles=quicker. If quicker=collide often=more collisions=faster rate of reaction

Concentration/pressure - if solution more concentrated=more particles colliding=successful collisions more likely 

Surface Area - if one reactants solid=breaking it up=larger surface:volume ration=particles around it more area to work on=more frequent collisions + rate of reaction faster

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Increasing the energy of collisions

effect of temp on reaction rate=also explained in terms of how much energy particles have when collide. Higher temp doesn't only increase freq of collisions but also energy as particles move faster.

Reactions only happen if particles collide with enough energy. Minimum amount of energy that particles must have in order to react= Activation Energy. At higher temp=more particles colliding with enough energy to make reaction happen.

Rate and Proportionality - rate of reaction=directly proportional to frequency of successful collisions.

Catalysts - many reactions can be speeded up by adding catalysts (catalyst=substance which can speed up reaction, without being changed/used up)

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Measuring rates of reaction

Measuring the formation of product

Precipitation - you can record visual change in reaction if inital solution=transparent + product is precipate which clouds solution. You can observe a mark through solution + measure how long it takes for mark to disappear. Reaction only works for reaction with visual change + results subjective=when mark disappears/changes colour.

Change in Mass - you can measure speed of reaction that produces gas using mass balance. Place reaction vessel on balance, then add reactants to conical flask + put cotton wool in neck. As gas released mass disappearing=easily measured. The quicker reading on balance drops, the faster reaction. If you take measurements during regular intervals=can plot graph of mass/time. This method=most accurate of three because mass balance accurate. But disadvantage as gas released straight into room=not good if gas dangerous.

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Measuring rates of reaction

Volume of gas given off - you can also measure rate of reaction that produces gas by using gas syringe to measure volume of gas given off. The more gas given off during given time=faster the reaction. You can take measurements at regular intervals + plot graph of volume/time. Gas syringes usually give volumes to nearest cm3, so are quite sensitive. Gas=isn't released in room=useful if gas poisonous. Have to be careful as if reaction too vigorous=can easily bow plunger out of end of syringe.

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Rate of reaction graphs

On a graph showing amount of product/reactant against time=rate of reaction shown by gradient (steepness) of line. The steeper the line=faster the rate. These graphs=not straight lines but start steep, get shallower and then level off. This is because reations start quick then slow down + eventually stop. 

Reactions start quickly because at beginning of reaction=lots of reactant particles around=collisions very frequent. As reaction progresses, reactants used up=no. of reactant particles decreases. Means collisions between reactant particles decrease + reaction slows down. Reaction stops when all particles from at least one of reactants used up.

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Rate of reaction graphs

On a graph showing amount of product/reactant against time=rate of reaction shown by gradient (steepness) of line. The steeper the line=faster the rate. These graphs=not straight lines but start steep, get shallower and then level off. This is because reations start quick then slow down + eventually stop. 

Reactions start quickly because at beginning of reaction=lots of reactant particles around=collisions very frequent. As reaction progresses, reactants used up=no. of reactant particles decreases. Means collisions between reactant particles decrease + reaction slows down. Reaction stops when all particles from at least one of reactants used up.

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Comparing rates of reaction

Comparing rates of reaction - can compare rate under different conditions by plotting series of lines on graph. All of lines=curves but exact shape of curve will depend on rate of reaction + amount of reactants you started with.

  • fastest reaction will be line with steepest slope at beginning + faster reaction goes, the sooner it finishes, which means line will become flat earlier
  • reactions that start off with same amount of reactants will give lines that finish at same level on graph

Mean rate from graphs - To find mean rate of whole reaction from graph of amount of substance/time, you just work out overall change in y and 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= change in y ÷ change in x

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Rates and tangents

To get an idea of how fast rate is at particular point in reaction you need to find out how steep curve is at point. And to do that need to be able to draw tangent to curve. A tangent is straight line that touches curve at particular point without crossing it.  

Calculating rates from tangents - as well as drawing tangent of curve, can also calculate gradient of tangent. The value you calculate will be equal to rate of reaction at particular point in time. Gradient = change in y ÷ change in x 

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Reaction Rate Experiments

Mg metal + HCl - can use reaction to investigate effect increased concentration on rate of reaction, equation= 2HCl + Mg --> MgCl2 + H2

Reaction gives of hydrogen gas=can follow volume of gas produced using gas syringe:

  • measure 50cm3 of dilute HCl using measuring cylinder + add to conical flask
  • add Mg ribbon to acid + quickly attach empty gas syringe to flask
  • start stopwatch
  • take readings of volume at regular intervals until volume hasn't changed
  • put results in table & plot graph time on 'x' + volume of gas produced on 'y'
  • repeat with more concentrated acid solutions + variables (mass, temp, vol of acid) kept same = fair test
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Reaction Rate Experiments...

Sodium Thiosulfate + HCl - both=clear solutions + react together to form precipate yellow of sulfur = reaction mixture becomes turbid (cloudy) as reaction continues: 2HCl + Na2S2O --> 2NaCl + SO2 + S + H2O

  • start by adding 50cm3 of dilute sodium thiosulfate to flask
  • place flask on piece of paper with black cross on it
  • add 10cm3 of dilute HCl to flask + give flask swirl to mix reactants
  • start stopwatch
  • watch black cross + time how long it takes to disappear
  • record results on table
  • repeat in exact same way + use results to calculate mean time for cross to disappear = results more accurate/precise
  • reaction can be repeated with solutions at different concentrations + when cross disappears=subjective=same person should do it=fair test
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