SC18 & SC19 - Rates of Reaction / Heat Energy Changes in Chemical Reactions ✓ Flashcards Preview

Edexcel GCSE Chemistry COPY > SC18 & SC19 - Rates of Reaction / Heat Energy Changes in Chemical Reactions ✓ > Flashcards

Flashcards in SC18 & SC19 - Rates of Reaction / Heat Energy Changes in Chemical Reactions ✓ Deck (27)
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1

SC18a - What is the rate of a reaction?

The speed at which reactants turn into products

2

SC18a - How can you use a gas syringe to investigate the rate of reaction?

  • Works for reactions where a gas is produced
  • Set up a liquid in a conical flask
  • Push a gas syringe to 0 and attach it to a bung
  • Add the granules of the other chemical into the conical flask and immediately attach the bung
  • Also set off a timer. Every 10 seconds record down the value red on your gas syringe
  • Repeat and take the average of concordant results
  • use the average results and plot them on a graph

3

SC18a - Describe a reactant/product time graph showing the rate of reaction

  • Concentration on y axis time on the x axis
  • Products start at 0 and have an increasing positive curve
  • Reactants start high and have an increasing positive curve till 0
  • Both curves will plateau (flatten out) at the same point

4

SC18a - How can you measure mass to investigate the rate of reaction?

  • Work for reactions where one of the products are gases
  • Place the conical flask on the balance at set it to 0
  • Add the liquid being used into the conical flask and then add the other chemical
  • Quickly record down the starting mass and cover the flask with cotton wool to stop acid form spitting
  • Set off timer and record down the mass every 10 seconds
  • Repeat and take the average of concordant results
  • Plot a graph using the average results

5

SC18b - What are the five main factors that affect rates of reaction?

  • Temperature
  • Pressure
  • Concentration
  • Surface areas/volume ratio
  • Presence of a catalyst

6

SC18b - How does temperature affect the rate of reaction?

  • An increase in temperature gives the reactant particles more kinetic energy meaning the reactant particles move more quickly
  • due to the energy of the particles increasing
  • there will be more frequent successful collisions between the particles 

  • therefore the rate of reaction increases

7

SC18b - What is activation energy?

The minimum amount of energy required for a reaction to occur

8

SC18b - How does concentration affect the rate of reaction?

  • If you increase the concentration of a reactant, there will be more of the chemical present
  • this increase in concentration means there will be more particles in a fixed volume
  • This means that successful collisions will be more frequent as particles are closer together

9

SC18b - How does pressure affect the rate of reaction?

  • An increase in pressure means the same amount of particles in a smaller volume
  • This means that that successful collisions will be more frequent as particles are closer together

10

SC18b - How does surface area/volume ratio affect the rate of reaction?

  • An increase of the surface area to volume ration means for a fixed volume, there is more surface area.
  • This leaves more of the surface exposed
  • This means that collisions have more surface to occur on and so successful ones occur more often

11

SC18b - How does the presence of a catalyst affect the rate of reaction?

  • A catalyst increases the rate of reaction without being used up by lowering the activation energy required
  • This means more particles will have the required activation energy and collisions will more frequently be successful

12

SC18b CP - Describe how you would investigate the affect of a change of surface on rates of reaction. [Use an upturned measuring cylinder and marble chips with hydrochloric acid]

  • Set up the apparatus with a clamp holding an upturned measuring cylinder over a trough containing water
  • Have a delivery tube attached to a bung lead up to the cylinder and record the starting value
  • Put 40cm³ of hydrochloric acid (Use the same concentration throughout) into a flask.
  • Measure out 5g of small marble chips
  • Add the marble chips to the flask and immediately close it with the bung
  • Start the timer and record the value of gas produced every 30 seconds until the reaction has finished
  • Repeat 3 times and take the average of concordant results. Use this to plot a graph
  • Repeat all steps but using large marble chips of the same mass and plot these results on a graph

13

SC18b CP - Describe how you would investigate the effect temperature has on the rate of reaction [Use the disappearing cross method and sodium thiosulfate with hydrochloric acid]

  • Measure out 50cm³ of sodium thiosulfate into a conical flask
  • Measure out 5cm³ of hydrochloric acid into a test tube
  • Place the conical flask and test tube in a water bath of a set temperature
  • After the chemicals have acclimatized (around 5 minutes), take them out and add the hydrochloric acid into the flask
  • Place it on a piece of white paper and immediately start the timer
  • Stop the timer once it has turned cloudy and the cross is no longer visible
  • Repeat with different temperatures of the water bath and plot a graph

14

SC18c - What is a catalyst?

Catalysts increase the rate of reaction without being used up

15

SC18c - How do catalysts work?

  • Catalysts provide an alternate reaction pathway by which a lower level of activation energy is required.
  • This means that more particles will be able to reach the required activation energy and therefore successful collisions will be more frequent

16

SC18c - How will the shape of a reaction profile change when a catalyst is used?

The peak will be a lot lower however it will being to rise and stop falling at the same places

17

SC18c - Why are catalysts desirable in industry?

  • By increasing the rate of reaction, there will be more products produced in a fixed amount of time.
  • Also, as catalysts don't get used up, they can be used multiple times.
  • This means that the process is more profitable

18

SC18c - How does a catalytic converter work?

  • Combustion inside a car produces toxic gases
  • A catalytic converter uses catalysts of palladium and platinum to convert theses into less harmful gases
  • The catalysts are spread over a thin honeycomb structure because:
    • Increased surface increases the efficiency of the cat con.
    • As little catalyst as possible is used as the metals are very expensive

19

SC18c - What are enzymes and how do they work?

  • Enzymes are large protein molecules that act as biological catalysts
  • They have specifically shaped active sites to the shape of their reactant molecules (substrates)
  • Changes in temperature can denature the active site, changing its shape
  • They are used in man things e.g. production of alcohol

 

You should know this in more detail for biology anyway but yeah....

20

SC19a - What are exothermic and endothermic reactions?

  • Exothermic reactions:
    • Energy from stores in bonds is transferred to surroundings
    • Measured temperature will be hotter
    • Products will have less energy
  • Endothermic reactions:
    • Energy is taken in from surroundings is transferred to stores in bonds
    • Measured temperature will be cooler
    • Products will have more energy

21

SC19a - Describe the shape of a simple reaction profile in exothermic and endothermic reactions

  • Exo:
    • Horizontal line for reactants
    • vertical arrow down
    • lower horizontal line for products
  • Endo:
    • Horizontal line for reactants
    • vertical arrow up
    • higher horizontal line for products

22

SC19a - Which two types of reactions are always exothermic?

  • Neutralisation: Acid + base
  • Displacement: More reactive element displaces a less reactive element that was part of a compound

23

SC19a - Which two types of reactions can be endothermic or exothermic?

  • Precipitation: Two soluble reactants form a soluble product #Dissolving: When you dissolve something #helpful

24

SC19a - How can you check if a reaction is exo/endothermic using a polystyrene cup and a thermometer?

  • Place one of your chemicals in the polystyrene cup (in a beaker for support) and measure the starting temperature.
  • Add the other chemical and then place the lid and push the thermometer through the top.
  • Record the final temperature once the reaction has stopped.
  • Lid used to limit heat loss and polystyrene is a poor conductor of heat
  • A reduction of heat means it is endothermic while an increase of heat means it is exothermic

25

SC19b - In terms of bonds broken/formed, describe how a reaction may be endothermic or exothermic

  • When bonds break, energy is released and transferred to surroundings (This happens to the reactants)
  • When bonds are formed, energy is taken in from surroundings (This happens to form products)
  • The energy released into the system (from bond breaking) - the energy used taken out of the system (to form bonds) = the total energy change
  • If the energy change is positive, the reaction is endothermic but if the energy change is negative, the reaction is exothermic

26

SC19b - What is the bond energy of a covalent bond?

  • The amount of energy required to break the covalent bonds in one mole (Measured in kJ/mol)
  • It is also the amount of energy required to form one mole of that covalent bond

27

SC19b - How would you label activation energy and overall energy change on a reaction profile?

  • Overall energy change is an arrow showing the vertical height change between reactants and products
  • Activation energy is the vertical arrow showing the difference in height between reactants and the top of the 'hump'