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Kinetics
AQA A-level chemistry kinetics year 13
| Term | Definition |
|---|---|
| Order + effect on rate of reaction | The extent to which each reactant affects rate of reaction. Rate of reaction is directly proportional to the concentration of reactant to the power of the order of that reactant |
| 0 order: effect on rate + concentration-time graphs + rate-concentration graphs | Rate throughout isn’t affected by concentration of the reactant. Concentration-time graphs show a straight line with a negative gradient and rate-concentration graphs show a horizontal line |
| 1st order: effect on rate + concentration-time graphs + rate-concentration graphs | Rate directly proportional to concentration of reactant, concentration-time graphs show a gentle negative curve, rate concentration graphs show a straight line with a positive gradient |
| 2nd order: effect on rate + concentration-time graphs + rate-concentration graphs | Rate directly proportional to square of concentration of reactant, concentration-time graphs show steep negative curve, rate-concentration graphs show steep positive exponential curve |
| Overall order of a reaction (and how it’s written) | Sum of the powers of the reactants in a rate equation e.g., rate = K[A]^m[B]^n K = constant [A] = concentration of substance A m = order of substance A in the reaction [B] = concentration of substance B n = order of substance B in the reaction |
| Initial rate of reaction | The speed at which reactants are converted into products at the very beginning of a reaction i.e., the tangent is drawn at T=0. T^-1 can be used as rate of reaction if the reactions have the same end point e.g., colour change |
| Rate constant | The speed at which a chemical reaction occurs, the higher the value the faster the reaction. Affected by temperature, concentrations of reactants, and presence of a catalyst |
| How to calculate rate constant and rate constant units (general equation) | K = rate/[X]^n Units = (moldm^-3s^1)/(moldm^3)^n n = order |
| Arrhenius equation | k = Ae^(-Ea/RT) k is the constant of the reaction representing successful collisions A is the pre-exponential factor representing all collisions e = mathematical constant Ea = activation energy R = gas constant T = absolute temperature in kelvin |
| How to plot a graph using the Arrhenius equation | Make the X axis K^-1 and the Y axis ln(k) Plot a line using the equation ln(k) = -EaT^1/R + ln(A) The by extrapolating the line, the ln(A) can be found by looking at the Y intercept |
| Rate determining step | The slowest step in a reaction mechanism that determines the overall rate of reaction. It has the highest activation energy and the slowest reaction time |
| How to find the rate determining step from a rate equation | Look at the rate equation and find the step of the mechanism which has those same concentrations of reactants. The orders of the reactants should correspond to the coefficients of the reactants in the RDS |
| How the orders of reactants affect the rate determining step | 0 order = isn’t included in the RDS 1st order = most likely to be included 2nd order = a small increase in concentration can lead to a large increase in rate of reaction so less likely to be included in the RDS |
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