GCSE Chemistry 2

• What is the rate of a chemical reaction?

  How fast the reactants are changed into products
• Examples of slow chemical reactions

  Rusting of iron

  Chemical weathering e.g. acid rain
• Example of moderate chemical reactions


  Reaction of magnesium with an acid to produce a gentle stream of bubbles
• Examples of fast chemical reactions

  Burning

  Explosions - over in a fraction of a second
• Rate of Reaction Graphs

  Steeper lines = faster rate - the quickest reactions have the steepest lines & become flat in the least time

  Shallower lines = slower rate - the slowest reactions have the shallowest lines & become flat in the most time
• What is collision theory?


  It affects the rate of a chemical reaction

  1) The collision frequency of reacting particles - the more collisions there are, the faster the reaction is

  2) The energy transferred during a collision - particles have to collide with enough energy for the collision to be successful
• What does the rate of reaction depend on?

  1) Temperature

  2) Concentration of a solution or the pressure of gas

  3) Surface area - this changes depending on the size of the lumps of a solid

  4) The presence of a catalyst
• Rate of Reaction - Increasing the Temperature

  1) Causes the particles to move faster - if they're moving faster, they're going to collide more frequently

  2) The faster the particles move, the more energy they have, so more of the collisions will have enough energy to make the reaction happen
• Rate of Reaction - Increasing the Concentration/Pressure

  1) A more concentrated solution will contain more particles in the same volume of water

  2) If the pressure of a gas is increased, there will be more particles occupying a smaller space

  3) This causes collisions between the reactant particles to be more frequent
• Rate of Reaction - Increasing the Surface Area


  Breaking a solid up into smaller pieces will increase its surface area to volume ratio - for the same volume of the solid, the particles around it will have more area to work on, thus there will be more frequent collisions
• Rate of Reaction - Using a Catalyst

  Not part of the overall reaction equation - doesn't get used up in the reaction itself

  They all decrease the activation energy needed for the reaction to occur - they do this by providing an alternative reaction pathway with a lower activation energy
• Equation for Rate of Reaction

  Rate of reaction = Amount of reactant used or amount of product formed / Time
• Measuring the Rate of Reaction - Precipitation & Colour Change

  1) Recording visual changes in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution

  2) Observing a mark through the solution & measure how long it takes for it to disappear - the faster it disappears, the faster the rate of reaction

  3) If the reactants are coloured & the products are colourless, time how long it takes for the solution to lose (or gain) its colour

  4) Results can be subjective & you can't plot a rate of reaction graph from the results
• Measuring the Rate of Reaction - Change in Mass

  1) Measuring the speed of a reaction that produces a gas can be carried out using a mass balance

  2) As the gas is released, the mass disappearing is quickly measured on the balance - the quicker the reading on the balance drops, the faster the reaction

  3) Measurements can be taken at regular intervals for a rate of reaction graph to easily find the rate

  4) Most accurate but the gas is released straight into the room
• Measuring the Rate of Reaction - The Volume of Gas Given Off

  1) Uses a gas syringe to measure the volume of gas given off

  2) The more gas given off during a given time interval, the faster the reaction

  3) Gas syringes usually give volumes accurate to the nearest cm³, so they're quite accurate

  4) Measurements can be taken at regular intervals for a rate of reaction graph to be plotted

  5) If the reaction is too vigorous, the plunger could easily be blown out of the end of the syringe
• How do you find the mean rate for the whole reaction?

  Work out the overall change in the y-value (amount of product formed or amount of reactant used) and divide it by the total time taken for the reaction
• How do you find the rate of reaction at a particular point?

  Finding the gradient of the curve at that point - do this by drawing a tangent to the curve and work out the gradient of the tangent
• Reversible reactions

  1) As the reactants react, their concentrations fall, so the forward reaction will slow down - but, as more and more products are made and their concentrations rise, the backward reaction will speed up

  2) After a while the forward reaction will be going at exactly the same rate as the backward one - the system is at equilibrium
• What happens at equilibrium during a reversible reaction?

  Both reactions are still happening, but there's no overall effect - the concentrations of reactants & products have reached a balance & won't change
• When will the reaction reach equilibrium?

  When the reaction takes place in a closed system - none of the reactants or products can escape & nothing else can get in
• What happens if the equilibrium lies to the left?

  The concentration of reactants is greater than that of the products
• What happens if the equilibrium lies to the right?

  The concentration of products is greater than that of the reactants
• What does the position of equilibrium depend on?

  1) The temperature

  2) The pressure (only affects the equilibria involving gases)

  3) The concentration of the reactants & products
• Are reversible reactions endothermic or exothermic?

  They can be both - if the reaction is endothermic in one direction, it will be exothermic in the other

  The energy transferred from the surroundings by the endothermic reaction is equal to the energy transferred to the surroundings during the exothermic reaction
• Le Chatelier's Principle

  The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change

  It's used to predict the effect of any changes you make to a reaction system
• Changes to Temperature

  1) If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat, therefore you'll get more products for the exothermic reaction and fewer products for the endothermic reaction

  2) If you increase the temperature, the equilibrium will move in the endothermic direction to try & decrease it, therefore, you'll get more products for the endothermic reaction and fewer products for the exothermic reaction
• Changes to Pressure

  Only affects an equilibrium involving gases

  Increasing the pressure will cause the equilibrium to try & reduce it, thus making it move in the direction where there are fewer gas molecules

  Decreasing the pressure will cause the equilibrium to try & increase it, thus making it move in the direction where there are more gas molecules
• Changes to Concentration


  Changing the concentration will cause the system to bring itself back to equilibrium

  Increasing the concentration causes the system to decrease it by making more products

  Decreasing the concentration causes the system to increase it by reducing the amount of reactants
• Reaction of Magnesium & Hydrochloric Acid to produce Hydrogen gas

  1) Add a set volume of dilute HCl to a conical flask & carefully place on a mass balance

  2) Add some magnesium ribbon to the acid & quickly plug the flask with cotton wool

  3) Start the stopwatch & record the mass on the balance - take readings at regular intervals

  4) Plot the results in a table & work out the mass lost for each reading. Plot a graph

  5) Repeat with more concentrated acid solutions, but keep variables like the amount of magnesium ribbon and the volume of acid the same
• Reaction of Sodium Thiosulfate & Hydrochloric Acid to produce a Cloudy Precipitate

  Both the solutions are colourless - they react together to form a yellow precipitate of sulfur

  1) Add a set volume of dilute Na2O3S2 to a conical flask

  2) Place the flask on a piece of paper with a black cross drawn on it - add some dilute HCl & start the stopwatch

  3) Watch the black cross disappear through the cloudy sulfur & time how long it takes

  4) Repeat the reaction with solutions of either reactant at different concentrations - only change one concentration at a time
• How can you find the speed of a reaction?

  Recording the amount of product formed or the amount of reactant used over time
• What is a successful collision?

  A collision that ends in the particles reacting to form products
• What do the particles need the activation energy for?

  To break the bonds in the reactants & start the reaction
• What factors will increase the rate of reaction?

  Factors that increase the number of collisions or the amount of energy particles collide with
• Thermal Decomposition of Hydrated Copper Sulfate - endothermic

  If you heat blue hydrated copper(II) sulfate crystals, it drives the water off & leaves white anhydrous copper(II) sulfate powder
• Thermal Decomposition of Hydrated Copper Sulfate - exothermic


  If you add a couple of drops of water to the white powder, you get the blue crystals back
• Are all reactions endothermic in one direction & exothermic in the other?

  Yes
• What is a hydrocarbon?

  Any compound that is formed from carbon and hydrogen atoms only
• What are alkanes?

  The simplest type of hydrocarbons

  They have single bonds between all the carbon atoms

  They are a homologous series
• Are alkanes saturated or unsaturated?

  They are saturated - each carbon atoms forms four single covalent bonds