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neurologyYou need to know

• The rate of reaction is the speed at which reactants are used up or products are formed in a chemical reaction.
• The mean rate of reaction can be calculated using `\text{mean rate} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}`.
• Common units for rate of reaction include grams per second, cubic centimetres per second, and moles per second.
• Rate of reaction data can be collected by measuring change in mass, volume of gas produced, or formation of a precipitate over time.
• On a graph of product formed against time, a steeper gradient means a faster rate of reaction.
• On a graph of reactant used up against time, a steeper downward gradient means a faster rate of reaction.
• The rate of reaction usually decreases over time because reactant particles are used up and collisions become less frequent.
• A tangent to a curve at a particular time gives the instantaneous rate of reaction at that time.
• The gradient of a tangent is calculated using `\text{gradient} = \frac{\text{change in y}}{\text{change in x}}`.
• The main factors that affect rate of reaction are temperature, concentration, pressure of gases, surface area of solids, and the presence of a catalyst.
• Collision theory states that reactions happen only when particles collide with enough energy and the correct orientation.
• Increasing temperature increases the rate of reaction because particles have more kinetic energy, move faster, and a greater proportion of collisions have energy greater than or equal to the activation energy.
• Increasing concentration increases the rate of reaction because there are more reactant particles in the same volume, so collisions happen more frequently.
• Increasing the pressure of reacting gases increases the rate of reaction because gas particles are closer together, so collisions happen more frequently.
• Increasing the surface area of a solid increases the rate of reaction because more reactant particles are exposed for collisions.
• A catalyst is a substance that increases the rate of a chemical reaction without being used up overall.
• A catalyst is not included as a reactant or product in the overall chemical equation because it is chemically unchanged at the end of the reaction.
• Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
• A reversible reaction is a reaction in which the products can react to reform the original reactants, and it is represented using the reversible reaction symbol `\rightleftharpoons`.
• In a reversible reaction, if the forward reaction is exothermic then the reverse reaction is endothermic by the same amount, and equilibrium is reached in a closed system when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.

• Collect rate of reaction data by measuring a suitable quantity, such as gas volume, mass loss, or precipitate formation, at regular time intervals.
• Calculate the mean rate of reaction from a table by dividing the change in quantity of reactant or product by the time taken, including correct units.
• Plot a rate of reaction graph with time on the x-axis and quantity of product formed or reactant used up on the y-axis, using accurate scales and labelled axes.
• Interpret rate of reaction graphs by comparing gradients and identifying when the reaction is fastest, slowing down, or complete.
• Draw a tangent to a curved rate graph at a chosen time by touching the curve at one point and matching the curve’s direction at that point.
• Calculate the instantaneous rate of reaction from a tangent by finding the tangent’s gradient using a clear triangle.
• Use collision theory to explain the effect of temperature, concentration, pressure, surface area, or a catalyst on rate of reaction.
• Identify a catalyst from reaction information by recognising that it speeds up the reaction and is not used up or shown in the overall chemical equation.
• Represent a reversible reaction using the reversible reaction symbol and describe the forward and reverse reactions.
• Explain equilibrium in a reversible reaction by stating that it occurs in a closed system when the forward and reverse reaction rates are equal.