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Edexcel Chemsitry > Topic 7: Rates of Reaction and Energy Changes > Flashcards

Topic 7: Rates of Reaction and Energy Changes Flashcards

1
Q

define rate of reaction

A

a mesure of how fast the reaction happens

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2
Q

how can rate of reaction be observed

A

by measuring how quickly the reactants are used up or how quickly the products are formed

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3
Q

formula for calculating rate of reaction

A
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4
Q

describe the protocol for a precipitation reaction to measure rate of reaction

A
  1. this method works for any reaction where mixing 2 see-through solutions produces a precipitate, which clouds the solution
  2. you mix the 2 reactant solutions and put the flask on a piece of paper that has a black cross on it.
  3. observe the mark through the mixture and masure how long it takes for the mark to be obscured. the faster it disappears the faster the reaction
  4. the result is subjective - different people might not agree on exactly when the mark ‘disappears’
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5
Q

describe the protocol for a reaction that measures change in mass (usually a gas given off) to measure rate of reaction

A
  1. you can measure the rate of reaction that produces a gas using a mass balance
  2. as the gas is released, the lost mass is easily measured on the balance. The quicker the reading on the balance drops, the aster the reaction.
  3. you know the reaction has finished when the reading on the balance stops changing
  4. you can use your results to plot a graph of change in mass against time
  5. this method does release gas prodiced straight into the room - so if the gas is harmful, you must take safety precautions, e.g. do the experiment in the fume cupboard
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6
Q

describe the protocol of a practical to find rate of reaction by measuring volume of gas given off

A
  1. this involves the use of a gas syringe to measure the volume of gas given off.
  2. the more gas given off during a set time interval, the faster the reaction
  3. you can tell the reaction has finished when no more gas is produced .
  4. you can use your results to plot a graph of gas volume against time lapsed
  5. you need to be careful that you’re using the right size gas syringe for your experiment - if your reactant is too vigorous, you can blow the plunger out of the end of the syringe
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7
Q

describe the practical for how to measure how surface area affects the rate of reaction

A

USE MARBLE CHIPS AND HYDROCHLORIC ACID
1. set the apparatus up as shown in the diagram on the right.
2. measure the volume of gas produced using a gas syringe. Take readings at regular time intervals and record the results in a table
3. you can plot a graph of your results - time goes on x-axis and volume goes on the y-axis
4. repeat the xperiment with exactly the same volume and concentration of acid and exactly the same mass of marble chips, but with the marble more crunched up
5. then repeat with the same mass of powdered chalk

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8
Q

how should the graph look after a practical on how surface area affects rate of reaction

A
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9
Q

analyse the data shown on a graph from the practical on how surface area affects rate of reaction

A
  1. the sooner the reaction finishes, the faster the reaction
  2. the steeper the gradient of the graph, the faster the rate of reaction. When the line becomes flat, no more gas is being produced and the reaction has finished
  3. using finer particles means that the marble has a larger surface area
  4. lines 1-3 on the graph show that the finer the particles are (and the greater the surface area of the solid reactants), the soonerthe reaction finishes and so the faster the reaction.
  5. line 4 shows the reaction if a greater mass of small marble chips is added. The extra surface area gives a faster recaction and there is also more gas evolved overall.
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10
Q

how does chnaging concentration of acid affect the rate of reaction (from volume of gas measuring ROR reaction)

A
  1. you can measure the effect of concentration on rate by following the same method. However, this time you repeat the experiment with exactly the same mass and surface area of marble chips and exactly the same volume of acid, but using different concentrations of acid
  2. lines 1-3 on the grpah show that a higher concentration gives a faster reaction, with the reaction finishing sooner
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11
Q

describe protocol for how reaction rate is affected by temperature

A
  • reaction between sodium thiosulfate and hydrochloric acid
  • measure out fixed volumes of sodium thiosulfate and hydrochloric acid, using a measuring cylinder
  • use a water bath to gently heat both solutions to the desired temperature before you mix them
  • mix the solutions in a conical flask. Place the flask over a black markon a piece of paper which can be seen through the solution. Watch the black mark disappear through the cloudy, yellow sulfur and time how long it takes to turn.
  • the reaction can be repeated for solutions at different temperatures
  • the depth and volumes of liquid must be kept the same each time. The concentrations of the solutions must also be kept the same.
  • you can use your results to measure hwat effect chnaging th etemperature has on rate of reaction. The shorter the length of the time taken for the mark to be obscured, the faster the rate.
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12
Q

interpret graph made after practical of measuring how reaction rate is effected by temperature

A
  • you can plot time taken for the mark to disappear against the temperature of the reacting solutions
  • if you look at the graph, you can see that the reactions that happened at lower temps took longer to obscure the mark, whereas the reactions happening at higher temps fiinished sooner
  • so increasing the temperature increases the rate of the reaction
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13
Q

how to calculate rate of reaction from a graph

A
  • find 2 points on the line that are easy to read the x and y values off
  • draw a line straight down from th ehigher point and straight across from the lower one to make a triangle
  • the height of you triangle = chnage in y. the base of your triangle = chnage in x
  • use the formula to work out gradient whihc is the rate
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14
Q

relationship between rate and gradient

A

rate of reaction = gradient

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15
Q

how to find rate when the graph has a curved line

A
  • if your graoh has a curve, the gradient and therefore rate is different at different pints along the curve
  • to find the gradient of the graph at a certain point, you have to draw a tangent at that point
  • a tangent is a line that touches the curve and has the same gradient as the line at that point
  • to draw a tangent place a ruler on the line of best fit at the point you are interested in, so you can see the whole curve. Adjust the ruler so the space between the ruler and the curve is the same on both sides of the point. draw a line along the ruler to make the tangent.
  • the rate at that point is then just the gradient of the tangent
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16
Q

what is a successful collision

A

a collision that ends in the particles reacting to form products

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17
Q

what does the rate of a chemical reaction depend on

A
  • the collision frequency of reacting particles - the more frequent the collision are the faster the reaction rate
  • the energy transfered during a collision. the minimum energy that particles need to react when they collide is called the activation energy. Particles need to collide with at least the activation energy for the collision to be successful
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18
Q

explain hwo increasing the number of collisions, the higher the rate of reaction

A

reactions happen if particles collide with enough energy to react. so if you increase the number of collisions or the energy with which the particles collide, the reaction happens more quickly

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19
Q

explain how increasing temperature increases rate of reaction

A
  • When the temperature is increased the particles move faster. If they move faster, they’re going to have more collisions.
  • Higher temperatures also increase the energy of the collisions, since the particles are moving faster. Reactions only happen if the particles collide with enough energy.
  • This means that at higher temperatures there will be more successful collisions (more particles will collide with enough energy to react).
    So increasing the temperature increases the rate of reaction.
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20
Q

explain how increasing concentration (or pressure) increases rate

A
  1. If a solution is made more concentrated it means there are more particles of reactant in the same volume. This makes collisions more likely, so the reaction rate increases.
    2) In a gas, increasing the pressure means that the particles are more crowded. This means that the frequency of collisions between particles will increase
    - so the rate of reaction will also increase.
21
Q

explain how smaller solid particles (or more surface area) increases rate of reaction

A

1) If one reactant is a solid, breaking it into smaller pieces will increase its surface area to volume ratio (i.e. more of
り
the solid will be exposed, compared to its overall volume).
2) The particles around it will have more area to work on, so the frequency of collisions will increase.
3) This means that the rate of reaction is faster for solids with a larger surface area to volume ratio

22
Q

what is a catalyst

A

a substance which increases the rate of reaction without being chemically changed or used up in the reaction

23
Q

how do catalysts work

A

by decreasing the activation energy by providing an alternative reaction pathway that has a lower activation enegry

24
Q

explain how catalysts increase the rate of reaction

A
  • they decrease the activation energy need for a reaction to occur by providing an alternative reaction pathway that has a lower activation energy so more of the particles have at least the minimum amount energy needed for a reaction to occur when the particles collide
25
Q

draw a reaction profile for a reaction that has been catalysed

A
26
Q

what is an enzyme

A

a biological catalyst

27
Q

what is a biological catalyst

A

they catalyse chemical reactions in living cells

28
Q

name some reactions that utilise biological catalysts

A

respiration, photosynthesis and proteinsynthesis

29
Q

define an exothermic reaction

A

one which gives out energy to the surroundings usually in the form of heat

30
Q

what kind of reaction is always exothermic

A

combustion reaction

31
Q

define an endothermic reaction

A

reaction that takes energy in from the surrounding

32
Q

draw a reaction profile for an exothermic reaction

A
  • products are at a lower energy than the reactants
  • the difference in height represents the energy given out in the reaction
33
Q

draw a reaction profile for an endothermic reaction

A
  • products are higher enegry than the reactants
  • the difference in height represents the energy taken in during the reaction
34
Q

show the activation energy in a recation profile

A

activation is the energy differenc between the reactants and the highest point on the curve

35
Q

define activation energy

A

the minimum amount of energy needed for bonds to break and a reaction to start

36
Q

how can you measure a temperature change using a practical

A
  • put a polystrene cup into a large beaker of cotton whool (the cotton whool gives insulation to help limit energy transfer to or from the reaction mixture)
  • add a known volume of your first reagent to the cup
  • measure the initial temp of the solution
  • add a measured mass/ volume of your second reagent and sitr the reaction mixture
  • put a lid on the cup to reduce any energy lost by evaporation
  • record the maximum or minimum temperature (depending on whether its increasing or decreasing) that the mixture reached during the reaction
  • calculate temp change
37
Q

how does dissolving salts in water alter the chnage in temperature of a reaction

A
  • measure temp chnage when dissolving salts in water by adding the salt to a polystrene cup of water and measuring the chnage in temperature when the salt has dissolved
  • dissolving ammonium chloride decreases the temperature of the reaction mixture - it endothermic
  • dissolving calcium chloride causes temp of rsolution to rise - it’s exothermic
38
Q

how can a neutralisation reaction alter the temperature change

A
  • in a neutralisation reaction, an acid and a base react to form a salt and water. most neutralisation reactions are exothermic e.g. HCl + NaOH -> NaCL + H2O
  • however the neutralisation recation between ethanoic acid and sodium carbonate is endothermic
39
Q

how can displacement reactions alter temp change

A
  • in a displacement reaction a more reactive element displaces a less reactive element in a compound. these types of reactions are accompanied by a release of energy - they are exothermic
  • zinc powder and copper sulfate react in a displacement reaction forming zinc sulfate and copper
40
Q

how do precipitation reactions alter temperature changes

A
  • precipitates are insoluble solids which can sometimes form when 2 solutions are mixed together
  • precipitation reactions are exothermic. e.g. the recation between lead(11) nitrate solution and potassium iodide forming a lead iodide precipitate would result in an increase in the temperature of the surroundings
41
Q

what happens during a chemical recation

A

old bonds are broken and new bonds are formed

42
Q

what type of process is bond breaking

A

endothermic as energy is needed to break them

43
Q

what type of process is bond making

A

exothermic and energy is released when new bonds from

44
Q

what is an endothermuc reaction in terms of bond energies

A

more energy is used to breka bonds than is released to make them

45
Q

what is an exothermic reaction in terms on bond energies

A

more energy is released when making bonds than is absorbed when breakng them

46
Q

formula for calulating overall energy change

A
47
Q

what does a positive energy change mean for the reaction

A

its endothermic

48
Q

what does a negative energy chnage mean for a reaction

A

its exothermic

49
Q
A
  1. work out energy required to break the original bonds in the recatants
  2. work out energy released by forming new bonds in the products
  3. work out overall change

Edexcel Chemsitry (9 decks)

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