Is the rate of change of the reactants into the products in a chemical reaction
Factors affecting rate of reaction
The rate depends on:
surface area or size of solid particles
concentration of solution
temperature
presence of a catalyst
Different methods of measuring rate, all of which measure a quantity against time
Reactions used in rate of reaction experiments
A metal reacting with dilute acid e.g zinc or magnesium with dilute hydrochloric acid or sulfuric acid. These produce hydrogen gas: Mg+2HCl-> MgCl2 + H2 or Zn+H2SO4->ZnSO4+H2
The reaction between sodium thiosulfate solution and HCl produces a precipitate of sulfur making solution go cloudy. Sulfur causes cloudiness.
Ways to measure the rate of reaction
Measuring a change in mass
Measuring gas volume
Measuring time for a reaction to be completed
Measuring production of a solid precipitate
Measuring a change in mass
The reaction between CaCO3 and HCl can be used to investigate how the size of solid particles affects the rate of the reaction
Mass is lost because CO2 escapes from the reaction vessel. Recording the loss over a certain time period gives an indication of the rate of reaction
Cotton wool stops any liquid lost in the flask. There is bubbling which causes solution to splash out
Change in mass equipment
Conical flask
Cotton wool
Top ban/electric balance
Stop clock
Change in mass results
The reaction occurs more rapidly with smaller chips. These have a much larger surface area to volume ratio. There is more contact between the surface of small chips and acid which causes greater rate of reaction
Same mass and same volume and same concentration of acid was used. Both reactions start and level off at the same masses
The steeper initial slope of curve for small marble chips show mass is decreasing quickly therefore rate of reaction is higher
Measuring gas volume
If a reaction produces gas, collecting gas in a gas syringe over a period of time is a good way to measure a rate of reaction. Volume is taken from readings on gas syringe at time intervals.
Gas volume equipment
Delivery tube
gas syringe
stop clock
conical flask
Gas volume results
The volume of gas produced increases more rapidly when small chips are used
Curves start and end at same gas volume because same volume and concentration of HCl is used
Both curves of small and large chips are steepest at the start. This indicates the reaction is faster at start and gradually gets slower because reactants are being used up. Rate of reaction decreases as reaction proceeds
Measuring time for a reaction to be completed
1. For a reaction that produces gas:
2. Reading the time when the mass first reaches its minimum on a graph of mass against time
3. Reading time when gas volume first reaches its maximum on a graph of gas volume against time
4. Timing the reaction until you see the production of gas has stopped
5. For a reaction in which a solid disappears: Timing the reaction until no more solid can be seen in reaction vessel
Sodium thiosulfate solution reacts with dilute hydrochloric acid and the reaction produces solid sulfur
If solutions are mixed in a conical flask placed on a piece of white paper with a black cross, then as the solid sulfur is produce, it obscures the cross
Time is measured until cross can no longer be seen from above
The higher the concentration of sodium thiosulfate solution of HCl acid, the less time taken for the cross to disappear, meaning a higher rate of reaction
Production of solid precipitate equipment
Conical flask
white paper
stop clock
sodium thiosulfate and HCl
Calculating rate from time
Rate = 1/time
The unit of rate is S-1 (small minus 1)
Planning an investigation
This involves several steps but making an initial prediction and ensuring the investigation is a fair test is important
For an investigation into effects of changing the concentration of HCl on the rate of reaction with Mg ribbon - the prediction could say as concentration increases, the rate of reaction increases
A fair test is used when one variable is changed, one is measured and all others are kept the same
Validity of data collection
Validity is part of the overall design of the experiment e.g using two concentrations to determine the link between concentration and rate of reaction. Validity can be improved by using a greater number and range of concentration
Reliability of data collection
Reliability depends on whether or not the same result could be obtained if the experiment was repeated. Measuring the gas volume or change in mass can produce more reliable results because time when the reaction finishes is determined by apparatus used
Anomaly
Any anomalles are indicated by points that don’t meet the line of best fit. It is a piece of data that doesn’t match the pattern shown
Explaining effects of temperature and concentration on reaction rate
Reactions can only happen when reacting atoms, molecules or ions collide with each other. Only some of the collisions result in a reaction - these are successful collisions
An increase in temperature increases the rate of most reactions
Activation energy
The minimum energy required in a collision in order to cause a reaction
At higher temperatures
Particles have more energy and move faster
This leads to more collisions between particles
And so more successful collisions in a given time - more particles have more energy than the activation energy
Which increases the rate of reaction
At a higher concentration of solution
There are more particles present in the same volume
Leads to more collisions between particles
More successful collisions in a given time
Increasing rate of reaction
Investigating the effect of the presence of a catalyst
Reaction commonly used for the effect of a catalyst is the decomposition of hydrogen peroxide using manganese (IV) oxide, MnO2 as a catalyst. Reaction can be monitored using a gas syringe to collect and measure volume of oxygen gas produced.
Presence of catalyst
The catalysed reaction happens at a faster rate, and the catalyst reaction at a higher temperature occurs at the highest rate of reaction
A change in concentration of hydrogen peroxide solution would change the volume at which the graph levels off
This is usually simple e.g if a concentration is doubled, the volume at which graph levels off doubles
Graph at high temperature when all factors are the same
Gas volume starts at 0
Gas volume is higher at each time interval
Line on graph levels off earlier
Line ends at the same final gas volume
Metal with acid reaction
Effects of changed of both temperature and concentration can be seen using two different reactions:
zinc with dilute acid
magnesium with dilute acid
Both of these produce hydrogen gas, so gas volume can be used to measure the rate of reaction
A change in temperature will alter the shape of the gas volume against time graph - it doesn’t make any difference whether the acid or metal is in excess
A change of concentration of acid will alter the shape of the gas volume graph but the final gas volume will be changed only if all acid is used up
Catalyst
A catalyst is a substance that increases the rate of a chemical reaction without being used up
Catalysts work
Catalysts work by providing an alternative reaction pathway of lower activation energy which increase the number of successful collisions in a given time - this increases the rate of reaction
The mass of a catalyst will be the same at the end of the reaction as the start as it is not used up
Transition metals and their compounds are often used as catalysts e.g manganese (IV) oxide in the catalytic decomposition of hydrogen peroxide and iron in the Harber process