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Chemistry > topic 6 - chemical change: rate & extent > Flashcards

topic 6 - chemical change: rate & extent Flashcards

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1
Q

how do you calculate the mean rate of reaction?

A

quantity of reactant used / time taken

quantity of product formed / time taken

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2
Q

what factors affect the rate of reaction?

A
  • temperature
  • the concentration of a solution or the pressure of a gas
  • the surface area of solid reactants
  • the presence of a catalyst
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3
Q

how does increasing the temperature increase the rate of reaction?

A
  • when the temperature is increased, the particles all move faster
  • if they’re moving faster, they’re going to collide more frequently
  • also the faster they move the more energy they have, so more of the collisions will have enough energy for the reaction to take place
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4
Q

how does increasing the concentration or pressure increase the rate of reaction?

A
  • if a solution is made more concentrated, it means there are more particles colliding in the same volume of the solvent
  • when the pressure of a gas is increased, this means that the same number of particles occupies a smaller space
  • this makes collisions between the reactant particles more frequent
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5
Q

how does increasing the surface area increase the rate of reaction?

A
  • if one of the reactants is a solid, then breaking it up into smaller pieces will increase its surface area to volume ratio
  • this means that for the same volume of the solid, the particles around it will have more area to work on - so there will be collisions more frequently
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6
Q

how does using a catalyst increase the rate of reaction?

A
  • a catalyst is a substance that speeds up a reaction, without being used up in the reaction itself. This means its not part of the overall reaction equation
  • different catalysts are needed for different reactions, but they all work by decreasing the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway with a lower activation energy
  • enzymes are biological catalysts
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7
Q

describe how to carry out a practical investigation to explore the effect of concentration on the rate of a reaction

A
  • use a measuring cylinder to put 10cm3 of sodium thiosulfate solution into a conical flask
  • place the conical flask onto a printed black cross
  • add 10cm3 of hydrochloric acid into the conical flask
  • swirl the solution and start a stopwatch
  • look down on top of the flask
  • after a certain time, the solution will turn cloudy
  • stop the clock when you can no longer see the cross
  • Carry out the experiment again using lower concentrations of sodium thiosulfate solution
  • repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution
  • do not include any anomalous results when calculating a mean
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8
Q

what is a hypothesis?

A
  • a proposal that could explain a factor of an observation
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9
Q

what is reproducibility?

A
  • a measurement is reproducible if it can be repeated by another person or using a different technique or equipment and still get the same result
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10
Q

what is the problem with the disappearing cross experiment?

A
  • different people have different eyesight. this means that some people can see the cross for longer than others so they may not get the same results
  • however, because all the students use the same size printed cross, this problem may not be too great
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11
Q

describe how to measure the rate by monitoring the volume of a gas

A
  • use a measuring cylinder to place 50cm3 of hydrochloric acid into a conical flask
  • attach the conical flask to a bung and delivery tube
  • place the delivery tube into a container filled with water
  • then place an upturned measuring cylinder also filled with water over the delivery tube
  • add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch
  • the reaction produces hydrogen gas which is trapped in the measuring cylinder
  • every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder
  • continue until no more hydrogen is given off
  • repeat the experiment using different concentrations of hydrochloric acid
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12
Q

what is the rate of a chemical reaction determined by?

A

the frequency of successful collisions

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13
Q

what is collision theory?

A
  • the theory that chemical reactions can occur only when reacting particles collide with each other and with sufficient energy
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14
Q

what is activation energy?

A
  • the minimum amount of energy that particles must have to react
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15
Q

describe how to investigate the effects of surface area on the rate of reaction

A
  • marble chips contain the chemical calcium carbonate
  • this reacts with hydrochloric acid and produces carbon dioxide gas
  • measure the volume of carbon dioxide gas and use this to determine the rate of reaction
  • change the surface area of the marble chips
  • measuring the volume of gas using a measuring cylinder may be difficult as the bubbles can be quite rapid -> more accurate results by using a gas syringe
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16
Q

describe how to investigate the effects of surface area on the rate of reaction by measuring the mass of carbon dioxide gas that’s lost

A
  • place the reaction on a balance
  • as carbon dioxide is produced, the mass decreases
  • calculate the rate of reaction
  • the cotton wool allows carbon dioxide gas to escape
  • it also prevents acid from splashing out of the flask
  • if any acid did splash out, it would cause the mass to fall more than it should - anomalous result
17
Q

what is the function of catalysts?

A
  • catalysts increase the rate of chemical reactions but are not used up during the reaction
18
Q

how are catalysts useful?

A
  • catalysts allow us to carry out reactions quickly without needing to increase the temperature.
  • this saves money -> reusable
19
Q

what does the rate of reaction depend on?

A
  • the number of particles that have enough energy to cross the activation energy barrier and collide successfully
20
Q

how do catalysts increase the rate of reaction?

A
  • catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy
  • when a catalyst is present, the particles require less energy to cross the activation energy barrier
  • this means that more particles can successfully collide per second
  • that increases the rate of reaction
21
Q

what do the arrows in a reversible reaction represent?

A
  • if a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction
22
Q

how can the direction of reversible reactions be changed?

A

changing the conditions

23
Q

what is the word equation for hydrated copper sulfate?

A

hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate ( white) + water

24
Q

what is equilibrium?

A
  • when the forward and the reverse reaction will take place at exactly the same rate
25
Q

what is the effect of changing conditions on equilibrium?

A
  • if a system is at equilibrium and a change is made to the conditions, then the system responds to counteract the change
26
Q

what is Le Chatelier’s principle?

A

the idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

27
Q

what would happen if we decrease the concentration of N2O4? (2NO2 ⇌ N2O4)

A
  • the system is no longer at equilibrium
  • this means that more NO2 will react to form N2O4 until equilibrium is reached again
28
Q

what happens if you change the temperature of a reversible reaction?

A
  • all reactions are exothermic in one direction and endothermic in the other
  • if you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. this means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction
  • if you raise the temperature, the equilibrium will move in the endothermic direction to try and decrease it. You’ll now get more products for the endothermic reaction and fewer products for the exothermic reaction
29
Q

what happens if you change the pressure of a reversible reaction?

A
  • changing pressure only affects an equilibrium involving gases
  • if you increase the pressure, the equilibrium shifts to the side with the smaller number of molecules
  • if you decrease the pressure, the equilibrium shifts to the side with the larger number of molecules

Chemistry (10 decks)

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