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 they move around more
• This means there will be more frequent successful collisions and more particles will have the required level of energy to react

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

• An increase in concentration means more particles in a fixed volume
• This means that 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

• Energy is transferred to the reactants to break their bonds, so breaking bonds is endothermic
• Energy is transferred to the surroundings as bonds form, so making bonds is exothermic

A reaciton is exothermic is more energy is given out making bonds than is needed to break bonds, since energy is transferred to the surroundings.

Vice versa applies for endo,

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’