Learning Objective To be able to: Describe how the rate of a reaction is changed, using collision theory Key Words: Energy, collisions, particles

Which jelly will dissolve quicker? Can you explain your answer? Monday, 05 August 2019 Collision Theory Date and title in books. You need a pen, pencil & ruler. Starter Activity – 5 minutes Which jelly will dissolve quicker? Can you explain your answer? One solid piece or 8 smaller pieces

Explain how the rate of reaction is changed Target 9-1 Identify ways we can speed up a reaction (F/H) Target 4-9 Describe how a reaction happens, using collision theory (H) Target 5-9 Explain how the rate of reaction is changed Extension: Application of Knowledge Target 8/9 Use mathematical analysis to explain why the rate of reaction has changed Learning Outcomes

Particles Colliding Hydrogen Chlorine Reactant particles collide Reactant particles move towards each other Product particles move apart

percentage completion of reaction Slower and slower! Reactions do not proceed at a steady state. They start off at a certain speed, then get slower and slower until they stop. As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down. 0% fast 25% slower 75% very slow 100% stopped reactant A reactant B product percentage completion of reaction

Changing the rate of reactions Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors speed up reactions? Increased temperature; increased concentration of dissolved reactants, and increased pressure of gaseous reactants; increased surface area of solid reactants; use of a catalyst.

Temperature The higher the temperature, the faster the particles move. This means they have more kinetic energy and so are more likely to react. They also move faster at higher temperature and so collide more frequently. So increasing the temperature increases the rate of reaction.

Temperature and particle collisions Boardworks GCSE Additional Science: Chemistry Changing Reaction Rates Teacher notes This simulation illustrates how increasing the temperature increases the number of collisions between particles. Explaining changes in the rate of reaction in terms of the collisions between reacting particles is higher tier for OCR Gateway GCSE Additional Science.

Surface area Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. slow rate This means that there is an increased area for the non-solid reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a faster rate of reaction. fast rate

Surface area and particle collisions Boardworks GCSE Additional Science: Chemistry Changing Reaction Rates Teacher notes This simulation illustrates how increasing the surface area increases the number of collisions between particles and solid reactants Explaining changes in the rate of reaction in terms of the collisions between reacting particles is higher tier for OCR Gateway GCSE Additional Science.

Increasing concentration

Increasing Concentration The more concentrated a solution, the more solute particles there are in a given volume. This means that it is more likely that reactant particles will collide with one another. More collisions, the faster the rate of reaction.

Reactions, particles and collisions Reactions take place when particles of reactants collide with a certain amount of energy. This energy is called activation energy, and is different for each reaction. The rate of a reaction depends on two things: the frequency of collisions between particles; the energy with which particles collide. If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

Activation Energy Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy (more next lesson!)