Rate and extent chemical change

• Rate of reaction formula

  Amount of reactant used / time

  OR

  Amount of product formed / time
• How to find rate of reaction graphically

  Draw tangents to curves and use the slope of the tangent.

  Calculate the gradient of a tangent to measure rate of reaction at a specific time.
• Factors which affect rate of reaction

  Concentration, pressure, surface area, temperature, catalysts.
• Collision theory

  Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
• Activation energy

  The minimum amount of energy that particles must have to react.
• How an increase in concentration increases rate of reaction

  There are more reactant particles in the same unit volume, so there are more frequent collisions.
• How an increase in pressure increases rate of reaction

  There are the same amount of reactant particles in a smaller unit volume, so there are more frequent collisions.
• How an increase in surface area increases rate of reaction

  There are more reactant particles able to react with each other, so there are more frequent collisions
• How increasing the temperature increases rate of reaction

  The reactant particles have more energy, so a larger proportion of reactant particles possess the activation energy, resulting in more frequent successful collisions.
• Catalysts


  Substances that speed up chemical reactions without being changed or used up during a reaction.
• How do catalysts increase rate of reaction

  They provide an alternative reaction pathway for a chemical reaction that has a lower activation energy, so a higher proportion of reactant particles possess the activation energy, so there are more frequent successful collisions.
• Reversible reactions

  When the products of the reaction can react to produce the original reactants.
• Energy changes in reversible reactions

  If a reversible reaction is endothermic one way, it is exothermic in the opposite direction.

  The same amount of energy is transferred each way.
• When does equilibrium occur?

  When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
• 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 the change.
• Effect of increasing concentration of reactants

  Position of equilibrium shifts towards products so more product is produced until equilibrium is reached again.
• Effect of increasing concentration of products

  Position of equilibrium shifts towards reactants so more reactant is produced until equilibrium is reached again.
• Effect of increasing temperature

  Equilibrium moves in the direction of the endothermic reaction.
• Effect of decreasing temperature


  Equilibrium moves in the direction of the exothermic reaction.
• Effect of increase in pressure

  Equilibrium shifts to the side of the equation with fewer moles of gas.
• Effect of decrease in pressure


  Equilibrium will shift to side of the equation with more moles of gas.
• What is the rate of a chemical reaction?

  A measure of how quickly the reactants are converted into products
• How can precipitation and colour change be used to measure the rate of a reaction?

  > Precipitation - measure the time it takes for a mark to disappear

  > Colour change - measure the time it takes for a colour change to occur
• What are the 2 issues with measuring the rate of a reaction by observing colour change and precipitation?

  > It is subjective - different people might interpret the exact point the solution changes colour differently

  > You can't use your results to plot a rate of reaction graph
• How can you calculate the rate of a reaction by measuring the change in mass?

  > Use a balance to monitor how the mass changes over time

  > Take measurements at regular intervals

  > Plot your results on a graph

  > Draw a tangent and calculate its gradient to find the rate at a particular time
• What is the most accurate way to measure the rate of a reaction and why?

  Change in mass, because the balance is the most accurate measuring device
• How can you use a gas syringe to measure the rate of a chemical reaction?

  > Use the gas syringe to monitor how the volume of gas changes over time

  > Take measurements at regular intervals

  > Plot your results on a graph

  > Draw a tangent and calculate its gradient to find the rate at particular time
• Give 4 factors that affect the rate of a chemical reaction?

  > Temperature

  > Concentration of solution or pressure of gas

  > Surface area

  > Presence of a catalyst
• Explain how increasing the temperature increase the rate of a reaction?

  > As the temperature increases, particles move around faster (have more kinetic energy)

  > This means they will collide more frequently

  > They will also collide with more energy, so the collision is more likely have enough energy to react successfully (activation energy)
• Explain how increasing concentration or pressure increases the rate of a reaction?

  > As the concentration of a solution increases, the number of particles per unit of volume will increase

  > This will increase the frequency of collisions, and so increase the rate of reaction
• Explain how increasing the surface area of a solid affects the rate of a reaction?

  > If a reactant is solid, breaking it into smaller pieces increases the surface area to volume ratio

  > This means, for the same volume of solid, the particles will have a greater area over which they can collide, so will collide more frequently

  > More frequent collisions result in a greater number of reactions
• What is collision theory?

  Collision theory states that for particles to react, they have to collide with sufficient energy, and at the right direction
• Why do particles need activation energy?

  They need energy to break the bonds of the reactants, so the reaction can begin
• How does a catalyst increase the rate of reaction?

  1) Catalysts speed up reactions by lowering the activation energy required for a reaction to occur

  2) They do this by giving an alternative route (reaction pathway) that requires less energy
• What is the formula to find the gradient of a line?

  change in y/change in x
• During a reversible reaction, what happens as the reaction progresses and the reactants react?

  > The concentrations of reactants fall

  > The forward reaction slows down

  > The concentrations of products rise

  > The backwards reaction speeds up
• What does it mean if the position of equilibrium to the right?

  The concentration of the products is greater than the concentration of the reactants
• Does equilibrium mean the concentration of products and reactants are equal?

  No

  An equilibrium may lie to the left or the right, and that tells us which has the greater concentration
• What is Le Chatelier's Principle?

  If you change the conditions of a reversible reaction at equilibrium, the system will try to counter that change in order to restore the equilibrium
• According to Le Chatelier's principle, what happens if you decrease the temperature


  The position of equilibrium will shift towards the exothermic direction to produce more heat (and counteract the decrease in temperature)