Topic 10 - Rates Of Reaction Flashcards
Activation energy
The energy needed to start a reaction.
Exothermic reaction
Transfers heat energy to the surrounding so the temperature of the surroundings increases.
Endothermic reaction
Absorbs heat energy from the surroundings so the temperature of the surroundings decreases.
Breaking bonds
Endothermic
Making bonds
Exothermic
Acronym to remember breaking bonds and making bonds?
Bendomex
Energy profile diagram:
Exothermic reaction
Overall, more heat energy is released in the making of bonds in the reactants, than is needed to break the bonds in the reactants.
Energy profile diagram:
Endothermic reaction
More heat energy is needed in the breaking of bonds in the reactants than is released in the making of the bonds on the reactants.
Bond energies
Total energy change = Total energy in - Total energy out
What is needed for a reaction to occur?
• For a reaction to occur particles need to collide.
• The particles can be a molecule, ion or atoms.
• The particles need to collide with enough energy for particles to reaction - the activation energy is the minimum energy needed.
Effective collision/Successful collision
Collisions between atoms or molecules that successfully transfer enough energy for the atoms or molecules to react together.
Four way in which a reaction can be increased?
The collision model helps explain this.
1) Increasing the temperature.
2) Increasing the surface area.
3) Increasing the concentration in a reacting solution.
4) Increasing the pressure in a reacting gas.
Concentration
• Increases the number of reactant particles in the same volume.
• Therefore collision occur more often
• Therefore rate increases.
Temperature
• The higher the temperature, the faster the particles move.
• Therefore, particles will collide more frequently.
• And with more energy meaning more successful collisions
• Therefore rate increases.
Surface area
• The greater the surface area, the more particles there are on the surface where collisions happen.
• Therefore, there will be more collisions per second.
• Therefore rate increases.
Mean rate of reaction
A measure of how quickly a reacting is used up, or a product is formed.
Quantity of reaction used / Time
OR
Quantity of product formed / Time
How to calculate rate of reaction at a certain time?
Draw a tangent on the curve at the specified time and use this equation to calculate the gradient.
Catalysts
A substance that speeds up the rate of a reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction.
How do catalysts reduce costs in industrial processes?
• By making products more quickly.
• By allowing reactions to occur at lower temperatures.
• Catalysts do not usually need to be replaced because they are not used up.
How does a catalyst increase the rate of reaction?
• A catalyst provides an alternative reaction route, which requires less activation energy.
• The catalyst does not alter the overall energy change.
• As less activation energy is needed to start the reaction, more reactant molecules have enough energy and so more collisions are successful.
What are enzymes?
Large complex protein molecules that act as catalysts in biological reactions.
How are enzymes used in the production of alcoholic drinks?
• Alcoholic drinks are produced using an enzyme in yeast.
• This enzyme catalyses the reaction in which glucose is converted into ethanol (alcohol) and carbon dioxide.
Core practical:
Rates of reaction with syringe
1) Set up gas collection apparatus. Clamp the syringe in place.
2) 50cm3 2M Hydrochloric Acid.
3) 5g of marble chips.
4) QUICKLY place bung into conical flask AND start the timer.
5) Measure and record volume on gas syringe every 10 s, for 100s.
6) Decant out any excess liquid, discard any solid and rinse.
7) Repeat the experiment with different sized marble chips or different concentration of acid.
Core practical:
Rates of reaction with syringe
Word equation and balanced equation
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Core practical:
Temperature and rate of reaction
1) Add 20cm3 of sodium thiosulfate into boiling tube.
2) Add 2cm3 of hydrochloric acid into boiling tube.
3) Place both boiling tubes into a rack in a water bath for 5 minutes.
4) Pour the sodium thiosulphate and acid into the conical flask.
5) Swirl flask Place on cross Start timer.
6) Stop timer when cross has disappeared – record time.
7) Repeat experiment using water baths at different temperatures.
□ 20OC □ 30OC □ 40OC □ 50OC
Core practical:
Temperature and rate of reaction
Word equation and balanced equation
Sodium thiosulfate + hydrochloric acid —> Sodium chloride + water + sulfur dioxide + sulfur
Na2S2O3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
The effect of temperature on the rate of reaction?
For a reaction to happen the reactant particles need to collide with enough energy to react. (Activation energy)
Increasing temperature increases the rate of reaction because when the temperature rises:
• The particles move faster and the particles collide more often.
• More of the collisions that happen are successful.
• There are more collisions every second.
Reversible reaction
Where the products can react and reform the reactants.
Dynamic equilibrium
A dynamic equilibrium occurs in a closed system, when forward and backward reactions happen at the same time and therefore there is no overall change in the concentrations of reactants or products.
There are no observable changes.
What must occur for dynamic equilibrium to occur?
• Reversible reaction.
• Closed system - none of the reactants or products can escape.
Haber process
During the Haber process, nitrogen and hydrogen react to form ammonia: N2 + 3H2 <—> 2NH3
The nitrogen is obtained from the air.
The hydrogen can be extracted from hydrocarbons from sources such as natural gas and crude oil.
The Haber process is carried out at 450°C with a pressure of 200 atmospheres and an iron catalyst.
What 3 things can change the position of equilibrium?
• Temperature
• Pressure
• Concentrations
Temperature
Increase in temperature means that equilibrium position shifts left because reverse reaction is endothermic.
Decrease in temperature means that equilibrium position shifts right because reverse reaction is exothermic.
Pressure
Increase in pressure favours side with fewer molecules.
Decrease in pressure favours side with more molecules.
Concentration
Increase in concentration favours side with lower concentration.
Decrease in concentration favours side with higher concentration.
Why is 450° chosen for the Haber process?
• High temperatures give a faster rate of reaction.
• Low temperatures favour the production of ammonia.
A compromise is reached.
Why is 200 atmospheres chosen?
• High pressures increase rate of reaction.
• However it can be dangerous and expensive.
A compromise is reached.
Why is an iron catalyst used?
Catalysts do not change the equilibrium concentrations of reacting substances in reversible reactions. However, they do reduce the time taken to reach equilibrium.
• Iron is a cheap catalyst used in the Haber process.
• It helps to achieve an acceptable yield in an acceptable time.
Le Chatelier’s Principal
When something changes in a reaction, the equilibrium will always shift to oppose the change.
