2.2 Flashcards
What is rate of reaction
Change in concentration of a reactant or product per unit time
Equation and units for rate in concentration terms
Rate = change in conc/time Units = mol dm-3 s-1
When is the rate of reaction usually fastest
Start of reaction
Each reactant has its greatest concentration
Why does rate slow down as the reaction proceeds
Concentration of reactants decreases
When does the rate of reaction become 0
When the reaction stops
When trying to calculate initial rate from a graph what do you do
Find the gradient of the tangent
Factors affecting reaction rates
- concentration of a solution
- surface area of a solid
- temperature of a reaction
- catalyst
- light
What is activation energy
Minimum energy required to start a reaction by breaking bonds
What do energy profiles compare
Enthalpy of the reactants with the enthalpy of the products
Look
Look
Effect of concentration on reaction rates
- increase in reactant concentration = increase in reaction rate
- more molecules in the same volume so distance between molecules is reduced and theres an increase in the number of collisions per unit time
- greater change of collisions with at least the activation energy and so rate increases
- gaseous reaction - increasing pressure has the same effect
Effect of surface area on rate of reaction
• reducing the particle size increases surface area and increases the rate of reaction
Effect of temperature on reaction rates
- temperature increase = reaction rate increases
- molecules have more kinetic energy and are moving faster
- more molecules have energy that is greater than the activation energy so more collisions take place in a certain length of time
Effect of higher temperature on Boltzmann energy distribution curves
- Peak moves right with a lower height
* more molecules have sufficient energy to react so increases rate significantly
Things to remember about Boltzmann energy distribution curves
- curves doesn’t touch the energy axis
- area under two curves are equal and proportional to the total number of molecules in the sample
- only molecules with an energy equal to or greater than the activation energy are able to react
What is a catalyst
Substance that increases the rate of a chemical reaction without being used up in the process
Increases the rate of reaction by providing an alternative route of lower activation energy
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Explain the effect of a catalyst using the Boltzmann energy distribution curve
At the same temperature a higher proportion of molecules will have sufficient energy to overcome the activation energy for a catalysed reaction
How does a catalyst affect equilibrium
Doesn’t affect the position of equilibrium
Position of equilibrium is reached more quickly
What is a homogenous catalyst
In the same phase as the reactants
Take an active part in a reaction rather than being an inactive spectator
Examples of a homogeneous catalyst
- concentrated sulfuric acid in the formation of an ester from a carboxylic acid and an alcohol
- aqueous iron (II) ions in the oxidation of iodine ions by peroxodisulfate (VI) ions
What is a heterogenous catalyst
Catalyst that is in a different phase from the reactants
Industrial they’re usually d-block transition metals (provide a reaction site for the reaction to take place- gas is absorbed onto the metal surface and react and the products de absorb from the surface) (larger SA the better)
Examples of a heterogeneous catalyst
- iron in Haber process for ammonia
- Vanadium (V) oxide in the Contact process for sulfuric acid
- Nickel in the hydrogenation of unsaturated oils for margarine
- Ziegler-Natta catalysts for high density poly(ethene)
Benefits of using enzymes as catalysts in industry
- lower temperature and pressures needed - save energy + cost
- operate in mild conditions
- dont harm fabrics or food
- biodegradable- disposing them is no problem
- no need for complex separation techniques as they allow reactions to take place which form pure products with no side reactions
Why are heterogeneous catalysts often favoured in industry
Easily separated from the products
How do catalysts save energy costs
Less energy is required for the molecules to react as activation energy is lowered
Ways of studying rates of reaction
- change in gas volume - gas syringe or the gas can be collected over water using an inverted burette
- change in gas pressure - manometer
- change in mass - weighing balance
- change in colour (colorimetry) - colorimeter
Give method of collecting gas that is produced when magnesium reacts with an acid
What does it show
- Shake the metal into the acid and start the stopwatch
- Measure amount of Hydrogen given off at constant intervals
- Stop the watch when hydrogen stops being produced
- Repeat the experiment with different concentrations of acid/ temperature of acid/ particle size of metal - keep other factors constant
- Draw a graph of the results
• shows rate change during chemical reaction as well as showing effect of concentration, temperature, particle size or catalyst on a chemical reaction
Suitable examples to use to investigate rate of reaction by a gas collection method
- calcium carbonate and hydrochloric acid
* decomposition of hydrogen peroxide
Other than using a gas syringe how can gas be collected
Over water using an inverted burette
Colour of iodine with starch solution
Strong blue
Thiosulfate ions and iodine
- rapid
* blue doesn’t appear until enough iodine is formed to react with all the thiosulfate
What is referee to as the ‘clock’ in the iodine clock experiment
Time taken for the solution to turn blue to measure the rate of iodide being oxidised
What is varied during an iodine clock experiment and what is it used to measure
Concentrations of the reactants one at a time
Measure the rate so that the dependence of rate on concentration of any reactant may be found
What must be kept constant during the iodine clock experiment and why
Temperature- rates vary rapidly with changes in temperature
How can precipitation reactions be used to measure rate of reaction?
Colourless solutions can turn more and more cloudy as precipitate forms
The time taken for a precipitate to form can be used to measure the rate of reaction
Why is a trial run before iodine clock and precipitation reactions carried out
To find what range of concentrations will be suitable
