A-Level化學(xué)有哪些知識(shí)點(diǎn)需要掌握?知識(shí)點(diǎn)那么多我要怎么復(fù)習(xí)才能高效呢？今天留學(xué)生輔導(dǎo)老師給大家說下，alevel考試輔導(dǎo)中，平衡知識(shí)的知識(shí)點(diǎn)復(fù)習(xí)總結(jié)。

Chemical Equilibria Revision

化學(xué)平衡知識(shí)點(diǎn)復(fù)習(xí)概要

01可逆反應(yīng)

Reversible Reactions

(1)可逆反應(yīng)

示意圖如下A reversible reaction is one where there is a forward and backward reaction occurring:

雙箭頭含義The double arrow signifies a reversible reaction.

動(dòng)態(tài)平衡：正向和逆向反應(yīng)速率相等的時(shí)候If in the above reaction the concentrations of A,B,C,D do not change,although the reaction is still in progress,then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.

(2)平衡常數(shù)Equilibrium Constants

所有的平衡反應(yīng)的性質(zhì)都可以用濃度平衡常數(shù)Kc來表示。Any dynamic equilibrium can be described in terms of its equilibrium constant,Kc.

The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation,divided by the product of molar concentrations of the reactants,each raised to the power of its coefficient in the stoichiometric equation.

具體方程關(guān)系如下：

So for the reaction:

The equilibrium constant is given by:

Where [ ] represents the concentration of the species in moldm-3.

對(duì)于由氣態(tài)物質(zhì)組成的反應(yīng)來說,可以用氣體的分壓來表示反應(yīng)的平衡常數(shù). For gaseous systems,we use Kp instead of Kc. Here,the species are shown in the equilibrium equation in terms of their partial pressures. (In a mixture of gases,the proportion of the total pressure due to a particular gas is dependent on its mole fraction).

關(guān)系如下: Kp=Kc*(RT)△n

(△n=反應(yīng)生成的物質(zhì)的量-反應(yīng)物的物質(zhì)的量)

02勒夏特列原理Le Chateliers principle

定義含義Le Chateliers principle states:

平衡的移動(dòng)總是向著逆轉(zhuǎn)反應(yīng)影響的方向。The position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.

This principle applies to any reaction that is in equilibrium.

(1)物質(zhì)濃度變化對(duì)于平衡的影響The effect of concentration changes on equilibrium

改變反應(yīng)物或者是生成物的濃度，不會(huì)影響這個(gè)反應(yīng)的平衡常數(shù)，但是會(huì)引起平衡移動(dòng)。Changing concentration of a reactant or product does not change the numerical value of the equilibrium constant,but it does change the position of the equilibrium.

In general,the position of the equilibrium is shifted towards the right if the concentration of a reactant is increased or to the left if the concentration of a product is increased.

At the start,when the change is made,the mixture is not at equilibrium,but equilibrium is eventually restored.

(2)壓強(qiáng)變化對(duì)于平衡的影響The effect of pressure changes on equilibrium

對(duì)于前后氣體分子個(gè)數(shù)不相等的反應(yīng)，壓強(qiáng)或者是體積變化也不會(huì)改變反應(yīng)的平衡常數(shù)，但是會(huì)引起平衡的移動(dòng)。For a reaction involving gases,altering the pressure may cause a change in the position of the equilibrium.

若反應(yīng)后氣體分子數(shù)增多，增大壓強(qiáng)，平衡向著逆向移動(dòng)。For a reaction where there is an increase in the number of moles from reactants to products,increasing the pressure moves the equilibrium to the left.

若反應(yīng)前后氣體分子數(shù)減少，增大壓強(qiáng)，平衡向著正向移動(dòng)。Where there is a decrease in the number of moles from reactants to products,increasing the pressure moves the equilibrium to the right.The equilibrium constant remains the same.

(3)溫度變化對(duì)于平衡的影響The effect of temperature changes on equilibrium

The change that takes place when temperature is changed depends upon whether the forward reaction is exothermic or endothermic.

If the forward reaction is exothermic then the backward one is endothermic.

吸熱反應(yīng)，升溫，平衡向正向移動(dòng)。If the temperature is increased,the equilibrium moves to the left,since an endothermic reaction will tend to reduce the temperature.

放熱反應(yīng)，升溫，平衡向逆向移動(dòng)。Conversely,if the temperature is decreased then the equilibrium,moves to the right.

(4)催化劑/稀有氣體對(duì)于平衡的影響The effects of catalysts on equilibrium

沒有影響!A catalyst or addition of noble gas has no effect on the position of the equilibrium. However,a catalyst does increase the rate of both the forward and backward reactions,decreasing the time taken to reach equilibrium.