Factor affecting reaction_rate

Factors affecting Reaction Rate影響反應速率的因素

29-3-2011

Some chefs prevent the browning by brushing the surface of the fruit with lemon juice.

有些廚師為防蘋果褐變，在蘋果表面上塗上檸檬汁。

Why they have different rate of reaction?

為什麼它們有不同的反應速率？

-Brainstorming Time-

How does a reaction successfully happen?

一個化學反應怎樣成功發生？

Collision Theory碰撞理論

For a chemical reaction to occur, the reactant particles must collide with each other. It is so-called effective collision.

一個化學反應需要發生，反應物粒子必須互相碰撞。這被稱之為有效碰撞。

1. Right Orientation

方位正確3. High kinetic energy enough

足夠高的動能

Collision Theory碰撞理論

反應物粒子互相靠近

有效碰撞

無效碰撞

没有發生反應

產生生成物

+

A BA B

AA

BB

B

B

A

A

AA

BB

AA B

B

Collision Theory碰撞理論

Therefore only small fraction of particles can produce effective collision.

因此只有少數粒子能產生有效碰撞。

速率 ∝ 有效碰撞的數目 單位時間

rate ∝ number of effective collisions

unit time

Factors affecting rate of reaction影響反應速率的因素

Concentration of reactants

反應物的濃度3. Temperature

溫度 Surface area of solid reactant

固體反應物的表面面積 Presence of catalyst

催化劑的使用

Investigating effect of concentration of reactant on reaction rate探究反應物的濃度對反應速率的影響

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Try to design an experiment to investigate the effect of concentration

on rate of reaction

嘗試設計一個實驗去探究濃度對反應速率的影響

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

• Conical flask 　錐形瓶

• Stopper and delivering tube 　膠塞及膠管

• Syringe and plunger 針筒及栓塞

• Stop-watch 計時器

柱塞

計時錶

Mg

thread 線

HCl (in excess 過量 )

H2 針筒

Plastic tube 膠管

Measure the volume of hydrogen gas evolved 量度反應所釋出的氫氣的體積

plunge

syringeStop-watch

• Concentration of acid 酸的濃度 (0.5 M 、 1.0 M 、 1.5 M 和 2.0 M)

Same mass of Mg and same volume of acid相同質量的鎂，和相同體積的酸

Control Experiment 　對照實驗

反應時間 (min)

氫氣的體積

(cm3)Vol. of H2

Time of reaction

Explanation

解釋

濃度較低 碰撞機會較低

濃度較高 碰撞機會較高

濃度上升

Conc. increase

Low concentration Lower chance to collide

High concentration Higher chance to collide

Reaction involving gases涉及氣體的反應

壓強較低 ∴ 碰撞機會較低

壓強較高 / 體積下降 ∴ 碰撞機會較高

壓強上升

Pressure increase

Low Pressure Lower chance to collide

High Pressure Higher chance to collide

Investigate the effect of Temperature on the rate of reaction探究溫度對反應速率的影響

S2O32–(aq) + 2H+(aq) → SO2(g) + S(s) + H2O(l)

交叉隨時間變得越來越模糊。

0.08012.565

0.05717.560

0.04025.055

0.02050.045

0.010100.035

0.004250.022

(s−1)

交叉被完全「遮蔽」的時間 (s)

反應混合物的溫度 (°C)

溫度 (°C)

1

交叉被「遮蔽」所需的時間

Explanation

解釋

反應物粒子的移動速率較慢

反應物粒子的移動速率較快

溫度上升

Temperature increase

Reactant particles move at lower speed

Reactant particles move at higher speed

Investigate the effect of surface area of solid reactant on rate of reaction探究固態反應物的表面面積對反應速率的影響

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

Investigate the effect of surface area of solid reactant on rate of reaction探究固態反應物的表面面積對反應速率的影響

塊狀石灰石 粉狀石灰石 Powdered limestone

Lumps of limestone

反應時間 ( 分 )

反應結束

反應結束

塊狀

石灰

石 粉

狀石

灰石

減少的質量

(g)

Experimental Result

實驗結果

Decrease in m

ass

Time of reaction

Reaction ends

Reaction ends

Lumps

of lim

estone

Powde

red lim

estone

Initial rate of powdered limestone is higher than that of lumps of limestone.粉狀石灰石的初速比塊狀石灰石的初速高。

The reaction of powdered limestone finishes earlier.粉狀石灰石的反應較早完結。

Experimental Result

實驗結果

Explanation

解釋

總表面積 = (2 × 2) × 6 cm2

= 24 cm2

總表面積 = (1 × 1) × 6 × 8 cm2

= 48 cm2

2 cm 1 cm

固體的表面面積較小 碰撞機會較低

固體的表面面積較大 碰撞機會較高

表面面積增加

Increase in surface area

Smaller surface area of solid

lower chance to collide

larger surface area of solid

higher chance to collide

Explanation

解釋

Check Point Lumps of coal can be barely ignited. However, coal dust

in coal mines is one of the causes of explosive combustion. Explain.

煤塊很難點燃，但煤礦中的煤粉卻是引致爆炸性燃燒的原因之一。試加以解釋

ANS: Coal dust is very finely divided state, with very large surface area in contact with oxygen in air. It can be ignited easily.

煤粉是極小的煤。由於煤粉與空氣中的氧的接觸表面面積極大，因此極之易燃。

Investigate the effect of catalyst on reaction rate探究催化劑對反應速率的影響

• Catalyst is a substance that changes(usually increases) the rate of a reaction without being used up.

• 催化劑是能改變 (通常提升 ) 反應速率但不會被消耗的物質。

2H2O2(aq) → 2H2O(l) + O2(g)

Decomposition of hydrogen peroxide過氧化氫的分解作用

2H2O2(aq) → 2H2O(l) + O2(g)MnO2(s)

vs.

計時錶

棉花塞子

少量固態MnO2

已知體積的標準 H2O2(aq)

電子天平

Cotton wool plug

Measured volume of standard H2O2(aq)Stop-watch

Small amount of solid MnO2

Electronic balance

Experimental setup

實驗設置

加入MnO2

没有加入MnO2

時間 (min)

反應混合物的質量

(g)

Mass of reaction m

ixture

Without MnO2

With MnO2

Time

Experimental Result

實驗結果

The industrial use of catalyst催化劑在化學工業上的應用

Haber process哈柏法

Contact process接觸法

N2(g) + 3H2(g) 2NH3(g)Iron powder鐵

粉 450°C 、 250 atm

2SO2(g) + O2(g) 2SO3(g)V2O5

450°C 、 1 atm

Chemical reaction within living cells are mostly catalysed by enzymes.

生物細胞內的化學反應大多數是被酶催化的

We call enzymes biological catalysts. 我們將酶稱為生物催化劑

Catalysts in biological systems生物體系上的催化劑

Catalysts in biological systems生物體系上的催化劑

Enzyme involved in the browning of fruits

導致水果褐變的酶

Enzyme in humans 人體內的酶人體內的酶

Enzyme in yeast

酵母內的酶酵母內的酶