rates of reaction

• Graph of quantity of product formed against time

  1. The slope of the line gives an idea of the rate of reaction

  2. Initially, the reaction is fast, then slows down, and finally stops
• Graph of quantity of reactant used against time

  Similar pattern to the graph of product formed: initially fast, then slows down, and stops
• Calculating mean rate of reaction

  Mean rate of reaction = Quantity of product formed / Time taken
• Mean rate of reaction calculation example: 60 grams of product formed in 15 seconds results in a mean rate of reaction of 4 grams per second
• If we measure the quantity of product formed by a reaction over time, we get a graph
• The slope of the graph gives us an idea of the rate of the reaction
• Initially, the reaction is extremely fast, then slows down, and finally stops
• The slope of the curve constantly changing indicates that the rate is also constantly changing
• How scientists solve the changing rate problem

  Draw a tangent which touches the curve at the point of interest
• Tangent

  A straight line that touches a curve at a specific point
• The slope of the tangent gives us an idea of the rate of reaction at that point
• The slope of the tangent can be used to calculate the rate of reaction at a specific point
• Constructing a triangle using the tangent helps calculate the rate of reaction
• Dividing the vertical side by the horizontal side of the triangle gives the rate of reaction
• Example calculation: 14 grams divided by 25 seconds equals 0.56 grams per second
• Collision Theory

  Chemical reactions can only take place when the reacting particles collide with each other and the collisions must have sufficient energy
• Rate of a chemical reaction

  • Determined by the frequency of successful collisions
• Rate of a chemical reaction

  • Frequency means the number of successful collisions per second
• Reactions are rapid initially


  Due to a large number of reactant molecules leading to a large number of collisions per second
• Over time reactions slow down


  Because the number of reactant molecules is running out, resulting in a smaller number of collisions per second
• Finally, reactions stop

  Because all of the reactant molecules have run out, leading to zero collisions per second
• Effect of concentration of reactants or pressure of reactant gases on the rate of a chemical reaction

  Higher concentration or pressure leads to a faster reaction rate
• Rate of reaction is proportional to the concentration of reactants
• Higher concentration results in a faster reaction rate
• Higher concentration leads to more product at the end of the reaction
• Investigating the effect of concentration on the rate of reaction is a required practical
• Practical investigation to explore the effect of concentration on the rate of the reaction

  1. Two different methods to carry out this practical

  2. Method 1: Disappearing cross reaction

  3. Method 2: Measuring the volume of gas produced by a reaction
• Hypothesis

  A proposal that could explain a fact or observation, must be testable
• Disappearing cross reaction

  Reacting sodium thiosulfate solution with hydrochloric acid to produce sulfur, making the solution go cloudy (turbidity)
• Disappearing cross reaction method

  1. Put 10 cm³ of sodium thiosulfate solution into a conical flask

  2. Place the flask on a printed black cross

  3. Add 10 cm³ of hydrochloric acid into the flask

  4. Swirl the solution and start a stopwatch

  5. Look down through the top of the flask until the solution turns cloudy

  6. Stop the clock when the cross is no longer visible

  7. Repeat with lower concentrations of sodium thiosulfate solution

  8. Calculate mean values for each concentration
• Reproducibility

  A measurement is reproducible if it can be repeated by another person or using different equipment and still get the same result
• Measuring the volume of gas produced by a reaction

  Reacting magnesium with hydrochloric acid to produce hydrogen gas, allowing measurement of the volume of gas produced
• Measuring gas volume method

  1. Place 50 cm³ of hydrochloric acid into a conical flask

  2. Attach the flask to a bong and delivery tube

  3. Place the delivery tube into a container filled with water

  4. Place an upturned measuring cylinder filled with water over the delivery tube

  5. Add a strip of magnesium to the acid and start a stopwatch

  6. Measure the volume of hydrogen gas in the cylinder every 10 seconds until no more gas is produced

  7. Repeat with different concentrations of hydrochloric acid
• Greater concentration of a chemical in a reaction


  Results in a faster reaction
• The finding that greater concentration leads to faster reactions is reproducible as shown by two different experiments
• Rate of chemical reactions

  1. Chemical reactions can only take place when the reacting particles collide with each other and the collisions must have sufficient energy

  2. The rate of a chemical reaction is determined by the frequency of successful collisions

  3. Frequency means the number of successful collisions per second
• Surface area of a solid reactant

  • Increasing the surface area of a solid reactant increases the rate of reaction
  • Smaller sized blocks of solid reactant have a greater surface area to volume ratio than larger blocks, leading to more collisions per second and increased rate of reaction
• Experiment setup

  • Marble chips containing calcium carbonate react with hydrochloric acid to produce carbon dioxide gas
  • Measuring the volume of carbon dioxide gas using a gas syringe
  • Measuring the mass of carbon dioxide gas lost on a balance
• Measuring the rate of reaction

  1. Volume of carbon dioxide gas can be measured using a gas syringe

  2. Mass of carbon dioxide gas lost can be measured on a balance
• Cotton-wool allows carbon dioxide gas to escape while preventing acid from splashing out of the flask to avoid anomalous results