Chemistry P2

• How to calculate the mean rate of reaction?

  mean rate of reaction = quantity of reactant used / time taken

  mean rate of reaction = quantity of product formed / time taken
• what is the measurement of a reactant or product measured?

  mass in grams or volume in cm^3
• what is the unit of rate of reaction?

  g/s or cm^3/s
• what are the factors which affect the rate of chemical reactions?

  the concentration of reactants in solution, the pressure of reacting gases, surface area of solid reactants, the temperature and the presence of catalyst
• Rate of reaction PRACTICAL?

  add a set volume of dilute hydrochloric acid to a conical flask and carefully place on a mass balance. Add some magnesium ribbon to the acid and quickly plug the flask with cotton wool. Start the stopwatch and record the mass on the balance. Take readings of the mass at regular interval. Plot a graph of ‘ volume of gas produced ‘ ( y axis ) against ‘ time ‘ ( x axis ). Repeat with more concentrated acid solution
• rate of reaction PRACTICAL?

  measure 10cm^3 sodium thiosulfate solution into the conical flask. dilute the solution by adding water. put the conical flask on the black cross. add some dilute hydrochloric acid to the flask and stir gently and start the stopwatch. Repeat using different volumes of sodium thiosulfate and water. This will change the concentration. repeat twice more. Calculate the mean time for each of the sodium thiosulfate concentrations
• safety precautions of rate of reaction PRACTICAL?

  wear safety glasses

  take care where using glassware
• What does the collision theory explain?

  how various factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with eachother and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.
• what is activation energy ?

  the minimum amount of energy that particles must have to react
• how to increase the rate of reaction?

  increasing the concentration of reactants in solutions, the pressure of reacting gases and the surface area of solid reactants increase the frequency of collisions and so increase the rate of reaction.
• what does increasing the temperature do?

  increasing the temperature increases the frequency of collisions and makes the collisions more energetic, and so increases the rate of reaction
• what do catalysts do?

  catalysts change the rate of chemical reactions but are not used up during the reaction. different reactions need different catalysts. enzymes act as a catalyst in biological systems. catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy
• how can a catalysed reaction be presented as a diagram?

  reaction profiles
• what are reversible reactions?

  when the product of the reaction can react to produce the original reactants. the direction of reversible reactions can be changed by changing the conditions
• revirsible reactions?

  if a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. the same amount of energy is transferred in each case
• when is equilibrium reached?

  when a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occurs at exactly the same rate
• what are the effects of changing conditions on equilibrium?

  the relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. the effects of changing conditions on a system at equilibrium can be predicted using Le Chateliers principle
• the effects of changing concentration?

  if the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again. If the conc of a reactant is inc, more products will be formed until equilibrium is reached again. If the conc of a product is dec, more reactants will react until equilibrium is reached again
• what happens if the temp of a system at equilibrium is inc?


  the relative amount of product at equilibrium increases for an endothermic reaction

  the relative amount of products at equilibrium decreases for an exothermic reaction
• what happens if the temp of a system at equilibrium is decreased?

  the relative amounts of products at equilibrium decreases for an endothermic reaction

  the relative amount of products at equilibrium increases for an exothermic reaction
• the effect of pressure changes on equilibrium?

  an increase in pressure causes the equilibrium position to shift toward the side with the smaller number of molecules as shown by the symbol equation for that reaction

  a decrease in pressure causes the equilibrium position to shift toward the side with the larger number of molecules as shown by the symbol equation for that reaction .
• What is crude oil?

  crude oil is a finite resource found in rocks. Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud. Crude oil is a mixture of a very large number of compounds. Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.
• what are most of the hydrocarbons in crude oil called?

  alkanes. the general formula for the homologous series of alkanes is C-n H-2n+2
• what are the first four members of the alkanes?

  Methane, ethane, propane, butane.
• what can the hydrocarbons in crude oil be separated into?

  fractions, each of which contain molecules with a similar number of carbon atoms by fractional distillation. the fractions can be processed to produce fuels and feedstock for petrochemical industry.
• what fuels are produced from crude oil?

  petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases
• what do the vast array of natural and synthetic carbon compounds occur due to?

  the vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds.
• how does fractional distillation work when separating hydrocarbon fractions?

  the oil is heated until most of its turned into gas. the gases enter a fractioning column. in the column there’s a temperature gradient ( its hot at the bottom and gets cooler as you go up ). the longer hydrocarbons have high boiling points. they condense back into liquids and drain out of the column early on, when they’re near the bottom. the shorter hydrocarbons have lower boiling points. they condense and drain out much later on, near to the top of the column where its cooler.
• what does the fractional distillations of crude oil result in?

  you end up with crude oil mixtures seperated out into different fractions. each fraction contains a mixture of hydrocarbons that all contain a similar number of carbon atoms, so they have similar boiling points
• properties of hydrocarbons?

  some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability. these properties influence how hydrocarbons are used as a fuels.
• what does the combustion of hydrocarbon fuels release?

  releases energy. during combustion, the carbon and hydrogen in the fuels are oxidised. the complete combustion of a hydrocarbon produces carbon dioxide and water.
• whats cracking?

  hydrocarbons can be broken down ( cracked ) to produce smaller, more useful molecules. cracking can be done by various methods including catalytic cracking and steam cracking
• whats the product of cracking?

  include alkanes and another type of hydrocarbon called alkenes.
• what are the general conditions of used for catalytic cracking?


  High temperature and pressure, presence of a catalyst, and a feedstock containing hydrocarbons.
• what are the general conditions used for steam cracking?

  High temperature and low pressure.
• are alkenes more reactive that alkanes?

  alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkene.
• whats the colour change when bromine water reacts with an alkene?

  Orange
• what is there high demand for?

  there is high demand for fuels with small molecules and so some of the products of cracking are useful as fuels
• what are alkenes used to produce?

  Alkenes are used to produce polymers and as starting materials for the production of many other chemicals
• What’s a pure substance?

  is a single element or compound , not mixed with any other substance. Pure elements and compounds melt and boil at specific temperatures. Melting and boiling point data can be used to distinguish pure substances from mixtures.