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

Question 1

SC18a - What is the rate of a reaction?

Answer 1

The speed at which reactants turn into products

Question 2

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

Answer 2

• 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

Question 3

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

Answer 3

• 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

Question 4

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

Answer 4

• 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

Question 5

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

Answer 5

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

Question 6

SC18b - How does temperature affect the rate of reaction?

Answer 6

• 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

Question 7

SC18b - What is activation energy?

Answer 7

The minimum amount of energy required for a reaction to occur

Question 8

SC18b - How does concentration affect the rate of reaction?

Answer 8

• 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

Question 9

SC18b - How does pressure affect the rate of reaction?

Answer 9

• 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

Question 10

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

Answer 10

• 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

Question 11

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

Answer 11

• 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

Question 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]

Answer 12

• 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

Question 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]

Answer 13

• 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

Question 14

SC18c - What is a catalyst?

Answer 14

Catalysts increase the rate of reaction without being used up

Question 15

SC18c - How do catalysts work?

Answer 15

• 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

Question 16

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

Answer 16

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

Question 17

SC18c - Why are catalysts desirable in industry?

Answer 17

• 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

Question 18

SC18c - How does a catalytic converter work?

Answer 18

• 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

Question 19

SC18c - What are enzymes and how do they work?

Answer 19

• 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….

Question 20

SC19a - What are exothermic and endothermic reactions?

Answer 20

• 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

Question 21

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

Answer 21

• 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

Question 22

SC19a - Which two types of reactions are always exothermic?

Answer 22

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

Question 23

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

Answer 23

• Precipitation: Two soluble reactants form a soluble product #Dissolving: When you dissolve something _{^{<strong>#helpful</strong>}}

Question 24

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

Answer 24

• 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

Question 25

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

Answer 25

• 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

Question 26

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

Answer 26

• 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

Question 27

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

Answer 27

• 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’