Rate of Reaction: Change in concentration of a reactant used up or product formed in a given
time.
• Calculating Rate of Reaction: Rate (mol dm-3
s
-1
) = change in
concentration (mol dm-3
) / time (s). Time on x axis.
• Working out rate of anything is change in dependent variable/ time
taken for change. Work out change between difference between two
vales.
• Curve: Plotted with concentration against time. Could also be
temperature against time. The graph is a curve, because of the
information below.
– At start curve steepest, rate of reaction fastest. Reactants at highest concentration. So
formation product greatest.
– Curve becomes less steep. Rate of reaction slows down because concentrations of reactants
decrease.
– Curve becomes straight line parallel to x axis, stays constant. Once one of the reactants has
been completely used up, rate of reaction is zero, reaction complete.
• Factors Affecting Rate of Reaction:
– Concentration or pressure or surface area of solid reactants. Surface area = more molecules
available to react so more successful collisions per unit time.
– Temperature.
– Use of catalyst.
Collision Theory: Two reactant particles must collide in correct orientation AND with sufficient
energy to overcome activation energy for reaction to occur. Those two conditions make an
effective collision.
• Increasing Concentration:
– An increase in concentration of reactant.
– Increases the number of particles per unit volume. Volume never area.
– So particles collide more frequently and successfully.
– So an increased rate of reaction.
• If mentions how does increasing concentration increases rate, talk about above and not
equilibrium.
• Increasing Pressure of Gas:
– When pressure of gas increases.
– Concentration of gas molecules increases per unit volume.
– So particles collide more frequently and successfully.
– So increased rate of reaction.
• Following Rate of Reaction: Can either monitor removal of reactant or formation of product at
regular intervals, depending on their physical states. Can also look at change in concentration, gas
volume, mass and colour. So change in volume/ mass/ conc against time give rate of reaction.
• Gas Collection: If equation has (g), then know method could be gas collection to measure gas
volume. Equipment- delivery tube.
• Curve:
– To calculate initial rate of reaction, tangent drawn on curve when time at 0. Line drawn all the
way down.
– Tangent should keep as close to curve as possible and be equal angles between tangent and
curve on both sides on point. The gradient of tangent gives reaction rate- change in y/ change
in x- draw a triangle if needed.
– To calculate rate at specific time, tangent only touches the specific point.
– Remember units. Need to draw tangent for a mark
• Loss of Mass: Rate of reaction can be determined monitoring loss in mass of reactants. Both
reactants added to conical flask on balance. Mass of flask and contents recorded initially and at
regular time intervals. Reaction complete when no more gas produced so no more mass lost.
• Increasing Surface Area: Powder instead of ribbons. Faster rate of reaction, however would reach
same maximum as same amount of product.
• Higher volume or moles (concentration) means higher maximum.
• Higher temperature means reaches the maximum quicker but same maximum.
• Gases have to be hot, effective after first 5 min, have enough energy to react.