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level: Controlling the rate

Questions and Answers List

level questions: Controlling the rate

QuestionAnswer
what is the relationship between rate of chemical reaction and concentration of reactantsrate of chemical reaction is proportional to concentration of reactants present
why does reaction rate slow down nearing the end of a reactionreactants are being used up
what is the formula for average rate of reactionrate=Δquantity/Δtime
what is the unit for reaction rate if change in quantity is measured in grams and time is measured in secondsg s-1
when can relative rate of reaction be calculatedif the change in quantity of a chemical is the same for a series of reactions
what can relative rate be used forto investigate the effect of factors such as temperature and concentration on the rate of reaction
what is relative ratethe rate of reaction at any one particular point since it is proportional to time it can be used to measure the rate where the change in quantity is difficult to obtain experimentally
what is the formula for relative raterelative rate= 1/t
what is the unit for relative rates-1 as no measurable change in quantity is being observed
what must happen in order for a chemical reaction to occurreactant particles must collide with each other
what conditions must occur for a collision to be successful (lead to a chemical reaction)reactant particles must collide with energy equal to or greater than the activation energy and with the correct collision geometry or angle
what is activation energythe minimum kinetic energy with which particles have to collide in order to react
what kind of graph is thispotential energy diagram
what kind of diagram is thisenergy distribution diagram
on a potential energy diagram, what represents the activation energythe distance between the starting enthalpy and the highest peak
is activation energy the same for every reactionno, activation energy varies from reaction to reaction eg. reacting an acid and an alkali has a very low activation energy but the reaction between nitrogen and oxygen has a very high activation energy
why do nitrogen and oxygen molecules not react in the air despite the many collisoinsthe activation energy for this reaction is so high that it is only under specific conditions of a lightning storm or in a car engine where it can be supplied by the high energy spark
how is an activated complex formedif reactant molecules collide with enough energy to overcome the repulsive forces (caused by outer electrons) the bonds between reactant atoms start to break and product bonds begin to form, giving rise to an activated complex energy is given out as new bonds are formed and the atoms are rearranged into the product molecule(s)
what is an activated complexan unstable arrangement of atoms, intermediate between reactants and products, formed at the maximum of the potential energy barrier during a reaction
is the activated complex stablethe activated complex is very unstable and exists for only a very short period
what is activation energy (alternative definition, refer to activated complex)the activation energy is the energy needed by colliding particles to form an activated complex
describe successful collision geometryfor a successful collision to take place the collision geometry must also be correct (the reactant molecules have to be facing the right way) so that the activated complex can be formed
which factors can affect the rate that reactant molecules collideparticle size/surface area concentration (and pressure for gases) temperature use of catalyst any factor that increases the frequency of collisions will also increase frequency of successful collisions and therefore increase the rate of reaction
what is the effect of particle size on reaction ratedecreasing particle size increases surface are agiving greater contact between reactants. This increases the rate of collisions and therefore successful collisions between reactant particles and the reaction rate increases
what is the effect of concentration on reaction rate in the dissolved (solution state)as the concentration of a reactant increases the reactant particles are closer together because there are more particles in a given volume, ie more moles per litre. This increases the rate of collisions and therefore successful collisions which increases reaction rate.
what is the effect of concentration and pressure in the gaseous stateas the pressure is increased reactant particles are pushed closer to each other so there are more particles in a given volume. This increases the rate of collisions and therefore successful collisions which increase rate.
what is the effect of temperature on reaction ratethe rate of reaction increases with increasing temperature because the average kinetic energy of the particles increases and a greater proportion of reactant particles collide with energy greater than or equal to the activation energy
what is temperaturea measure of the average kinetic energy of all the particles in a substance
For many reactions a 10°C rise in temperature doubles the reaction rate. Why are most reactions very sensitive to changes in temperature, making increasing temperature a very effective way in increasing reaction rate?at lower temperatures only a small fraction of the particles have very low or very high Ek and the majority have E close to the average Ek. At lower temps, the proportion of particles that have energy equal or greater than the Ea is shown by the small blue shaded area If temperature is increased, then the fraction of particles with low Ek decreases, fraction with high Ek increase. At this temp, proportion of particles that have energy =< Ea is shown by both shaded areas. This shows that there is a large increase in the proportion of particles that have enough energy to form an activated complex.
what is the effect of use of catalyst on reaction rateA catalyst provides an alternative reaction pathway with a lower activation energy catalyst provides a surface for reactions to take place on. Reactant molecules are held at a favourable angle for collisions to ocur and the reactant bonds are weakened , lowering the activation energy and increasing the likelihood of successful collisions.
what is a catalysta substance that speeds up a chemical reaction without being chemically changed
catalysts allow chemical reactions to occur at lower temperature, and are therefore used in the chemical industry to make manufacturing processes more economical. give some examples of catalysts used in industryiron: used to make ammonia by the haber process platinum: used in the manufacture of nitric acid in the ostwald porocess rhodium and platinum: in catalytic converters nickl: to make margarine by hardening vegetable oil vanadium (v) oxide: in the contact process, to make sulphuric acid
what is enthalpythe heat stored by a substance it is given the symbol H
what is enthalpy change (∆H)the energy difference between the products and the reactants and is the overall enthalpy change for a reaction ∆H = H products - H reactants
what does it mean if the energy of the products on a potential energy diagram is lower than that of the reactantsenergy is released to the surroundings as the reaction proceeds, it is an exothermic reaction.
what sort of ∆H value are exothermic reactions assigned-∆H
what does it mean when the energy of products is higher than that of the reactants on a potential energy diagramenergy is absorbed from the surroundings as the reaction proceeds.
what sort of ∆H values are endothermic reactions assigned+∆H
is bond breaking exothermic or endothermicendothermic (think trying to snap a pencil, it requires energy) +∆H
is bond making exothermic or endothermicexothermic (-∆H)