Key terms: Rate of Reaction

Learn these terms word-perfect, because the exam often asks for a one-mark definition and rewards the exact wording. Each entry gives the term in all three languages and a short definition; use the list to test yourself by covering the definitions. The terms are grouped to match the content standards.

The concept and units of rate (7.1)

• Rate of reaction / Kadar tindak balas / 反应速率, the change in the amount of a reactant or a product per unit time. It can be measured as a volume, mass or concentration divided by time.
• Average rate / Kadar purata / 平均速率, the total change in a quantity divided by the total time taken over a period of the reaction.
• Instantaneous rate / Kadar serta-merta / 瞬时速率, the rate at a particular moment, found from the gradient of a tangent to the curve at that point.
• Gradient / Kecerunan / 梯度, the steepness of a graph; on a volume-time or mass-time curve the gradient at a point equals the rate at that moment.
• Plateau / Plato / 平台, the flat part of a rate curve where the gradient is zero, showing that the reaction has stopped and no more product forms.

Factors and how rate is followed (7.2, 7.1)

• Concentration / Kepekatan / 浓度, the amount of solute in a given volume of solution; a higher concentration gives more particles per unit volume and a faster rate.

• Surface area / Luas permukaan / 表面积, the total exposed area of a solid reactant; a smaller particle size (a powder) gives a larger surface area and a faster rate.

• Pressure / Tekanan / 压力, for gases, increasing the pressure pushes particles closer together, raising the collision frequency and the rate; it does not affect reactions between solids and solutions.

• Gas syringe / Picagari gas / 气体注射器, apparatus used to measure the volume of gas released as a reaction proceeds, so the rate can be followed.

• Precipitate / Mendakan / 沉淀, an insoluble solid formed in a solution during a reaction; in the sodium thiosulfate reaction the sulfur precipitate makes the mixture cloudy, hiding a mark under the flask.

Collision theory (7.3)

• Collision theory / Teori perlanggaran / 碰撞理论, the model that reacting particles must collide, and that only collisions with enough energy and the correct orientation lead to a reaction.
• Effective collision / Perlanggaran berkesan / 有效碰撞, a collision that results in a reaction, because the particles have energy equal to or greater than the activation energy and collide in the correct orientation.
• Activation energy / Tenaga pengaktifan / 活化能, the minimum energy that colliding particles must have for a collision to be effective and a reaction to occur.
• Frequency of effective collisions / Kekerapan perlanggaran berkesan / 有效碰撞的频率, the number of effective collisions per unit time; the rate of reaction depends directly on this quantity.
• Kinetic energy / Tenaga kinetik / 动能, the energy a particle has because of its motion; at a higher temperature particles have more kinetic energy, so more collisions exceed the activation energy.
• Energy profile diagram / Gambar rajah profil tenaga / 能量剖面图, a diagram showing how the energy changes during a reaction, with the activation energy drawn as the height of the barrier the particles must overcome.

Catalysts (7.4)

• Catalyst / Mangkin / 催化剂, a substance that alters the rate of a chemical reaction while remaining chemically unchanged in mass and composition at the end.
• Positive catalyst / Mangkin positif / 正催化剂, a catalyst that speeds a reaction up by providing an alternative path with a lower activation energy.
• Alternative reaction path / Laluan tindak balas alternatif / 替代反应路径, the lower-activation-energy route a catalyst provides, along which a larger fraction of collisions is effective.

Calculations and graphs (7.5)

• Molar volume of a gas / Isipadu molar gas / 气体摩尔体积, the volume occupied by one mole of any gas at a stated condition; at room conditions it is 24 dm³ mol⁻¹, used to turn moles of gas into a volume.
• Limiting reactant / Bahan tindak balas penghad / 限量反应物, the reactant that is completely used up first; it fixes the amount of product and therefore the height of the plateau, while any other reactant is in excess.
• Manipulated variable / Pemboleh ubah dimanipulasi / 操纵变量, the factor deliberately changed in a rate experiment, such as temperature or concentration.
• Responding variable / Pemboleh ubah bergerak balas / 反应变量, the quantity measured in a rate experiment, such as the volume of gas or the time taken.
• Controlled variable / Pemboleh ubah dimalarkan / 固定变量, a factor kept constant so the test is fair and only the manipulated variable affects the result.

A few more useful terms

• Reactant / Bahan tindak balas / 反应物, a starting substance that is used up during a reaction; its amount falls with time as the reaction proceeds.
• Product / Hasil tindak balas / 生成物, a substance formed in a reaction; its amount rises with time until the reaction stops.
• Enzyme / Enzim / 酶, a biological catalyst, usually a protein, that speeds up a specific reaction in living things without being used up.
• Orientation / Orientasi / 取向, the direction in which particles approach each other when they collide; only the correct orientation, together with enough energy, gives an effective collision.
• Fair test / Ujian adil / 公平检验, an experiment in which only the manipulated variable is changed and all other variables are controlled, so the result is reliable.

How to use this glossary

Cover the definitions and try to state each term from memory, then check the wording. The five terms most often asked as one-mark definitions are rate of reaction, average rate, activation energy, effective collision and catalyst, so make those word-perfect first. Notice how the definitions connect: activation energy and orientation define an effective collision, whose frequency sets the rate, which a catalyst raises by lowering the activation energy through an alternative path. Once the terms link up in your mind, the collision-theory explanations in the rest of the chapter become much easier to write. A one-to-one teacher can quiz you on these definitions and correct any loose wording before the SPM Chemistry papers, because a precise definition is often the difference between a full and a partial mark.