Rates of Reaction and Energy Changes

Rate of Reaction

The rate of a reaction depends on the collision frequency of reacting particles and the energy transferred in the collision. The minimum energy for a successful reaction to occur during a collision is called the activation energy. Graphs of mass, volume and concentration of reactant or product against time show different rates of reaction

Condition	Effect
Temperature	When the temperature increases, the particles move faster due to increased kinetic energy, leading to a higher frequency of effective collisions with energy greater than or equal to the energy required to cause a reaction, leading to an increased rate of reaction
Surface Area	When a solid is broken down the surface area to volume ratio increases. There is more area for particles to collide with. An increased frequency of collision leads to an increased rate of reaction
Concentration or Pressure	If a solution is more concentrated, there are more particles in the same volume. This means that the frequency of collisions will increase so the rate of reaction increases
Catalyst	A catalyst is a substance that speeds up the rate of reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction Catalysts offer an alternative pathway for the reaction which has a lower activation energy. As a result, more of the particles will have at least the minimum amount of energy needed for a reaction to occur when particles collide, so the rate of reaction increases Enzymes are biological catalysts that speed up reactions in organisms such as photosynthesis, respiration and protein synthesis. Enzymes from yeast cells are used in the fermentation process for the production of ethanol for use in alcoholic beverages

CORE PRACTICAL: Investigating Reaction Rates

Method 1: Measuring Volumes of Gas

A – Set up conical flask with delivery tube to gas syringe or measuring cylinder inverted in water

B – Measure volume of hydrochloric acid and pour into flask

C – Add mass of marble chips of particular surface area. Immediately close the flask and start time

D – Note the volume of gas produced at a given time interval until the reaction is finished

E – Repeat using different sized chips but the same total mass

This method is accurate as it produces quantitative results. However, some gas may dissolve in the water

Method 2: Precipitation

A – Pour sodium thiosulfate into conical flask then clamp in a water bath at the same temperature

B – Measure out acid in small test tube then warm in water bath

C – Remove flask then place over a cross on white paper

D – Add acid to the sodium thiosulfate then start time

E – Record time for cross to disappear and the final temperature

F – Repeat at different temperatures

This method is subjective as the cross is no longer seen at different times.

Method 3: Measuring Change of Mass is not a core practical but is still required on the spec. The two reactants are placed in a conical flask with cotton wool preventing anything from spraying out of the flask while gas can escape. Change in mass is measured using a mass balance

Heat Energy Changes in Chemical Reactions

Exothermic Reactions transfer thermal energy from the stores of energy in chemical bonds to the surroundings. Endothermic Reactions take in thermal energy from the surroundings to stores of energy in chemical bonds. The breaking of bonds is endothermic and the making of bonds is exothermic. If more heat energy is released in forming bonds in the products than is required in breakings bonds in the reactants, then the reaction is exothermic. If the opposite is true, then the reaction is endothermic.

Energy changes in precipitation reactions and salts dissolving in water depends on the type of salt or substance involved. However, neutralisation and displacement reactions are always exothermic

The energy needed to break one mole of a particular covalent bond is its bond energy, measured in kilojoules per mole. In the GCSE Paper 2 Exam, a table of values for bond energy will be supplied. The question will ask you to calculate the overall energy change in the reaction to show if it is exothermic or endothermic. First, calculate the energy needed to break the bonds by multiplying the number of each bond type there is on the reactants side of the equation by its bond energy. Then, use the same method to calculate the energy released when making bonds. Work out the energy needed – energy released for the final energy change, keeping in +/- symbols as this indicates whether the reaction is endothermic or exothermic. If it is negative, then heat is supplied to the surroundings so the reaction is exothermic