| what is the relationship between rate of chemical reaction and concentration of reactants | rate of chemical reaction is proportional to concentration of reactants present |
| why does reaction rate slow down nearing the end of a reaction | reactants are being used up |
| what is the formula for average rate of reaction | rate=Δquantity/Δtime |
| what is the unit for reaction rate if change in quantity is measured in grams and time is measured in seconds | g s-1 |
| when can relative rate of reaction be calculated | if the change in quantity of a chemical is the same for a series of reactions |
| what can relative rate be used for | to investigate the effect of factors such as temperature and concentration on the rate of reaction |
| what is relative rate | the 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 rate | relative rate= 1/t |
| what is the unit for relative rate | s-1
as no measurable change in quantity is being observed |
| what must happen in order for a chemical reaction to occur | reactant 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 energy | the minimum kinetic energy with which particles have to collide in order to react |
| what kind of graph is this | potential energy diagram |
| what kind of diagram is this | energy distribution diagram |
| on a potential energy diagram, what represents the activation energy | the distance between the starting enthalpy and the highest peak |
| is activation energy the same for every reaction | no, 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 collisoins | the 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 formed | if 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 complex | an 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 stable | the 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 geometry | for 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 collide | particle 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 rate | decreasing 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 state | as 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 rate | the 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 temperature | a 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 rate | A 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 catalyst | a 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 industry | iron: 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 enthalpy | the 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 reactants | energy 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 diagram | energy is absorbed from the surroundings as the reaction proceeds. |
| what sort of ∆H values are endothermic reactions assigned | +∆H |
| is bond breaking exothermic or endothermic | endothermic
(think trying to snap a pencil, it requires energy)
+∆H |
| is bond making exothermic or endothermic | exothermic (-∆H) |
