‘As the concentration of sodium thiosulfate solution doubles, the rate of reaction doubles.’ Explain the student’s conclusion in terms of particles.
the number of particles per unit volume increases so the frequency of collisions increases so the number of particles increases
Gold ions are used as a catalyst. How does a gold atom (Au) become a gold ion (Au3+)?
It loses 3 electrons
Carbon dioxide has a very low boiling point. Explain why.
because carbon dioxide is simple molecular there are weak intermolecular forces
Rate of reaction
Amount (e.g. grams, cm3) of reactant used or product formed / time
Rate of reaction (mol/s)
Moles of reactant used or product formed / time
Units for rate of reaction
g/s
cm3/s
mol/s
Common ways of measuring rate of reaction
Loss in mass of reactants
Volume of gas produced
Time for a solution to become opaque
Measuring rate by monitoring mass loss
Place the reaction flask on a balance. In these reactions (e.g. metal carbonate + acid) a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light). Plot a graph of mass vs time.
Measuring rate by monitoring volume of gas
Connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals. Plot a graph of volume vs time.
Measuring rate by monitoring disappearance of a cross
Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
Finding rate of reaction at time t from graph of amount of reactant vs time
Pick a point corresponding to the time t, and find the tangent to the curve at this point. The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values over change in x values.
Factors affecting rate of reaction
Concentration of reactants
Pressure of gases (volume)
Surface area
Temperature
Catalysts
Collision theory
Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy)
Increasing temperature
Faster reaction
Increasing concentration
Faster reaction
Increasing pressure of gases
Faster reaction
Increasing surface area
Faster reaction
Catalyst
Changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
Enzyme
A molecule that acts as a catalyst in a biological system.
Reversible reaction
A reaction where the products can react backwards to produce the original reactants.
Dynamic equilibrium
In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
Le Chatelier's Principle
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.