SC18 & SC19 - Rates of Reaction / Heat Energy Changes in Chemical Reactions ✓ Flashcards
SC18a - What is the rate of a reaction?
The speed at which reactants turn into products
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
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
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
SC18b - What are the five main factors that affect rates of reaction?
- Temperature
- Pressure
- Concentration
- Surface areas/volume ratio
- Presence of a catalyst
SC18b - How does temperature affect the rate of reaction?
- 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
SC18b - What is activation energy?
The minimum amount of energy required for a reaction to occur
SC18b - How does concentration affect the rate of reaction?
- 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
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
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
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
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
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
SC18c - What is a catalyst?
Catalysts increase the rate of reaction without being used up
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
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
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
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
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….
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
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
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
SC19a - Which two types of reactions can be endothermic or exothermic?
- Precipitation: Two soluble reactants form a soluble product
- Dissolving: When you dissolve something <strong>#helpful</strong>
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
SC19b - In terms of bonds broken/formed, describe how a reaction may be endothermic or exothermic
- 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,
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
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’
