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Rates/Energetics
Rates of reaction + Energetics
| Question | Answer |
|---|---|
| Define rate of reaction. | The rate of a reaction is a measured change in the concentration of a reactant or product over time. |
| State the formula for the rate of reaction. | = change in concentration of reactant or product / time taken |
| What does the collision theory state? | For a reaction to occur successfully: 1) Particles must collide 2) Particles must collide with sufficient energy 3) Particles must collide with correct orientation |
| List the 4 main factors which affect the rate of a chemical reaction. | 1) Concentration 2) Temperature 3) Surface area (particle size) 4) Presence of a catalyst |
| Describe the effect of concentration on rate of reaction. | The greater the concentration (greater number of particles), the greater the chances for effective collisions. |
| Describe the effect of temperature on rate of reaction. | Increased temperature increases kinetic energy of particles leading to greater chances of effective collisions |
| Describe the effect of surface area (particle size) on rate of reaction | Smaller particles have a larger surface area exposed to the other reactant therefore increasing the chances for effective collisions |
| Describe the effect of presence of a catalyst on rate of reaction. | These usually increase the rate of a reaction without themselves being changed therefore increasing the chances for effective collisions. |
| What is described as an effective collision in a reaction? | One which results in a reaction occurring (i.e. product being formed) |
| What is shown on a rate curve? | A measured property (a responding variable on the Y axis) against time (the manipulated variable on the X axis). Many varieties exist e.g. concentration vs time, mass vs time, volume vs time, concentration vs 1/time, rate vs time.... |
| What does the steepest part of the curve indicate? | Reaction is occurring at the fastest rate |
| What does a less steep curve indicate? | Reaction is slower. |
| What does the flat part of a curve indicate? | Reaction is complete |
| State 4 actions which can be taken to make the rate curve more steep (increase rate). | 1) Increasing concentration of one reactant 2) Increasing temperature 3) Decreasing the particle size (i.e. Increasing surface area) of a solid reactant 4) Adding a catalyst |
| State 4 actions which can be taken to make the rate curve less steep (decrease rate). | 1) Decreasing concentration of one reactant 2) Decreasing temperature 3) Increasing the particle size (i.e. decreasing surface area) of a solid reactant 4) Adding an inhibitor ( a negative catalyst) |
| What is an exothermic reaction? | - One which releases energy to the surroundings. - The energy content of the reactants is higher than that of the product since energy is released to the environment. |
| What is an endothermic reaction? | - One which absorbs energy from the surroundings. - The energy content of the products is higher than that of the reactants since energy is absorbed from the surroundings. |
| Give 3 examples of exothermic reactions. | Burning fossil fuels, neutralization reactions, respiration, dissolving NaOH or H2SO4 in water. |
| Give 3 examples of endothermic reactions. | Thermal decomposition, photosynthesis, dissolving NH4Cl or KNO3 in water. |
| Distinguish between exothermic reactions in terms of enthalpy change (delta H value). | Exothermic reactions: Enthalpy change is NEGATIVE (-ve) Endothermic reactions: Enthalpy change is POSITIVE (+ve) |
| Define enthalpy (H). | The energy content of a substance. |
| Give the formula of enthalpy change (delta H). | = Total enthalpy (H) of products - Total enthalpy (H) of reactants |
| T or F. In endothermic reactions, energy absorbed to break bonds in the reactants is greater than the energy released in forming new bonds in the products. | T |
| T or F. In exothermic reactions, energy absorbed to break bonds in the reactants is less than the energy released in forming the new bonds in the products. | T |
| Define activation energy (Ea) | This is the minimum amount of energy required by reactants in order to start breaking bonds and forming products. |
| What is the effect of adding a catalyst on the activation energy of a chemical reaction? | They lower the activation energy and therefore increase the rate of a reaction. |
| T or F. Enthalpy cannot be measured directly however heat of reaction can be. | T |
| Define specific heat capacity (c). | the quantity of heat required to raise the temperature of a unit mass of a substance by 1℃ or 1 K. Unit : Joule per gram per degree Celsius |
| Give the value of specific heat capacity of water. | 4.2 Jg-1oC-1 |
| State the formula for heat of a reaction. | q = mc delta T where q: heat of reaction m : mass of reactants (specifically mass of acid + alkali in heat of neutralization; mass of water in heat of solution) c : specific heat capacity delta T : change in temperature |
| Define heat of neutralization. | This is the heat change which occurs when 1 mol of water is produced in a reaction between an acid and an alkali. |
| Define heat of solution. | This is the heat change which occurs when 1 mol of a solute dissolves in such a volume of solvent that further dilution by the solvent produces no further heat change. |
| What is the unit for heat of a reaction? | Joule per mole OR kilojoule per mole |
| What is a calorimeter? | It is an insulated container which prevents heat released by a reaction from escaping to the surroundings and heat from the surroundings from being absorbed. Double styrofoam cups with a lid, thermometer and stirrer can be used as a simple calorimeter. |
| Describe how the heat of a reaction can be measured using a simple calorimeter. | 1) Measure fixed volumes of reactants 2) Measure initial temperature with a thermometer 3) Mix the reactants together in the calorimeter and measure the temperature periodically until it remains constant (this is the final temp.) |
| State the 3 assumptions made when performing heat of reaction calculations. | 1) The density of a dilute aqueous solution is the same as water i.e. 1 g cm-3 2) The specific heat capacity of a dilute aqueous solution is the same as water 4.2 J g-1 degree C-1 3) Negligible heat is lost or absorbed from the surroundings |
| Describe the dynamics involved in the process of forming a solution (solvation). | 1) Solute-solute bonds break (energy is absorbed to do so) 2) Solvent-solvent bonds break (energy is absorbed) 3) Solute-solvent interactions are formed (energy is released).This is called solvation. |
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CSEC Chemistry 11th Grade
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